Two or three simultaneous disorders can be present in a mixed acid-base disorder, but there can never be two primary respiratory disorders. Uncovering a mixed acid-base disorder is clinically important and requires a methodical approach to acid-base analysis (see box Step-by-Step Analysis of Acid-Base Status). Once the primary disturbance has been determined, the clinician should assess whether the compensatory response is appropriate (Table 21–11). An inadequate or an exaggerated response indicates the presence of another primary acid-base disturbance.
STEP-BY-STEP ANALYSIS OF ACID-BASE STATUS
Step 1: Determine the primary (or main) disorder—whether it is metabolic or respiratory—from blood pH, HCO3–, and PCO2 values.
Step 2: Determine the presence of mixed acid-base disorders by calculating the range of compensatory responses (see Table 21–11).
Step 3: Calculate the anion gap (see Table 21–12).
Step 4: Calculate the corrected HCO3– concentration if the anion gap is increased.
Step 5: Examine the patient to determine whether the clinical signs are compatible with the acid-base analysis.
The anion gap should be calculated for two reasons. First, it is possible to have an abnormal anion gap even if the sodium, chloride, and bicarbonate concentrations are normal. Second, an anion gap larger than 20 mEq/L suggests a primary metabolic acid-base disturbance regardless of the pH or serum bicarbonate level because a markedly abnormal anion gap is never a compensatory response to a respiratory disorder. In patients with an increased anion gap metabolic acidosis, clinicians should calculate the corrected bicarbonate. In increased anion gap acidoses, there should be roughly a millimole for millimole decrease in HCO3– as the anion gap increases. A corrected HCO3– value higher or lower than normal (24 mEq/L) indicates the concomitant presence of metabolic alkalosis or normal anion gap metabolic acidosis, respectively.