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?