Acid-base regulation is the maintenance of normal hydrogen ion concentration. The normal plasma hydrogen ion concentration is 40 nmol/L, which corresponds to a pH of 7.4 (the negative log of its concentration). Any decrease in blood pH represents an increase in hydrogen ion concentration (acidemia), while an increase in blood pH represents a decrease in hydrogen ion concentration (alkalemia). Bicarbonate (HCO3–) is the primary acid buffer in the body and the relationship between acid and base behavior in the body is shown by the Henderson-Hasselbalch equation below. Note that because carbon dioxide is primarily measured as a partial pressure, the denominator contains a conversion factor.
To best evaluate acid-base status, a blood gas and chemistry panel are required. Venous blood pH, typically 0.03–0.04 units lower than arterial blood pH, closely approximates arterial blood pH. An arterial blood gas should be obtained if more accurate assessment of blood pH and PCO2 is required. Bicarbonate (HCO3–) is calculated from the Henderson-Hasselbalch equation below; therefore, the bicarbonate value in the electrolyte panel is typically used.
Primary acid-base disorders are secondary to changes in either serum bicarbonate or PCO2. The first step is to look at the pH of a blood gas (venous or arterial). If changes in pH are secondary to changes in HCO3–, a metabolic disorder is present. If changes in pH are secondary to changes in PCO2, a respiratory disorder is present. 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). Normally, the kidneys compensate for respiratory acid-base disorders and the lungs compensate for metabolic disorders in order to maintain pH in a narrow physiologic range. For example, with metabolic acidosis (low pH, low HCO3–), alveolar ventilation increases (PCO2 decreases), returning pH close to the normal range. Similarly, with respiratory acidosis (low pH, high PCO2), the kidneys excrete H+ (HCO3– increase) to return the pH close to normal range. Such compensation can only bring the pH toward normal, though it can never completely correct it. In order to normalize pH, the primary disorder must be corrected.
One respiratory or metabolic disorder with its appropriate compensatory response is a simple acid-base disorder. A mixed acid-base disorder is when multiple simple disorders are present simultaneously. Diagnosing an acid-base disorder requires a systematic approach (see box Step-by-Step Analysis of Acid-Base Status). Once the primary disorder has been identified, the clinician should assess whether the compensatory response is appropriate (Table 21–11). An inadequate or an exaggerated response indicates the ...