A 72-year-old male with diabetes mellitus is evaluated in the emergency room because of lethargy, disorientation, and long, deep breaths (Kussmaul respirations). Initial chemistries on venous blood demonstrate high glucose at 380 mg/dL (normal up to 120) and a pH of 7.3. Recalling the normal bicarbonate (22 to 28 mM) and PCO2 (33 to 45 mmHg) values, which of the additional test results below would be consistent with the man’s pH and breathing pattern?
Correct Answer: A bicarbonate of 15 mM and PCO2 of 30 mmHg
Description: The man is acidotic as defined by the pH lower than the normal 7.4. His hyperventilation with Kussmaul respirations can be interpreted as compensation by the lungs to blow off CO2, lower PCO2, increase ratio, and raise pH. The correct answer therefore includes a low PCO2, eliminating choices c & d. Using the Henderson-Hasselbalch equation indicates that the pH minus the pK for carbonic acid (7.3 - 6.1 = 1.2) equals log / or log . These values correspond to those in choice b. The man has compensated his metabolic acidosis (caused by the accumulation of ketone bodies such as acetoacetic acid) by increasing his respiratory rate and volume.
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