pH = power of Hydrogen defined by the Henderson-Hasselbalch equation:
pH=pK+log ([HCO3−]/0.03 PCO2)
Acidosis = decrease in arterial pH
Respiratory acidosis – caused by CO2 retention. This increases the denominator in the Henderson-Hasselbalch equation and depresses the pH.
Hypoventilation and ventilatory failure can cause respiratory acidosis.
Metabolic acidosis – reduced pH not explained by increased PCO2. Caused by a primary fall in the numerator [HCO3−] of the Henderson-Hasselbalch equation.
It is usually associated with an increased anion gap (see later).
Alkalosis = increase in arterial pH
Respiratory alkalosis – the decreased PCO2 explains the increased pH.
Seen in alveolar hyperventilation.
Metabolic alkalosis – raised pH out of proportion to changes in PCO2.
It is associated with hypokalemia, exogenous alkali administration, or volume contraction (e.g., severe prolonged vomiting) when the plasma bicarbonate concentration rises.
Anion gap: helps differentiate between acid gain and HCO3−loss.
Anion gap=[Na+]−([Cl−]+[HCO3−])Normal gap 8−12 mEq/L
See Figure 22-1.
Anion gap acidosis: gap >12 mEq/L
Caused by decrease in HCO3−balanced by an increase in unmeasured acid ions, not by an increase in chloride.
Causes include salicylates, methanol, paraldehyde, ethylene glycol, lactic acidosis, ketoacidosis (from diabetes or starvation), and uremia.
Non–anion gap acidosis: gap 8 to 12 mEq/L
Caused by a decrease inHCO3− balanced by an increase in chloride.
Causes include renal tubular acidosis, carbonic anhydrase inhibitor, and diarrhea.
Table 22-1 gives normal values for infants and children.
Fencl-Stewart approach to understanding acid-base balance: looks for unmeasured anions, which could contribute to a pH disturbance.
BEexplained=H2O+Cl−+albumin
H2O contribution = 0.3 (Na− + 140)
Cl− contribution = 102 − (Cl− × 140/Na)
Albumin contribution = 3.4 (4.5-albumin)
BE: base excess
*Get BEmeasured from blood gas
FIGURE 22-1
Anion gap.