Metabolic acidosis in the intensive care unit

A metabolic acidosis is a process whose net effect would be the lowering of the pH of the body if it were unopposed. Normal blood pH is between 7.38 and 7.42, corresponding to a hydrogen ion concentration of 42 to 38 nanomoles/liter. Conceptually, a metabolic acidosis is the result of the accumulation of nonvolatile acid or the loss of serum bicarbonate, usually through the gastrointestinal or genitourinary systems. Because of the very nature of the illnesses found in critically ill patients it is understandable that metabolic acidosis is a common derangement in the intensive care unit. Metabolic acidosis results in a compensatory increase in minute ventilation; near-complete respiratory compensation can be expected within hours. A decrease in pH sensitizes peripheral chemoreceptors, which triggers an increase in minute ventilation. This compensatory hyperventilation is relatively slow and is not complete for 12-24 hours [5]. The expected PCO2 for any given degree of metabolic acidosis can be predicted using the formula: PCO2 = (1.5 x [HCO3]) + 8 +/- 2 [3]. In mechanically ventilated patients, inadequate minute ventilation will prevent this compensation.