21-15: Acid-Base Disorders

Assessment of a patient’s acid-base status requires measurement of arterial pH, partial pressure of CO₂ (PCO₂), and plasma bicarbonate (HCO₃^–). Blood gas analyzers directly measure pH and PCO₂. The HCO₃^– value is calculated from the Henderson–Hasselbalch equation:

Note on a chemistry panel it is typically the total venous CO₂ (TCO₂) that is measured as an assessment of serum HCO₃^–, although this actually measures the sum of dissolved CO₂, carbonic acid and serum bicarbonate. Typically, this is within 1–2 mEq/L of the actual serum HCO₃^– value, and a good approximation in clinical practice.

Venous blood gases can provide useful information for acid-base assessment since the arteriovenous differences in pH and PCO₂ are small and relatively constant. Venous blood pH is usually 0.03–0.04 units lower than arterial blood pH, and venous blood PCO₂ 7 mm Hg or 8 mm Hg higher than arterial blood PCO₂. Calculated HCO₃^– concentration in venous blood is at most 2 mEq/L higher than arterial blood HCO₃^–. Arterial and venous blood gases will not be equivalent during a cardiopulmonary arrest; arterial samples should be obtained for the most accurate measurements of pH and PCO₂.

There are two types of acid-base disorders: acidosis and alkalosis. These disorders can be either metabolic (decreased or increased HCO₃^–) or respiratory (decreased or increased PCO₂). Primary respiratory disorders affect blood acidity by changes in PCO₂, and primary metabolic disorders are disturbances in HCO₃^– concentration. A primary disturbance is usually accompanied by a compensatory response, but the compensation does not fully correct the pH disturbance of the primary disorder. If the pH is < 7.40, the primary process is acidosis, either respiratory (PCO₂ greater than 40 mm Hg) or metabolic (HCO₃^– less than 24 mEq/L). If the pH is > 7.40, the primary process is alkalosis, either respiratory (PCO₂ less than 40 mm Hg) or metabolic (HCO₃^– greater than 24 mEq/L). One respiratory or metabolic disorder with its appropriate compensatory response is a simple acid-base disorder.