Assessment of a patient’s acid-base status requires measurement of arterial pH, PCO2, and plasma bicarbonate (HCO3–). Blood gas analyzers directly measure pH and PCO2. The HCO3– value is calculated from the Henderson–Hasselbalch equation:
The total venous CO2 measurement is a more direct determination of HCO3–. Because of the dissociation characteristics of carbonic acid (H2CO3) at body pH, dissolved CO2 is almost exclusively in the form of HCO3–, and for clinical purposes the total carbon dioxide content is equivalent (± 3 mEq/L) to the HCO3– concentration:
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 is 7 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.
TYPES OF ACID–BASE DISORDERS
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.