Review Question - QID 104285

This patient'sarterial blood gas is consistent with metabolic acidosis with appropriate respiratory compensation, likely secondary to diabetic ketoacidosis.

In patients with suspected acid-base disorders, it is important to analyze the arterial blood gas systematically. If the pH is low, the patient has acidemia. The presence of a low pH and low bicarbonate signifies that metabolic acidosis is the primary process. To determine whether or not the decrease in PCO2 represents an appropriate respiratory compensation, one should employ Winters formula: PCO2 = (1.5 x HCO3-) + 8 ± 2. If PCO2 is within the range determined by the formula, it is considered to represent appropriate respiratory compensation. If the observed PCO2 is higher than that determined by the formula, there is mixed metabolic and respiratory acidosis; similarly, if the PCO2 is lower than that calculated by the formula, there is a mixed metabolic acidosis and respiratory alkalosis.

Illustration A depicts the nomogram of acid-base disorders, which shows the 90% confidence intervals of the normal respiratory and metabolic compensations for primary acid-base disturbances. Illustration B shows how to calculate the appropriate compensation for each of the acid-base disorders.

Incorrect Answers:
Answer 2: In primary respiratory acidosis, the PCO2 would be elevated, not decreased.
Answer 3: A mixed acidosis would be diagnosed if the PCO2 was higher than expected based on Winters formula.
Answer 4: Metabolic alkalosis is diagnosed when the pH is higher than 7.4 in the setting of a primary elevation of HCO3-.
Answer 5: Respiratory alkalosis is diagnosed when the pH is higher than 7.4 in the setting of a primary decrease of PCO2.