Metabolic Alkalosis after Diuretics

A 3-month-old, 4-kg, male infant presented for repair of an atrioventricular canal (see Chapter 51 ). The patient's congenital heart failure had been controlled with digitalis and furosemide. Before induction of anesthesia, blood gas analysis showed pHa = 7.53, PaCO₂ = 44 mm Hg, BE = 14 mEq/L, bicarbonate = 35.9, and PaO₂ = 110 mm Hg—a severe metabolic alkalosis with a minimal respiratory acidosis typical of a partially compensated metabolic disturbance. After induction of anesthesia, blood gas analysis showed pHa = 7.61, PaCO₂ = 35 mm Hg, BE = 14 mEq/L, bicarbonate = 34, and PaO₂ = 60 mm Hg. After completion of the procedure and cessation of cardiopulmonary bypass, the blood gas analysis showed pHa = 7.40, PaCO₂ = 33 mm Hg, BE = -4 mEq/L, and bicarbonate = 20.

The hypochloremic alkalosis that accompanies prolonged administration of furosemide explains the first set of results. After induction, the hyperventilation decreased the PaCO₂ but had no effect on the base excess. Two major factors explain the change during the procedure. Hemodilution tends to return the pHa toward neutral (6.8 at body temperature), and low perfusion during bypass frequently produces a metabolic acidosis that, in this case, also helped to overcome the initial metabolic alkalosis.