The ultimate goal of fluid and electrolyte therapy is to maintain a balance between the intracellular and extracellular compartments. This means that often, in caring for the critically ill patient, fluid replacement and electrolyte balance become an afterthought of therapy instead of a goal. However, maintaining this balance is essential for an optimal outcome.1
As management surrounding volume replacement during pregnancy evolve, it is important that the clinician implement best practices regarding fluid and volume replacement in the critically ill patient. Delay in replacement in the severely hypovolemic patient can lead to irreversible shock and multisystem organ failure. In addition, the use of vasopressors should be delayed until adequate volume replacement has taken place as they do not correct the primary problem and further reduce tissue perfusion.
Many disease states in pregnancy can change the intracellular and extracellular compartments. Preeclampsia is a prime example of how changes in the dynamics of the intracellular and extracellular compartments can affect overall fluid physiology. Common therapies used during pregnancy such as intravenous tocolysis can change electrolyte balance. The purpose of this chapter is to explain basic fluid and electrolyte balance and to provide guidelines for initiating the therapy.
The human body is composed mostly of water. Approximately, 50% of total body weight in an average female is body water (total body water [TBW] = 1/2 weight [kg]). Given that, total blood volume increases by 50% during pregnancy with only 20% of that increase attributed to an increase in red cell mass, one can extrapolate that 60% to 65% of the total body weight in pregnancy can be attributed to TBW. The intracellular fluid compartment (ICF) contains 66% of TBW. The extracellular compartment is composed of 34% TBW. The extracellular fluid compartment (ECF) is composed of both intravascular and the interstitial components. Most of the ECF is interstitial (26%); plasma composes 8% (Fig. 25-1). Other examples of ECF include cerebrospinal fluid, synovial fluid, and secretions from the gastrointestinal tract. The fluid compartments are separated by semipermeable membranes. Water and smaller molecules may pass through the membranes; larger colloid substances and proteins are confined to the intravascular space. Hydrostatic pressure assists in maintaining an overall fluid balance as water moves by osmosis, from the area with the lowest concentration of plasma proteins (interstitial compartment) to the area with the highest concentration of plasma proteins (blood).
Osmotic activity is the expression of the concentration of solute or the density of solute particles in a fluid. In the ECF, osmotic activity can be defined as the sum of the individual osmotic activities of each solute in the fluid. Plasma osmolality can be calculated using the following formula:
where BUN is blood urea nitrogen.
Colloid osmotic pressure (COP) is produced by serum albumin (60%-80%) with fibrinogen and the globulins accounting for the remainder. Normal COP is decreased in pregnancy. While COP can be measured with electronic equipment, the equation in Table 25-1 below can estimate COP by using total protein (TP) in g/dL (see Table 25-1).
In the obstetric patient, depletion of intravascular volume can occur in two ways: overall fluid loss, eg, vomiting, diarrhea, perspiration, or by shifting of the intravascular volume to the interstitial space. Overall fluid loss may be manifested by a clinical examination that reveals orthostasis, poor skin turgor, dry mucous membranes, and a thready pulse. These clinical findings respond to rehydration (Table 25-2).
Shifting of intravascular fluid occurs in disease states such as preeclampsia. Evaluation of volume status in this subset of patients may be much more difficult. In critically ill patients, the evaluation of volume status can be obtained by the judicious use of central pressure catheters (Table 25-3). The central venous pressure (CVP) may be used to evaluate long-term trends in fluid status; however, in patients with cardiac disease or preeclampsia, the CVP may not reflect left ventricular filling. For acute management of the disease states previously mentioned, a pulmonary artery catheter can be more useful in guiding therapy. The left ventricular end-diastolic pressure is best reflected by the pulmonary artery occlusion (wedge) pressure (PAOP). Numerous formulas have been used to evaluate the effect of volume replacement on ventricular filling pressures; we present the 7-3 rule as a handy guideline (see Table 25-3).
| Measure hemodynamic parameters |
| Fluid bolus of 200-500 mL of isotonic fluid |
| Measure PAOP 30 min after fluid challenge |
| PAOP increase >7 mm Hg → no more fluid |
| PAOP increase 2-7 mm Hg (over baseline) → wait 10 min |
| If PAOP still >3 mm Hg → stop |
| If PAOP ≤3 mm Hg → continue fluid administration |
The majority of intravenous fluid therapy is directed at expanding the plasma component of the ECF. Fluids used to optimize volume status are broadly classified as crystalloids and colloids. Some of the most common questions asked are: which fluid should I use, how much should I use, and when should I use it? Fluid therapy remains a controversial topic with variable recommendations for the volume and type of fluid given.2 Three issues need to be addressed in the setting of hypovolemia, the rate of replacement, the type of fluid used, and the correction of tissue hypoperfusion.
The goal of this section is to review the pharmacology of the available colloid and crystalloid therapies and discuss their pros and cons.
As sodium is the major cation and determinant of osmotic pressure in the ECF, it is not surprising that most crystalloid preparations contain mixtures of sodium chloride and other physiologically active solutes. As crystalloid fluids are designed to expand the interstitial space, only about 20% remain in the vascular space. The remaining 80% enters the interstitial compartment over 25 to 30 minutes. Indications for the use of crystalloid include extracellular losses, as in dehydration, acute hemorrhage, and acute volume replacement.
Normal saline, 0.9%, is isotonic to normal body fluid, and, therefore, does not alter osmotic movement of water across the cell membrane. It is the most common fluid used for acute volume expansion. However, when administered in large amounts, normal saline can cause a normal anion gap metabolic acidosis.
There are variations of this fluid such as one-half normal saline, 0.45%; however, these hypotonic fluids have no place in acute volume expansion. Infusing normal saline may produce a hyperchloremic metabolic acidosis; however, this occurrence is rare.
Ringer lactate is an isotonic fluid that may be used interchangeably with normal saline in the treatment of hypovolemia or shock. Ringer lactate is a balanced electrolyte solution that substitutes potassium and calcium for some of the sodium. Lactate is added as a buffer. The concern that acidosis may be worsened by the installation of Ringer lactate during shock is unfounded; however, it is recommended that extreme caution should be taken while using the fluid in patients with diabetes or renal failure.
Hypertonic sodium-containing solutions (600-2400 mOsm/L) may be used in patients in whom a large volume load is contraindicated. Some studies have noted that the infusion of a hypertonic solution may reduce interstitial edema as well as create a positive inotropic effect. Care should be taken to use a slow infusion rate and to monitor sodium regularly to avoid hypernatremia.
Plasmalyte is a highly buffered fluid that has a pH equivalent to plasma. Although theoretically its adjusted pH may be preferential to saline or Ringer lactate, there are no definitive studies that show a great difference in its effects on vascular volume.3
Dextrose, 5% in water, is isotonic; but unlike normal saline, it penetrates the cell leaving behind the infused water. It provides a carbohydrate source during brief periods of fasting. One liter of a 5% solution contains approximately 170 kcal and a 10% solution contains 340 kcal. When dextrose is added to normal saline or to Ringer lactate, it raises the osmolality of the fluid to roughly twice that of plasma. This can promote significant changes in serum osmolality when large volumes of fluid are infused.
Colloids are large molecular weight substances that do not pass readily across capillary walls. The rationale for the use of colloids is to expand the vascular volume and to decrease the amount of fluid that leaves the intravascular space. The duration of plasma expansion may last from 2 to 36 hours. There is no evidence that colloid containing solutions are more effective in preserving pulmonary function. A recent review in the Cochrane database does not support the routine use of colloid for volume resuscitation.4
Albumin, which is synthesized in the liver, is the major oncotic protein of plasma. Responsible for about 80% of the COP of plasma, it also serves as the major transport protein for drugs and ions. The preparation is available as 5% (50 g/L) and 25% solution (250 g/L) in isotonic saline. The 5% solution has a COP of 20 mm Hg, which is equivalent to plasma COP; the 25% solution has a COP of 70 mm Hg. An infusion of 100 mL of 25% albumin will expand the plasma volume to about 500 mL. The effect of the albumin infusion persists for 24 to 36 hours. Contrary to popular belief, albumin will eventually pass into the interstitial space. This may produce delayed pulmonary edema in patients at risk. Caution should also be exercised when using large volumes since dilutional coagulopathy can occur. In a well-executed multicenter trial fluid resuscitation with albumin versus saline, there was a trend to higher mortality in patients with head trauma.5
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