Biophysical Definitions
Molecular Weight
One mole of an element or compound is the atomic weight or molecular weight, respectively, in grams. For example, 1 mol of sodium is 23 g (atomic weight Na = 23) and 1 mol of sodium chloride is 58.5 g (atomic weight Cl = 35.5; 35.5 + 23 = 58.5). A ‘normal’ (molar) solution contains 1 mol/L of solution. Therefore a ‘normal’ solution of sodium chloride contains 58.5 g and is a 5.85% solution. This is very different from a physiological ‘normal’ solution of sodium chloride, where the concentration of sodium chloride (0.9%) is adjusted so that the sodium has a similar concentration as the total number of cations in plasma (154 mmol/L). The concentrations of biological substances are usually much weaker than molar. However, commonly used intravenous solutions that combine sodium chloride with glucose often contain sodium chloride 0.18% (sodium 30 mmol/L and chloride 30 mmol/L) and glucose 4%. Injudicious use of excessive volumes of this combination with 30 mmol NaCl will quickly lead to hyponatraemia.
The conventional nomenclature for decreasing molar concentrations is given below. The same prefixes may be used for different units of measurement:
1 equivalent (Eq) = 1 mol divided by the valency, (valency is the capacity of the atom to combine with a hydrogen atom). Thus 1 Eq of sodium (valency 1) = 23 g, and 1 mol of sodium = 1 Eq, that is, 1 mmol = 1 mEq.
However, 1 Eq of calcium (valency 2, mol wt 40) = 20 g. 1 mol of calcium = 2 Eq, and 1 mmol Ca 2+ = 2 mEq Ca 2+ .
Measurements in medicine are wherever possible being made in Systeme Internationale (SI) units. Under this system, the concentration of biological materials is expressed in the appropriate molar units (often mmol) per litre (L).
The units used in the measurement of osmotic pressure are considered below.
Distribution of Water and Electrolytes
A normal 70 kg man is composed of 60% water, 18% protein, 15% fat and 7% minerals. Obese individuals have relatively more fat and less water. Of the 60% (42 L) of water, 28 L (40% of body weight) are intracellular; the remaining 14 L of extracellular water are made up of 10.5 L of interstitial fluid (extracellular and extravascular) and 3.5 L of blood plasma. The total blood volume (red cells and plasma) is 8% of total body weight, or about 5.6 L.
The reference method of measuring total body water (TBW) is to give a subject deuterium oxide (D 2 O), ‘heavy water’, and measuring how much it is diluted. Extracellular fluid volume can be measured with inulin by the same principle. Intracellular fluid volume = TBW (D 2 O space) less extracellular fluid volume (inulin space). Intravascular fluid volume can be measured with Evans blue dye. Total blood volume can be calculated knowing intravascular fluid volume and the haematocrit. Interstitial fluid volume = extracellular fluid volume (inulin space) less intravascular fluid volume. TBW and distribution can now be measured noninvasively with bioelectric impedance analysis (BIA).
The distribution of electrolytes and protein in intracellular fluid, interstitial fluid and plasma is given in Fig. 10.1 . Note that, for reasons of comparability, concentrations are expressed in milliequivalents per litre (mEq/L) of water, not millimoles per litre (mmol/L) of plasma.
The major difference between plasma and interstitial fluid is that interstitial fluid has relatively little protein. As a consequence, the concentration of sodium in the interstitial fluid is less and therefore, so is the overall osmotic pressure (see below). There are further major differences between intracellular fluid and extracellular fluid. Sodium is the major extracellular cation, whereas potassium and, to a lesser extent, magnesium are the predominant intracellular cations. Chloride and bicarbonate are the major extracellular anions; protein and phosphate are the predominant intracellular anions.
Anion Gap
In considering the composition of plasma for clinical purposes, account is often taken of the ‘anion gap’. This is calculated by subtracting the concentrations of the two principal anions, chloride (100 mmol/L) and bicarbonate (24 mmol/L), from the principal cation, sodium (136 mmol/L). This leaves a positive balance of 12 mmol/L: normal range 8 to 16 mmol/L. The gap is considered to exist because of the occurrence of unmeasured anions, such as protein and lactate, which balance the number of cations. An increase in the anion gap suggests that there are more unmeasured anions present than usual. This occurs in such situations as lactic acidosis, or diabetic ketoacidosis, where the lactate and acetoacetate are balancing the excess sodium ions. A more complete explanation of the anion gap would be to consider both the unmeasured cations as well as the unmeasured anions, as in Table 10.1 . Situations where the anion gap is increased include ketoacidosis, lactic acidosis and hyperosmolar acidosis, and poisoning with salicylate, methanol, ethylene glycol and paraldehyde, and hypoalbuminaemia. A decreased anion gap occurs in bromide poisoning and myeloma, which is characterised by excess immunoglobulin.
| Cation | Anion | |||
|---|---|---|---|---|
| Na + | 136 | Cl − | 100 | |
| — | HCO 3 − | 24 | ||
| — | ||||
| 136 | 124 | |||
| Gap | 12 | |||
| — | — | |||
| 136 | 136 | |||
| The gap consists of unmeasured cations and anions: | ||||
| K + | 4.5 | Protein | 15 | |
| Ca 2+ | 5 | PO 4 3− | 2 | |
| Mg 2+ | 1.5 | SO 4 2− | 1 | |
| Organic acids | 5 | |||
| — | — | |||
| 11 | 23 | |||
| — | — | |||
| 147 | 147 | |||
Cellular Transport Mechanisms
These mechanisms account for the movement of substances within cells and across cell membranes. The transport mechanisms to be considered include diffusion, solvent drag, filtration, osmosis, non-ionic diffusion, carrier-mediated transport and phagocytosis. Not all of these mechanisms will be considered in detail.
Diffusion is the process whereby a gas or substance in solution expands to fill the volume available to it. Relevant examples of gaseous diffusion are the equilibration of gases within the alveoli of the lung, and of liquid diffusion, the equilibration of substances within the fluid of the renal tubule. An element of diffusion may be involved in transport across all cell membranes because there is a layer of unstirred water up to 400 μm thick adjacent to biological membranes.
If there is a charged ion that cannot diffuse across a membrane which other charged ions can cross, the diffusible ions distribute themselves as in the following example:
The cell is permeable to K + and Cl − but not to protein. Since intracellular potassium (K i ) is about 157 mmol/L and extracellular potassium (K 0 ) is 4 mmol/L, the Gibbs–Donnan equilibrium would predict that the ratio of chloride concentration outside the cell to that inside should be 157/4 (i.e. about 40). In fact, there is almost no intracellular chloride so that the ratio in vivo is even greater than 40. This is because factors other than simple diffusion affect both potassium and chloride concentrations.
Solvent drag is the process whereby bulk movement of solvent drags some molecules of solute with it. It is of little physiological importance.
Filtration is the process whereby substances are forced through a membrane by hydrostatic pressure. The degree to which substances pass through the membrane depends on the size of the holes in the membrane. Small molecules pass through the holes, larger molecules do not. The renal glomerular basement membrane (GBM) has holes too small for blood cells and the majority of plasma proteins to traverse, but large enough to allow filtration of most other blood constituents. The GBM also acts as a charge-selective filtration barrier, favouring the passage of positively charged molecules.
Osmosis describes the movement of solvent from a region of low solute concentration, across a semipermeable membrane to one of high solute concentration. The process can be opposed by hydrostatic pressure; the pressure that will stop osmosis occurring is the osmotic pressure of the solution. This is given by the formula:
where P = osmotic pressure, n = number of osmotically active particles, R = gas constant, T = absolute temperature, V = volume. For an ideal solution of a non-ionised substance, n/V equals the concentration of the solute. In an ideal solution, 1 osmol of a substance is then defined such that:
So for an ideal solution of glucose:
However, sodium chloride dissociates into two ions in solution. Therefore, for sodium chloride:
Calcium chloride dissociates into three ions in solution. Therefore, for calcium chloride,
However, the molecules or ions of all solutions aggregate to a certain degree so that interaction occurs between the ions or molecules, and they each do not behave as osmotically independent particles and do not form ideal solutions.
Freezing point depression by a solution is also caused by the number of osmotically active particles. The greater the concentration of osmotically active particles, the greater the freezing point depression. In an ideal solution, with no interaction, 1 mol of osmotically active particles per litre depresses the freezing point by 1.86°C. Therefore, an aqueous solution which depresses the freezing point by 1.86°C is defined as containing 1 osmol/L. One which depresses the freezing point by 1.86°C/1000 (i.e. 0.00186°C) contains 1 mosmol/L. Plasma (osmotic pressure 300 mosmol/L) has a freezing point of (0 to 0.00186 × 300) °C = −0.56°C.
Osmolarity defines osmotic pressure in terms of osmoles per litre of solution. Since volume changes at different temperatures, osmolality which defines osmotic pressure in terms of osmoles per kilogram of solution is preferred, though not always employed. The major osmotic components of plasma are the cations sodium and potassium, and their accompanying anions, together with glucose and urea.
The concentration of sodium is about 140 mmol/L. This, and the accompanying anions, will therefore contribute 280 mosmol/L. The concentration of potassium is about 4 mmol/L, which, with its accompanying anions, will give 8 mosmol/L. Glucose and urea contribute 5 mosmol/L each to a total of 300 mosmol/L in normal plasma. During pregnancy, due to an expansion of plasma volume this falls to below 290 mosmol/L. The mechanism of plasma volume expansion appears to relate to a resetting of the hypothalamic thirst centre, so that in early pregnancy women still feel thirsty at a lower plasma osmolality than when not pregnant.
We are now in a position to consider some of the forces acting on water in the capillaries ( Fig. 10.2 ). The capillary membrane behaves as if it is only permeable to water and small solutes. It is impermeable to colloids such as plasma protein. There is a difference of 25 mmHg in osmotic pressure between the interstitial water and the intravascular water due to the intravascular plasma proteins (see above). This force (oncotic pressure) will tend to drive water into the capillary. At the arteriolar end of the capillary, the hydrostatic pressure is approximately 37 mmHg; the interstitial pressure is 1 mmHg. The net force driving water out is therefore 37 − 1 − 25 = 11 mmHg, and water tends to pass out of the arteriolar end of the capillary. At the venous end of the capillary, the pressure is approximately 20 mmHg lower, at around 17 mmHg. The net force driving water in the capillary is therefore 25 + 1 − 17 = 9 mmHg. Fluid therefore enters the capillary at the venous end. Factors which would decrease fluid reabsorption and cause clinical oedema are a reduction in plasma proteins, so that the osmotic gradient between the intravascular and interstitial fluids might be only 20 mmHg, not 25 mmHg, or a rise in venous pressure, for example, due to increased pressure on the inferior vena cava in the third trimester of pregnancy, so that the pressure at the venous end of the capillary might be 25 mmHg, rather than 17 mmHg.
Non-ionised diffusion is the process whereby there is preferential transport in a non-ionised form. Cell membranes consist of a lipid bilayer with specific transporter proteins embedded in it. Lipid-soluble drugs (e.g. propranolol) can cross the lipids of the blood–brain barrier or the placenta by non-ionised diffusion. But small hydrophilic molecules such as O 2 can also diffuse across the lipid bilayer, which is also permeable to water.
Carrier-mediated transport implies transport across a cell membrane using a specific carrier. If the transport is down a concentration gradient from an area of high concentration to one of low concentration, this is known as facilitated transport (e.g. the uptake of glucose by the muscle cell) facilitated by the participation of insulin in the transport process. If the carrier-mediated transport is up a concentration gradient from an area of low concentration to one of high concentration, this is known as active transport (e.g. the removal of sodium from muscle cells by the ATPase-dependent sodium pump). The channel may be ligand gated where binding of external (e.g. insulin as earlier) ligands or an internal ligand opens the channel. Alternatively the channel may be voltage gated, where patency depends on the transmembrane electrical potential; voltage gating is a major feature of the conduction of nervous impulses.
Phagocytosis and pinocytosis involve the incorporation of discrete bodies of solid and liquid substances, respectively, by cell wall growing out and around the particles so that the cell appears to swallow them. If the cell eliminates substances, the process is known as exocytosis; if substances are transported into the cell, the process is endocytosis. In endocytosis, the Golgi apparatus is involved in intracellular transport and processing to varying extents depending on whether exocytosis is via the non-constitutive pathway (extensive processing) or the constitutive pathway (little processing). Similarly, endocytosis may involve specific receptors for substances such as low-density lipoproteins (receptor-mediated endocytosis) or there may be no specific receptors (constitutive endocytosis).
Acid–Base Balance
Normal Acid–Base Balance
A simple knowledge of chemistry allows some substances to be easily categorised as acids or bases. For example, hydrochloric acid is clearly an acid and sodium hydroxide is a base. But when describing acid–base balance in physiology, these terms are used rather more obscurely. For example, the chloride ion may be described as a base. A more applicable definition is to define an acid as an ion or molecule which can liberate hydrogen ions. Since hydrogen ions are protons (H + ), acids may also be defined as proton donors. A base is then a substance which can accept hydrogen ions, or a proton acceptor. If we consider the examples below, hydrochloric acid dissociates into hydrogen ions and chloride ions, and is therefore a proton donor (acid). If the chloride ion associates with hydrogen ions to form hydrochloric acid, the chloride ion is a proton acceptor (base). Ammonia is another proton acceptor when it forms the ammonium ion. Carbonic acid is an acid (hydrogen ion donor); bicarbonate is a base (hydrogen ion acceptor). The H 2 PO 4 − ion can be both an acid when it dissociates further to HPO 4 2− and a base when it associates to form H 3 PO 4 :
pH
The pH is defined as the negative log 10 of the hydrogen ion concentration expressed in mol/L. A negative logarithmic scale is used because the numbers are all less than 1 and vary over a wide range. Since the pH is the negative logarithm of the hydrogen ion concentration, a low pH number (e.g. pH 6.2) indicates a relatively high hydrogen ion concentration (i.e. an acidic solution). High pH numbers (e.g. pH 7.8) represent a lower hydrogen ion concentration (i.e. alkaline solutions). Because the pH scale is logarithmic to the base 10, a 1-unit change in pH represents a 10-fold change in hydrogen ion concentration.
The normal pH range in human tissues is 7.36 to 7.44. Although a neutral pH (hydrogen ion concentration equals hydroxyl ion concentration) at 20°C has the value 7.4, water dissociates more at physiological temperatures, and a neutral pH at 37°C has the value 6.8. Therefore, body fluids are mildly alkaline (the higher the pH number, the lower the hydrogen ion concentration).
A pH value of 7.4 represents a hydrogen ion concentration of 0.00004 mmol/L as seen in the following example:
Partial Pressure of Carbon Dioxide
In arterial blood, the normal P co 2 value is 4.8 to 5.9 kPa (36 to 44 mmHg). It is a coincidence that the figures expressing P co 2 in mmHg are similar to those expressing the normal range for pH (7.36 to 7.44).
Henderson–Hasselbalch Equation
This equation describes the relationship of hydrogen ion, bicarbonate and carbonic acid concentrations (see Equation (3) below). It can be rewritten in terms of pH, bicarbonate and carbonic acid concentrations, as in Equation (4) , but carbonic acid concentrations are not usually measured. However, because of the presence of carbonic anhydrase in red cells, carbonic acid concentration is proportional to P co 2 ( Equation (1) ). Equation (4) can therefore be rewritten in terms of pH, bicarbonate and P co 2 ( Equation (5) ). All these data are usually available from blood gas analyses. If we know any two of these variables, the third can be calculated.
Carbonic anhydrase:
By the Law of Mass Action:
By taking logarithms of the reciprocal:
K ′ is a constant equal to 6.1 ⁎ *
⁎ * For Eq. (5) , because of the action of carbonic anhydrase, [H 2 CO 3 ] is proportional to P a co 2 . For the given constants of Eq. (5) , P co 2 is expressed in mmHg.
:*For Equation (5), because of the action of carbonic anhydrase, [H2CO3] is proportional to Paco2. For the given constants of Equation (5), Pco2 is expressed in mmHg.
Control of pH
The Henderson–Hasselbalch equation, expressed in Equation (5) , indicates that the variables controlling pH are P co 2 and bicarbonate concentration. Ultimately, P co 2 is controlled by respiration. Short-term changes of pH may therefore be compensated for by changing the depth of respiration. Bicarbonate concentration can be altered by the kidneys, and this is the mechanism involved in the long-term control of pH. Further details of these mechanisms are given on pp. 25 and 201.
Buffers
A buffer solution is one to which hydrogen or hydroxyl ions can be added with little change in the pH.
Consider a solution of sodium bicarbonate to which is added hydrochloric acid ( Fig. 10.3 ). The hydrogen ions of the hydrochloric acid react with bicarbonate ions of the sodium bicarbonate to form carbonic acid. Carbonic acid does not dissociate so readily as hydrochloric acid. Therefore the hydrogen ions are buffered. Reading from right to left in Fig. 10.3 , we have a solution that starts as 100% bicarbonate ions and becomes 100% carbonic acid as hydrochloric acid is added. Initially, in the pH range 9 to 7, a very small change in bicarbonate concentration, requiring the addition of only a few hydrogen ions, is associated with a large change in pH. However, in the steep part of the curve, between pH 5 and 7, a considerable quantity of hydrogen ions can be added, as indicated by a marked fall in the proportion of bicarbonate remaining, with relatively little change in pH. It is in that pH range that the buffering ability of bicarbonate is greatest.
The pH at which 50% of the buffer is changed from its acidic to its basic form (or vice versa) is known as the p K . For bicarbonate the p K is 6.1, making bicarbonate rather poor as a buffer for body fluids, since the p K is considerably towards the acidic side of the physiological pH range (7.36 to 7.44). The buffer value of a buffer (mmol of hydrogen ion per gram per pH unit) is the quantity of hydrogen ions which can be added to a buffer solution to change its pH by 1.0 pH unit from p K + 0.5 to p K −0.5.
In blood, the most important buffers are proteins. These are able to absorb hydrogen ions onto free carboxyl radicals, as illustrated in Fig. 10.4 . Of the proteins available, haemoglobin is more important than plasma protein, partly because its buffer value is greater than that of plasma protein (0.18 mmol of hydrogen per gram of haemoglobin per pH unit, vs 0.11 mmol of hydrogen per gram of plasma protein per pH unit), but also because there is more haemoglobin than plasma protein (15 g haemoglobin per 100 mL vs 3.8 g of plasma protein per 100 mL). These two factors mean that haemoglobin has six times the buffering capacity of plasma protein. In addition, deoxygenated haemoglobin is a weaker acid and a more efficient buffer than oxygenated haemoglobin. This increases the buffering capacity of haemoglobin where it is needed more, after oxygen has been liberated in the peripheral tissues.
Buffer Base and Base Excess
The buffer base is the total number of buffer anions (usually 45 to 50 mEq/L of blood) and consists of bicarbonate, phosphate and protein anions (haemoglobin and plasma protein).
Base excess is the difference between the actual buffer base and the normal value for a given haemoglobin and body temperature. It is negative in acidosis and is then sometimes expressed as a positive base deficit, and positive in alkalosis. It gives an index of the severity of the abnormality of acid–base balance.
Standard bicarbonate
This is the carbon dioxide content of blood equilibrated at a P co 2 of 40 mmHg and a temperature of 37°C when the haemoglobin is fully saturated with oxygen. In general it represents the non-respiratory part of acid–base derangement and is low in metabolic acidosis and raised in metabolic alkalosis. The normal value for the standard bicarbonate is 27 mmol/L.
Abnormalities Of Acid–Base Balance
These are usually divided into acidosis (pH <7.36) and alkalosis (pH 7.44). In addition, we consider respiratory acidosis and alkalosis where the primary abnormality is in respiration (carbon dioxide control) and metabolic acidosis and alkalosis, which are best defined as abnormalities that are not respiratory in origin. Only initial, single abnormalities will be considered. For these single uncomplicated abnormalities, respiratory and metabolic acidosis and alkalosis can be defined according to Table 10.2 , which gives the values of pH and P co 2 characterising each abnormality.
| pH | P co 2 (kPa) | P co 2 (mmHg) | |
|---|---|---|---|
| Normal | 7.36–7.44 | 4.8–5.9 | 36–44 |
| Respiratory acidosis | <7.36 | >5.9 | >44 |
| Respiratory alkalosis | >7.44 | <4.8 | <36 |
| Metabolic acidosis | <7.36 | <5.9 | <44 |
| Metabolic alkalosis | >7.44 | >4.8 | >36 |
Respiratory Acidosis
There is a low pH and a high P co 2 . Here the basic abnormality is a failure of carbon dioxide excretion from the lungs. Carbon dioxide dissolves in the blood, and in the presence of carbonic anhydrase, carbonic acid is formed which dissociates into hydrogen ions and bicarbonate (Equations (1) and (2), p. 5). Respiratory acidosis may arise from abnormalities of respiration, which may range from impaired respiratory control due to excessive sedation, to chronic pulmonary disease. In the long term, respiratory acidosis is compensated by bicarbonate retention in the kidneys, which increases pH towards normal values.
Respiratory Alkalosis
There is a high pH and a low P co 2 . This is induced by hyperventilation, whatever the cause. Perhaps the commonest clinical presentation is anxiety, where the acute fall in hydrogen ion concentration due to blowing off carbon dioxide may cause paraesthesiae, or even tetany. Tetany occurs because more plasma protein is ionised when the pH is high. This protein binds more calcium, lowering the ionised (metabolically effective) calcium level. However, respiratory alkalosis is also seen in the early stages of exercise, at altitude and in patients who have had a pulmonary embolus. In pregnancy, there is hyperventilation but the kidney excretes sufficient bicarbonate to compensate fully for the fall in carbon dioxide, and there is therefore no change in pH.
Metabolic Acidosis
There is a low pH and the P co 2 is not elevated. This may occur because of excessive acid production, impaired acid excretion or excessive alkali loss. Examples of excess acid production are diabetic ketoacidosis and methanol poisoning, in which methanol is metabolised to formaldehyde, which subsequently forms formic acid.
Failure of acid excretion occurs in chronic renal failure, and more specifically in renal tubular acidosis, where the patients are not initially uraemic but acid excretion by the kidney is impaired. Acetazolamide is a diuretic drug which inhibits ammonia formation within the kidney, and this too causes metabolic acidosis. Excess alkali loss is seen in patients who have a pancreatic fistula or prolonged diarrhoea, since both the bodily fluids lost are alkaline.
Women with diabetes are prone to ketosis and ketoacidosis in pregnancy but even non-diabetic women may become ketotic or ketoacidotic in pregnancy during periods of starvation, particularly in the third trimester. During the second half of normal pregnancy, a relatively insulin resistant state develops secondary to increased levels of placentally derived counter-regulatory hormones such as glucagon, human placental lactogen and cortisol. In the absence of glucose and sufficient glycogen stores during fasting, adipose tissue is utilised as an energy source with fatty acid oxidation to acetyl CoA. Acetyl CoA can then be utilised by the citric acid cycle to produce ATP. If, however, the capacity of the citric acid cycle is overwhelmed, acetyl CoA will instead be converted to ketones including acetone, acetoacetate and β-hydroxybutyrate. Whilst a healthy adult would only become ketoacidotic after prolonged periods of fasting (i.e. >14 days), in the third trimester, pregnant woman can develop ketoacidosis in as little as 24 hours. When assessing ketoacidosis as a cause of metabolic acidosis, a finger prick test of capillary ketones, measuring β-hydroxybutyrate, are preferred to urinary ketones, which measure acetoacetate. β-Hydroxybutyrate is present in higher concentrations than acetoacetate and capillary testing can more accurately track response to treatment.
Metabolic Alkalosis
The pH is high and the P co 2 is not reduced. This may occur due to prolonged vomiting. The mechanism is less to do with the loss of acidic fluid and more to a loss of fluid volume and a compensatory activation of the renin–angiotensin–aldosterone system. Sodium is reabsorbed at the renal tubules at the expense of potassium and hydrogen ions. Metabolic alkalosis also occurs in excessive alkali ingestion, seen in patients who take antacids for peptic ulceration. Metabolic alkalosis frequently accompanies hypokalaemia.
Cardiovascular System
Cardiac disease is now the leading cause of maternal mortality in the United Kingdom as described in successive Mothers and Babies: Reducing Risk through Audits and Confidential Enquiries across the UK (MBRRACE) reports. This section will detail the physiology of both cardiac output and the conduction system in a normal pregnancy as well as examining normal pregnant haemodynamics and the potential changes that can occur in cardiac disease.
Conduction System Of The Heart
The heart has its own unique electrical conduction tissue ( Fig. 10.5 ) which allows orderly coordinated activity between atria and ventricles to ensure maximum efficiency and cardiac output. The electrical impulse is generated by the sino-atrial (SA) node which is located high in the right atrium at the entry of the superior vena cava. The impulse is then transmitted across both atria by crossing adjoining cardiomyocytes of the smooth muscle via gap junctions resulting in atrial contraction. There is an electrical seal allowing no conduction between the atria and ventricles which in the normal heart is broken only by the atrioventricular (AV) node. The electrical impulse once arrived at the AV node is stored for a few milliseconds to allow maximum ventricular filling from the atria. The AV node, which sits in the AV ring, conducts the impulse through specialised conduction tissue called the His–Purkinje system. The His bundle divides into a right and left branch which innervate the right and left ventricles respectively. The right bundle is a relatively narrow group of fibres. The left bundle is a much wider sheet of fibres and divides further into fascicles. Thus right bundle branch block due to damage to the right bundle occurs relatively easily and is not necessarily of pathological significance. Left bundle branch block implies considerable additional damage to the underlying myocardium to interrupt such a wide sheet of fibres and is always pathological. Interruption or damage to the normal conduction system can lead to varying degrees of heart block. In the event of failure of the SA or AV node, the ventricular tissue has the ability to contract under its own intrinsic rate, although this is usually at a much slower rate than normal.
Some patients have additional electrical pathways which cross the AV seal and can conduct impulses antegradely (from atria to ventricles) and retrogradely (from ventricles to atria). By having an additional pathway to the AV node, impulses can pass from atria to ventricles and back again to create a circuit which causes tachyarrhythmias. The most common example of this is Wolff–Parkinson–White (WPW) syndrome that predisposes to supra-ventricular tachycardia.
Factors Affecting Heart Rate
The activity of the SA node is controlled neurogenically by the sympathetic and parasympathetic nervous systems, directed by the vasomotor and cardio-inhibitory centres, respectively (see later). At rest, the dominant tone is parasympathetic, mediated via the vagus nerve (a muscarinic effect; Table 10.3 ).
| Location | Receptor | Comments |
|---|---|---|
| Heart muscle and conducting tissue | Cholinergic |
|
| α-Adrenergic | Nil | |
| β 2 -Adrenergic |
| |
| Blood vessels | Cholinergic (vasodilator) |
|
| α-Adrenergic (vasoconstrictor) | All tissues | |
| β 1 -Adrenergic (vasodilator) |
|
In addition, the discharge rate from the SA node and therefore heart rate is increased by the direct actions of thyroxine, high temperature, β-adrenergic activity, and block of the dominant parasympathetic tone by atropine. Conversely, it is decreased by hypothyroidism, hypothermia, and β-adrenergic blockade. SA node activity is also decreased in ischaemia, and under these circumstances other intrinsic pacemakers (AV node, ventricles) take over pacemaker activity, albeit at a slower rate.
Cardiac Chambers
Table 10.4 shows the normal dimensions for the cardiac chambers outside of pregnancy. In pregnancy, the chambers increase to accommodate the increased circulating volume with the largest changes being seen in the left and right atrium (an increase of 5 and 7 mm, respectively) (Campos, 1996).
| Control | Weeks 8–12 | Weeks 20–24 | Weeks 30–34 | Weeks 36–40 | Change cf Control | |
|---|---|---|---|---|---|---|
| LVEDd | 40.1 | 41.1 | 42.7 | 43.0 | 43.6 | 3.5 |
| LA | 27.9 | 29.6 | 31.5 | 33.1 | 32.8 | 4.9 |
| RVEDd | 28.5 | 30.1 | 31.9 | 35.5 | 35.5 | 4.4 |
| RA | 43.7 | 42.8 | 47.4 | 50.8 | 50.9 | 7.2 |
Electrocardiogram
Fig. 10.6 shows a normal electrocardiogram (ECG). The P wave is atrial depolarisation which leads to atrial contraction while the QRS complex is ventricular depolarisation which leads to ventricular contraction. The T wave is secondary to ventricular repolarisation. Atrial repolarisation is not seen on the surface ECG as it occurs at the same time as ventricular depolarisation, and it is too small an electrical signal to be seen within the QRS. The normal ECG is recorded at a speed of 25 mm/s, so each small square represents 0.04 seconds and each large square represents 0.2 seconds. In the vertical axis, the ECG is calibrated so that 1 cm equals 1 mV. In order to calculate the heart rate, divide 300 by N , where N is the number of large squares between successive R waves. In the event of atrial fibrillation (AF), where it is variable, an average is taken.
The normal PR interval is between 0.12 and 0.20 ms. If there is a delay, then there is a delay in conduction between the atria and ventricles and this is known as first-degree heart block. If the PR interval is short, then the electrical impulse is being transmitted between the atria and ventricles through a much faster pathway than normal, which implies aberrant conduction. This is typically seen in WPW syndrome and leads to a rapid inflection on the upstroke of the R wave known as a delta wave.
The normal QRS width should be no greater than 0.12 seconds (three small squares) and any longer is due to a delay in the impulse travelling along the His–Purkinje system. This is known as bundle branch block and, depending upon which bundle is involved, leads to a different morphology of the QRS seen best in lead V1. The QT interval is between 0.30 and 0.45 seconds and is dependent upon heart rate. It is increased in hypocalcaemia, hypokalaemia, rheumatic carditis and with a large number of drugs. It is decreased in hypercalcaemia, hyperkalaemia and digoxin.
Pressure And Saturation In The Cardiac Chambers
Blood enters the right side of the heart via the inferior and superior vena cava ( Fig. 10.7 ). That which comes from the head is more desaturated than that from the rest of the body due to increased consumption by the brain, and normal mixed venous oxygen saturation in the right atrium (RA) is usually around 60%. If there is oxygenated blood abnormally entering the atrium due to a shunt or atrial septal defect, then this will lead to a step up in the saturations if sampled from high to low RA and will lead to an increased mixed venous saturation. True mixed venous blood, however, is best taken from the pulmonary artery (PA) as blood from the coronary sinus enters the right atrium and with streaming, which occurs in the right atrium and ventricle, blood is not fully mixed until it reaches the PA. Blood in the left side of the heart is 96% saturated with oxygen, giving a P o 2 of 90 to 100 mmHg (100 mmHg = 13.3 kPa). There is no difference in saturation in blood in the left atrium (LA) and left ventricle (LV).
All pressures in the circulation should be measured relative to a fixed reference point, ideally the level of the right atrium. The normal ranges are shown in Table 10.5 . Using this reference point, the mean right atrial pressure is usually between 2 and 6 mmHg (average 4 mmHg). This is determined indirectly by assessing the jugular venous pressure, and more directly by measurement of central venous pressure. The pressure in the LA is approximately 10 to 15 mmHg, and this can be measured using a Swan–Ganz catheter. The catheter is placed in the PA either under direct radiological vision or the balloon tip inflated and the device floated through the right heart via a central vein. Once in the PA, the inflated balloon can be wedged into a branch of the distal PA. Providing there are no significant reasons for pressure across the lung capillaries to be raised then the pressure reflects that of the LA. The same Swan–Ganz catheter can also be used for measuring cardiac output by the thermodilution method which involves injecting a bolus of cold saline into the PA and recording the area under the curve of the temperature change over time. Essentially, the higher the cardiac output, the quicker the cold saline is replaced with warm blood and hence the area under the curve will be reduced.
| Normal Pressure (mmHg) | Normal Saturation (%) | |
|---|---|---|
| Right atrial pressure | 2–6 | |
|
| Mixed venous saturations |
|
| 70–75 |
| Pulmonary capillary wedge | 6–12 | |
| Left ventricle end-diastolic pressure (EDP) | <12 | 95–100 |
| Cardiac output (L/min) | 4.0–8.0 | |
| Cardiac index (L/min per m 2 ) | 2.8–4.2 |
Haemodynamic Events In The Cardiac Cycle And Their Clinical Correlates
This section describes events in the left side of the heart, although the events occurring on the right side of the heart are similar. However, left atrial systole occurs after right atrial systole and LV systole precedes right ventricular systole.
At the very beginning of ventricular systole, the mitral valve is open; the pressure in the LA is somewhat greater than that in the LV. As ventricular systole continues, the pressure in the LV exceeds that in the LA, thus closing the mitral valve. Shortly afterwards, the pressure in the LV exceeds that in the aorta, and this opens the aortic valve; ejection of blood then occurs from the LV. As the ventricle starts to relax, the pressure in the LV falls below that in the aorta; initially, the aortic valve stays open because of the forward kinetic energy of the ejected blood. With a further fall in pressure in the LV, the aortic valve then closes. As the pressure in the LV continues to fall below and becomes lower than that in the LA, the mitral valve opens, and blood passes from the atrium to the ventricle.
In the period of rapid passive filling (early in diastole) blood falls from the atria to the ventricles. However, the remaining one-third of ventricular filling is caused by atrial systole (active filling), which, in turn, causes the a wave in the jugular venous pressure trace. The c wave coincides with the onset of ventricular systole, making the tricuspid valve bulge into the atrium and raising the pressure there. The v wave is due to the filling of the atrium while the tricuspid valve is shut, and the upward movement of the tricuspid valve at the end of ventricular systole. Active filling constitutes approximately 5% of cardiac output in a normal heart and is lost in AF. This may not be noticed by women with normal LV function. However, in patients with a fixed cardiac output (e.g. mitral stenosis) it may reduce cardiac output significantly.
During the early part of ventricular systole, both the mitral and aortic valves are closed. The volume of blood within the ventricle must then remain the same. This is therefore known as the period of isovolumetric contraction. As the ventricle relaxes, there is a similar period when both aortic and mitral valves are closed: the period of isovolumetric relaxation.
In those with normal hearts, valve closure is associated with heart sounds, but valve opening is not. The first sound is caused by mitral valve closure, and the second sound by aortic valve closure. Patients with abnormal valves may have an ejection click (aortic stenosis) at aortic valve opening, or an opening snap (mitral stenosis) at mitral valve opening. The third heart sound occurs at the period of rapid ventricular filling; the fourth heart sound is related to atrial systole. The fourth heart sound is therefore absent in patients with AF. Heart sounds, other than the first and second, are usually considered pathological, although the third heart sound in particular is very commonly heard in pregnancy and in young people.
The electrical events of the electrocardiograph precede mechanical ones. Thus, the P wave representing atrial depolarisation occurs before the fourth heart sound, and the QRS complex representing ventricular depolarisation occurs at the onset of ventricular systole. The T wave (ventricular repolarisation) is already occurring at the height of ventricular systole.
Alterations in heart rate are associated with changes in the length of diastole rather than the length of systole. This can be a problem in patients where filling of the ventricles is impaired, as in mitral stenosis; such patients are very intolerant of rapid heart rates.
Since right ventricular systole occurs a little later than left, the second sound is split, the second component being due to the closure of the pulmonary valve. During inspiration, the delay of ejection of blood from the right side of the heart is even greater, so that splitting of the second sound widens.
Control Of Cardiac Output
Cardiac output (CO) is the product of stroke volume (SV) and heart rate (HR), where stroke volume is the volume of blood ejected by the heart per beat and is normally 70 mL.
Normal resting cardiac output is 4.5 L/min in females and 5.5 L/min in males. While this can be a useful measurement, it does not take into account the differences between individuals and thus an 80-year-old small woman does not have the same cardiac output as a 90-kg large man. The cardiac index is therefore a measurement which is corrected for surface area and is thus more accurate than cardiac output. It is calculated as the CO divided by the body surface area in square metres, and normal is 3.2 L/min/m 2 .
Cardiac output can be affected by either changes in heart rate or contractility. Starling’s law states that the force of contraction is proportional to the initial muscle fibre length. This initial fibre length is in turn dependent upon the degree of stretch of the ventricular muscle, or the amount that the ventricle is dilated in diastole (i.e. the venous return). As end-diastolic volume increases, the force of contraction increases until a maximum is reached after which the heart will start to fail ( Fig. 10.8 ).
Factors affecting end-diastolic volume (also called preload) are those factors that control effective blood volume (i.e. the total blood volume), body position (pooling of blood in the lower limbs in the upright posture) and pumping action of muscles in the leg which encourages the venous return. Venous tone also affects the effective blood volume. The veins are the capacitance vessels of the circulation. If venous tone is increased, venous return is also increased. Intrathoracic pressure is also important. If intrathoracic pressure is high, as in patients who are being artificially ventilated, blood does not return so effectively to the heart. When patients have a pericardial effusion, intrapericardial pressure may be high, the heart cannot dilate and ventricular filling is impaired, so cardiac output falls. Atrial systole, as described above, contributes to one-third of ventricular filling.
Fig. 10.8 shows one curve relating ventricular performance to end-diastolic volume. However, one can also draw a series of such curves ( Fig. 10.9 ) showing how ventricular performance may be increased without change in end-diastolic volume. Such an increase from a lower to higher curve represents an increase in contractility. This is seen in treatment with digoxin and other ‘inotropic’ agents such as β-adrenergic catecholamines (e.g. adrenaline (epinephrine) and isoprenaline). A decrease in contractility is seen with drugs such as β-adrenergic blocking agents (e.g. propranolol) which pharmacologically depress myocardial activity and in pathological states such as hypoxia, hypercapnia, acidosis and in patients who have lost myocardial tissue after a myocardial infarction or with systolic hypertension. Systemic arterial pressure is a major component of afterload, the resistance against which the heart must work to pump out blood.
Changes In Blood Volume And Cardiac Output During Pregnancy
During pregnancy, plasma volume increases from the non-pregnant level of 2600 mL to about 3800 mL ( Fig. 10.10 ). This increase starts in early pregnancy and reaches a maximum around 32 weeks’ gestation. The red cell mass also increases steadily until term from a non-pregnant level of 1400 mL to 1650 to 1800 mL. However, since plasma volume increases proportionately more than red cell mass, the haematocrit and haemoglobin concentration fall during pregnancy. A haemoglobin level of 105 g/L is typical during healthy pregnancy.
Cardiac output also rises by about 40% from about 4.5 to 6 L/min. This rise starts in early pregnancy and reaches a maximal plateau between 24 and 30 weeks of gestation. The rise is maintained through labour, and declines to pre-pregnancy levels over the next 2 to 6 weeks after childbirth. If the patient is studied lying supine, the gravid uterus constricts the inferior vena cava, and decreases the venous return, thus falsely decreasing cardiac output. This is also the mechanism of supine hypotension seen when some pregnant women lie flat on their backs at the end of pregnancy and can contribute to fetal distress.
The gestational increase in cardiac output is through a combination of increased heart rate by about 20%, and an increase in stroke volume. The increase in cardiac output is more than is necessary to distribute the extra 30 to 50 mL of oxygen consumed per minute in pregnancy. Therefore, the arteriovenous oxygen gradient decreases in pregnancy.
Fig. 10.11 indicates the distribution of the increase in cardiac output seen in pregnancy. At term, about 400 mL/min goes to the uterus and about 300 mL/min extra goes to the kidneys. The increase in skin blood flow could be as much as 500 mL/min. The remaining 300 mL would be distributed among the gastrointestinal tract, breasts and the other extra metabolic needs of pregnancy, such as respiratory muscle and cardiac muscle. Cardiac output, renal blood flow and blood flow to most other maternal organs occurs before an increase in uterine blood flow. It is the mother preparing her body for the demands of an enlarging placenta and fetus in the second half of pregnancy.
Blood Pressure Control
Blood pressure is proportional to cardiac output and peripheral resistance. Peripheral resistance is controlled neurogenically by the autonomic nervous system, and directly by substances that act on blood vessels: (e.g. angiotensin II, serotonin, kinins, nitric oxide, endothelial-derived hyperpolarising factor, catecholamines, adenosine, potassium, H + , P co 2 , P o 2 and prostaglandins).
From the Poiseuille formula the flow ( f ) in a tube of radius ( r ) and length ( L ) is governed by the relation:
where P is the pressure gradient and η the viscosity of the fluid. Flow and peripheral resistance are therefore extremely sensitive to blood vessel radius. A 5% increase in vessel radius increases flow and decreases resistance by 21%. In blood, which is not a Newtonian fluid, viscosity rises markedly when the haematocrit rises above 45%. Such a marked increase in viscosity therefore causes a considerable reduction in blood flow.
Autonomic Nervous System and Blood Pressure Control
Receptors involved in blood pressure control in blood vessels and the heart are shown in Table 10.3 . Both cholinergic and α- and β-adrenergic receptors are involved. The major tonic effect is adrenergic vasoconstriction, and vasodilatation is largely achieved by a reduction in vasoconstrictor tone rather than active vasodilatation.
The action of the autonomic system in controlling blood pressure is governed by the cardioinhibitory and vasomotor centres. The cardioinhibitory centre is the dorsal motor nucleus of the vagus nerve. Impulses pass from the cardioinhibitory centre via the vagus nerve to the heart, causing bradycardia and decreasing contractility. These effects reduce cardiac output and therefore blood pressure. The input to the cardioinhibitory centre is from the baroreceptors (see later). An increase in baroreceptor firing rate stimulates the cardioinhibitory centre and so produces reflex slowing of the heart and a reduction in blood pressure. The cardioinhibitory centre also receives inputs from other centres, so that pain and emotion can both increase vagal tone. If the vagal stimulation caused by pain and/or emotion is severe enough, blood pressure is decreased to the point where cerebral perfusion is impaired and the subject faints.
Sympathetic output to the heart and blood vessels is controlled by the vasomotor centre. The input to the vasomotor centre is from the baroreceptors; a fall in baroreceptor activity is associated with increased output from the vasomotor centre, thus increasing blood pressure. The vasomotor centre also receives fibres from the aortic carotid body chemoreceptors so that a fall in the P o 2 or pH or a rise in the P co 2 will stimulate the vasomotor centre and cause a rise in blood pressure. In addition, baroreceptors in the floor of the fourth ventricle, which are sensitive to cerebrospinal fluid (CSF) pressure, innervate the vasomotor centre. These act so that a rise in CSF pressure causes an equal rise in blood pressure (Cushing reflex). Pain and emotion can also stimulate the vasomotor centre as well as the cardioinhibitory centre. Therefore, these stimuli can cause a rise in blood pressure, as well as a fall in blood pressure.
The carotid sinus baroreceptor is located at the bifurcation of the internal carotid artery. Fibres of the glossopharyngeal nerve carry impulses at frequencies that, within certain limits, are proportional to the instantaneous pressure in the carotid artery. In experimental animals at pressures below 70 mmHg, the receptors do not fire at all. Between 70 and 150 mmHg the receptors fire with increasing frequency as the blood pressure rises. This frequency reaches a maximum at 150 mmHg. Therefore, the carotid sinus baroreceptors can modulate blood pressure between 70 and 150 mmHg, but not outside this range. In patients with hypertension, the baroreceptors adapt and shift upwards the pressures over which they respond.
Local Control of Blood Flow
Metabolites that accumulate during anaerobic metabolism cause vasodilatation. This allows tissues to autoregulate their blood flow; vasodilatation allows an increased blood flow and decreases the tendency for anaerobic metabolism. The metabolites involved are hydrogen ions, potassium, lactate, adenosine (in heart but not skeletal muscle) and carbon dioxide. In addition, hypoxia itself causes vasodilatation.
Another form of autoregulation is the myogenic reflex. If the perfusion pressure in the arteriole decreases, thus tending to decrease local blood flow, the smooth muscle in the arteriole relaxes allowing vasodilatation and an increase in local blood flow. The converse occurs at high perfusion pressures: arteriolar smooth muscle then contracts, causing vasoconstriction, and a reduction in blood flow to offset the high perfusion pressure. Note that these changes induced by the myogenic reflex maintain local blood flow but will exacerbate changes in systemic blood pressure.
Other substances affecting the blood vessels locally are prostaglandins derived enzymatically from fatty acids. The cyclooxygenase pathway creates either prostaglandins or thromboxane from the intermediate phospholipase A2, whereas the lipoxygenase pathway forms leukotrienes. The cyclooxygenases (COX1 and COX2) are located in blood vessels, the kidney and stomach. Technically, prostaglandins are hormones though are rarely classified as such but are known as mediators which have profound physiological effects. Prostaglandins are found in virtually all tissues and act on a variety of cells but most notably endothelium, platelets, uterine and mast cells. Prostaglandin E and prostaglandin A cause a fall in blood pressure by reducing splanchnic vascular resistance. Prostaglandin F causes uterine contraction and bronchoconstriction. Prostacyclin, the levels of which increase considerably in pregnancy and which is produced by blood vessels and the fetoplacental unit, causes a marked vasodilatation, which will cause a fall in blood pressure unless the cardiac output also increases. Thromboxane derived from platelets causes vasoconstriction.
Nitric oxide is an endothelium-derived vasodilator generated from the conversion of l-arginine to L-citrulline by nitric oxide synthase, whilst endothelin is a 21-amino-acid peptide derived from endothelium that is a powerful vasoconstrictor. Another potent vasoconstricting agent is angiotensin II, produced under the influence of renin. Renin is an enzyme largely produced by the juxtaglomerular apparatus of the kidney, but also by the pregnant uterus. It cleaves the peptide bond between the leucine and valine residues of angiotensinogen forming the decapeptide angiotensin I, which itself has no biological activity. The stimuli to renin secretion are β-adrenergic agonists, hyponatraemia, hypovolaemia, whether induced by bleeding or changes in posture, and pregnancy. A similar but smaller rise in renin levels is also seen in patients taking oestrogen-containing contraceptive pills. Angiotensin I is then converted to the intensely vasoconstrictive angiotensin II in the lungs, by angiotensin-converting enzyme (ACE), which removes a further two amino acid residues. Angiotensin II has a number of effects throughout the body other than its vasoconstrictive properties. It has prothrombotic potential due to its adhesion and aggregation of platelets and production of PAI-1 and PAI-2. It also affects blood volume in a number of ways. Angiotensin II increases thirst sensation, decreases the response to the baroreceptor reflex and increases the desire for salt. It has a direct effect on the proximal tubules of the kidney to increase Na + absorption as well as complex and variable effects on glomerular filtration and renal blood flow. In addition, angiotensin II also stimulates aldosterone production from the zona glomerulosa of the adrenal gland, and this will, in turn, cause a rise in blood volume, and blood pressure over the longer term, by sodium retention. In the luteal phase of the menstrual cycle, elevated plasma angiotensin II levels are responsible for the elevated aldosterone levels found.
All three levels of the renin–angiotensin–aldosterone system (RAAS) are now being targeted by drugs in order to reduce blood pressure. The action of angiotensin II is blocked by angiotensin receptor-blocking drugs (ARBs), whereas the ACE is inhibited by the ACE inhibitors (ACEis) and other similar drugs. Most recently, direct renin inhibitors, such as aliskiren, are now available for use alone or in direct combination with an ACEis or ARB. Unfortunately, ACEis and ARBs are associated with major congenital anomalies of the fetus and should not be taken by the pregnant mother.
Blood Pressure Changes In Pregnancy
The marked gestational rise in cardiac output is associated with an initial fall rather than a rise in maternal blood pressure. This can be explained by a decrease in total peripheral vascular resistance, which accommodates the increased blood flow to the uterus, kidney, skin and other organs (see Fig. 10.11 ).
The decreased peripheral vascular resistance does not always keep strictly in proportion with the increase in cardiac output and a fall in blood pressure if often noted in early pregnancy as peripheral resistance falls by more than cardiac output rises. There is then only a small reduction of 1 mmHg in the median systolic and diastolic blood pressure from 12 to 19 weeks. Blood pressure subsequently rises until term with the median systolic blood pressure increasing by up to 9 mmHg and median diastolic blood pressure by up to 10 mmHg to a maximum in healthy pregnancy of 145/95 mmHg at term. Heart rate typically increases during pregnancy with 10% of healthy pregnant women having a heart rate >100 bpm at 18 weeks’ gestation and >105 bpm at 28 weeks’ gestation ( Fig. 10.12 ). Other factors affecting blood pressure are posture and uterine contractions, which act via the changes in cardiac output already described. Uterine contractions expel blood from the uterus, increase cardiac output and increase blood pressure. The supine position, by causing vena caval obstruction, decreases cardiac output and will decrease blood pressure.
Endothelium in Pregnancy
The endothelium is a single cell layer that lines the internal surface of all blood vessels and plays a far more important role than that of a barrier between intra- and extravascular spaces. The endothelium controls vascular permeability, it determines vascular tone of the underlying smooth muscle and plays a major role in the inflammatory response. In normal pregnancy, the endothelium undergoes many subtle changes in function which contribute to the maintenance of normal cardiovascular function in mother and fetus. The onset of similar cardiovascular changes during the luteal phase of the menstrual cycle suggests that maternal rather than feto-placental factors initiate the vasodilatation associated with early pregnancy. There is now clear evidence that maternal endothelium plays a major role in this adaptation of the cardiovascular system to pregnancy.
Endothelium As A Barrier
The endothelium provides a passive barrier between blood and extravascular compartments, and prevents easy passage of erythrocytes and leucocytes. Transduction of fluid and small molecules occurs in accordance with the balance of Starling’s forces (see p. 12); hydrostatic (blood) pressure favours fluid transfer out of the vessel and plasma oncotic pressure provides the predominant breaking force which limits outward flow. It is also now accepted that an almost invisible layer positioned above the cells in the lumen, the glycocalyx, provides another ‘ultrafilter’, which contributes to the molecular selectivity of the endothelium. The high incidence of oedema in normal pregnancy is likely to be the result of increased fluid transfer across the endothelium. It is currently uncertain whether the oedema arises from a simple increase in the balance of transcapillary hydrostatic pressure favouring outward fluid transduction or from a combination of this and increased fluid conductivity.
Endothelium As A Modulator Of Vascular Tone
The endothelium ( Fig. 10.13 ) synthesises a number of potent vasoactive factors that can influence the tone of the underlying vascular smooth muscle. Vasodilators include nitric oxide, prostacyclin and endothelium-derived hyperpolarising factor. Constrictor factors include endothelin, angiotensin and thromboxane. All of these factors are involved in the vasodilatation of healthy pregnancy.
Endothelium-Derived Vasodilators
Nitric Oxide
Nitric oxide (NO) is an inorganic molecule synthesised within the endothelium to relax underlying vascular smooth muscle. Endothelial nitric oxide synthase (eNOS) is one of three NOS isoforms that catalyses the conversion of l-arginine to NO and the co-product l-citrulline. Nitric oxide evokes relaxation in vascular smooth muscle through activation of soluble guanylate cyclase and subsequent stimulation of cyclic guanosine monophosphate (cGMP). Increased activity of the l-arginine–NO pathway is evident in healthy human pregnancy but does not appear to be diminished in the vasoconstricted state of pre-eclampsia.
Nitric oxide has a short half-life and cannot easily be measured directly. Other indirect methods have therefore been employed to evaluate its role in pregnancy. In human pregnancy, urinary concentrations of cGMP increase early in pregnancy and remain elevated until term. It is unclear whether plasma cGMP changes during normal pregnancy. A confounding issue is that cGMP is also a second messenger for atrial natriuretic peptide (ANP). However, the circulating concentration of ANP does not rise until the third trimester, long after the increase in urinary cGMP.
In vivo studies provide the most compelling evidence that NO synthase is upregulated in the maternal peripheral circulation during normal pregnancy. Infusion of the NO synthase inhibitor, l-NMMA, into the brachial artery causes a greater reduction of hand and forearm blood flow in pregnancy compared with that in non-pregnant women. Normal pregnancy is also associated with enhanced endothelium-dependent flow-mediated vasodilatation in the brachial artery and isolated vessels. All of these studies support the view that basal and stimulated NOS activity contributes to the fall in peripheral vascular resistance during a healthy pregnancy. Furthermore, circulating levels of an endogenous inhibitor to NOS, asymmetrical dimethylarginine (ADMA), fall during a healthy pregnancy in association with a gestational fall in blood pressure.
Prostacyclin
Prostacyclin (PGI 2 ) is a vasodilator derived from the arachidonic acid pathway after conversion by cyclo-oxygenase. In common with NO, PGI 2 has a short half-life and evaluation of PGI 2 synthesis depends on the measurement of stable metabolites (e.g. 6-oxo-PGF 1 ). The high circulating concentrations of these metabolites during pregnancy does not necessarily indicate that PGI 2 is the predominant vasodilator in pregnancy. This conclusion is upheld by studies in pregnant animals and women in which infusion of the cyclo-oxygenase inhibitor indometacin was shown not to affect blood pressure or peripheral vascular resistance. In sheep, PGI 2 biosynthesis seems to be increased preferentially in the uterine circulation during pregnancy, possibly in response to elevated angiotensin II. Pregnancy in the ewe is also associated with a dramatic rise in the expression of COX-1 mRNA and protein in the uterine artery endothelium.
Endothelium-Derived Hyperpolarising Factor
Nitric oxide and prostacyclin do not account for all agonist-induced endothelium-derived vasodilatation. The residual vasodilatation is abolished by potassium channel blockers or by a depolarising concentration of potassium ions, so this factor has become known as endothelium-derived hyperpolarising factor (EDHF). As the name implies, it causes hyperpolarisation of the underlying vascular smooth muscle. Hyperpolarisation, in turn, provokes relaxation. While the existence of an EDHF is indisputable, its variable nature and mechanisms of action has meant that any singular and distinct chemical identification is not possible. For this reason, it is more appropriate to consider EDHF as representing a mechanism of action, rather than a specific factor.
EDHF is most evident in small arteries where it is influential in controlling organ blood flow and blood pressure, especially when NO production is compromised. Intriguingly, there are gender differences with the effects of EDHF. For example, in mice where eNOS and COX-1 have been deleted, blood pressure changes little in females, but males become hypertensive. Due to the nature of its actions, EDHF has not been widely studied in humans. Nitric oxide is, however, undoubtedly the predominant endothelium-derived relaxing factor. Increased synthesis of a vascular EDHF has been described in animal and human pregnancy, and so may play a role in peripheral vasodilatation.
Vascular Endothelial Growth Factor
Vascular endothelial growth factor (VEGF) has potent angiogenic and mitogenic actions. It induces nitric oxide synthase in endothelial cells and is likely to play a part in decreasing vascular tone and blood pressure in healthy pregnancy. The VEGF family of proteins includes VEGF/VEGF-A, VEGF-B, VEGF-C, VEGF-D and VEGF-E. VEGF is a homodimeric 34- to 42-kDa glycoprotein, which in normal tissues is expressed in a number of cell types, including activated macrophages and smooth muscle cells. VEGF-A is expressed in syncytiotrophoblast cells and, along with VEGF-C, is also present in the cytotrophoblast. VEGF interacts through three different receptors: VEGFR-1 (FMS-like tyrosine kinase 1, Flt-1), VEGFR-2 (KDR/Flk-1) and VEGFR-3 (Flt-4), which mediate different functions within endothelial cells. VEGFR 1 and 3 are expressed on invasive cytotrophoblast cells in early pregnancy.
Soluble Flt-1 (sFlt-1) is a soluble form of VEGFR-1 secreted by endothelial cells, monocytes and the placental trophoblast. sFlt-1 can bind, and therefore inactivate, VEGF and placental growth factor (PlGF) thereby mediating an anti-angiogenic effect. sFlt-1 is present in only small concentrations in the serum of non-pregnant females and males. Higher levels are detectable in healthy pregnant women towards term but become pathologically elevated prior to the clinical onset of pre-eclampsia.
There have been conflicting results relating to changes in VEGF levels in pregnancy, as a consequence of difficulties in measuring free as opposed to bound VEGF. Levels appear to be lower in the vasoconstricted state of pre-eclampsia.
Placental Growth Factor
PlGF is a member of the VEGF family and is also distantly related to the platelet-derived growth factor (PDGF) family. PlGF is a 149-amino-acid mature protein with a 21-amino-acid signal sequence and a centrally located PDGF-like domain. It shares a 42% sequence homology with VEGF, and the two are structurally similar. PlGF has angiogenic properties, enhancing survival, growth and migration of endothelial cells in vitro , and promotes vessel formation in certain in-vivo models. It is thus regarded as a central component in regulating vascular function.
PlGF was first identified in the human placenta and is expressed in greatest quantities under normal conditions. It is important in placental development, as it is present in high concentrations within villous cytotrophoblastic tissue and the syncytiotrophoblast. PlGF concentrations increase throughout pregnancy, peaking during the third trimester, and falling thereafter, probably as a consequence of placental maturation. Low levels of PlGF are evident up to 6 weeks before the clinical onset of pre-eclampsia.
Thromboxane
Human pregnancy is associated with increased synthesis of the vasoconstrictor prostanoid, thromboxane (TXA 2 ), as assessed by measurement of its stable systemic metabolite 2,3-dinor-TXB 2 . Thromboxane is mainly derived from platelets and increases three- to fivefold during pregnancy.
Endothelin
The family of endothelins includes endothelin-1 (ET-1) which plays the predominant physiological role in the control of vascular tone. ET-1 is cleaved from a larger precursor polypeptide, big-endothelin, by the action of membrane-bound enzymes, the endothelin-converting enzymes. The plasma concentration of ET-1 is very low or undetectable in maternal plasma and not affected by healthy pregnancy. Endothelin may however play a role in constriction of the umbilical circulation at birth. Paradoxically, binding of endothelin to a receptor subtype, the ET B receptor, in the endothelium can lead to vasodilatation through stimulus of nitric oxide release. Studies in rats have suggested that this mechanism may play a role in the increase in renal blood flow in pregnancy.
Angiotensin II
Angiotensin II (AII) was once considered to be synthesised predominantly in the pulmonary circulation, in which ACE activity is high, but it is now known to be widely synthesised in endothelium. In a normal pregnancy, despite a dramatic increase in activity of the renin–angiotensin–aldosterone axis, there is a well-documented blunting of the pressor response to angiotensin II, which may contribute to lowering of peripheral vascular resistance.
Oestrogen and the Endothelium
High oestrogen levels have far-reaching systemic effects on pregnant women. They include changes to serum lipoprotein concentrations, coagulation factors, antioxidant activity and vascular tone. Oestrogen has two direct effects on blood vessels: rapid vasodilatation (5 to 20 min after exposure) and chronic (hours to days) protection against vascular injury and atherosclerosis. The rapid vasodilatory effects of oestrogen are non-genomic (i.e. they do not involve changes in gene expression of vasodilator substances). There are two functionally distinct oestrogen receptors (ERs), α and β. ER-α a receptors on the endothelial cell membrane can directly activate NOS. A study of ER knockout mice has confirmed a role for ERs in NO synthesis. The non-genomic mechanism by which oestrogen rapidly activates NOS has not been fully elucidated. Animal studies suggest that involvement of the endothelium in the vasodilatation induced by longer-term exposure to oestrogen is similar to that seen during pregnancy. Enhanced NO-mediated relaxation in the sheep uterine artery induced by oestrogens is associated with greater NOS enzymatic activity.
Clinical evidence that supports a vasodilatory role for oestrogens has mainly come from studies on postmenopausal women given exogenous oestrogen. For example, 17β-oestradiol potentiates endothelium-dependent vasodilatation in the forearm and coronary arteries of postmenopausal women. Oestrogen can also act directly on vascular smooth muscle, independent of the endothelium, by opening calcium-activated potassium channels. Furthermore, 17β-oestradiol may also decrease synthesis of the superoxide free radical, and thereby prolong the half-life of pre-existing NO.
Much less is known about the vascular effects of progesterone. Circulating progesterone levels increase by a similar amount to 17β-oestradiol and may play a role in reducing pressor responsiveness to angiotensin II.
Endothelium And Haemostasis
In anticipation of haemorrhage at childbirth, normal pregnancy is characterised by low-grade, chronic activation of coagulation within both the maternal and utero-placental circulations. The endothelium is directly involved in promoting a procoagulant state in healthy pregnancy. During the third trimester, plasma levels of endothelium-derived von Willebrand factor are elevated, promoting coagulation and platelet adhesion. Circulating levels of clotting factors, especially fibrinogen, factor V and factor VIII, are increased, while there is a gestational fall in the level of the endogenous anticoagulant, protein S. Furthermore, endothelial production of both plasminogen activator inhibitor (PAI-1) and tissue plasminogen activator (t-PA) are increased during pregnancy, with the effect of both inhibition and promotion of fibrinolysis, respectively. The procoagulant state of the endothelium therefore is to some extent compensated by upregulation of the fibrinolytic system.
Endothelium And Inflammation
A healthy pregnancy stimulates a generalised inflammatory response. Not only do peripheral blood leucocytes develop a more inflammatory phenotype than in non-gravid women, but the expression of leucocyte adhesion molecules on the endothelium also increases. It has recently been shown that these inflammatory changes are even more pronounced during pre-eclampsia. Further details of the complex immune interactions involving many different immune cell types can be found in Chapter 8 .
Pre-Eclampsia
Relative to the vasodilated, plasma-expanded state of women in healthy pregnancy, pre-eclampsia is a vasoconstricted, plasma-contracted condition with evidence of intravascular coagulation. Whereas healthy maternal endothelium is crucial for the physiological adaptation to normal pregnancy, the multiple organ failure of severe pre-eclampsia is characterised by widespread endothelial cell dysfunction. The endothelium of women destined to develop pre-eclampsia both fails to adapt properly and can be further damaged during a pre-eclamptic pregnancy. Prior to the onset of clinically identifiable disease, women destined to develop pre-eclampsia show evidence of poor placentation, high uteroplacental resistance and abnormal placental function. Measurement of sFlt-1 and PlGF in mid-pregnancy, in particular the sFlt-1:PlGF ratio can be helpful in the assessment of a woman with suspected pre-eclampsia. A normal ratio is reassuring with a high negative predictive value for pre-eclampsia in the subsequent 1 to 2 weeks. Pre-eclampsia eventuates when reduced placental perfusion is associated with endothelial abnormalities in the mother. Women with risk factors for cardiovascular disease such as hypertension, diabetes mellitus and hyperlipidaemia are predisposed to pre-eclampsia and at risk of these conditions following a pre-eclamptic pregnancy.
Endothelial Dysfunction in Pre-Eclampsia
Damaged endothelial cells in pre-eclampsia ( Fig. 10.14 ) cause increased capillary permeability, platelet thrombosis and increased vascular tone. Evidence of endothelial cell damage prior to clinical manifestation of pre-eclampsia can be demonstrated by the presence of markers of endothelial cell activation. Specifically, levels of fibronectin and factor VIII-related antigen are elevated. Furthermore, women with endothelial cell damage secondary to pre-existing hypertension or other microvascular disease have a higher incidence of pre-eclampsia than normotensive women.
