A previously healthy 55-year-old man undergoes elective right hemicolectomy for a stage I (T2N0M0) cancer of the cecum. His postoperative ileus is somewhat prolonged, and on the fifth postoperative day his nasogastric tube is still in place. Physical examination reveals diminished skin turgor, dry mucous membranes, and ohostatic hypotension. Peinent laboratory values are as follows: Aerial blood gases: pH 7.56, PCO2 50 mm Hg, PO2 85 mm Hg. Serum electrolytes (mEq/L): Na + 132, K+ 3.1, Cl - 80; HCO3 - 42. Urine electrolytes (mEq/L): Na + 2, K+ 5, Cl - 6. What is the patient's acid-base abnormality?

The patient has a metabolic alkalosis secondary to gastric losses of HCl, with compensatory hypoventilation as reflected by the elevated aerial pH and PCO2 and suppoed by the absence of clinical lung disease. The PCO2 would be normal if the metabolic alkalosis was uncompensated. A respiratory acidosis with metabolic compensation would be characterized by decreased pH, increased PCO2 levels, and increased bicarbonate levels. Mixed acid- base abnormalities should be suspected when the pH is normal, but the PCO2 and bicarbonate levels are abnormal or if the compensatory responses appear to be excessive or inadequate. The combination of respiratory acidosis and respiratory alkalosis is impossible.