Causes of Metabolic Acidosis in Extracorporeal Circulation at Normothermia.

The development of a mild metabolic acidosis during general anaesthesia has been reported by many workers (Bunker, Brewster, Smith, and Beecher, 1952; Pontius, Watkins, Manheim, Allen, Sauvage, and Gross, 1958; Robertson and Frazer, 1958). Likewise the early onset of a slight but definite metabolic acidosis has been observed in dogs during cyclopropane anaesthesia and the degree of acidosis was increased by thoracotomy (unpublished observations). Metabolic acidosis has also been described as a serious complication of open heart surgery (Kolff, Effler, and Groves, 1960). During operation with the aid of an extracorporeal circulation there is a relatively long anaesthetic phase before the start of the bypass procedure. Hyperventilation and the consequent respiratory alkalosis have been advocated as a compensation for the metabolic acidosis produced by low flow perfusion (Keats, Kurosu, Telford, and Cooley, 1958). Subsequently it has become the custom in many clinics for the anaesthetist to hyperventilate the patient before the start of the bypass procedure (Ito, Faulkner, and Kolff, 1957; Norlander, Pitzele, Edling, Norberg, Crafoord, and Senning, 1958; McGoon, Moffitt, Theye, and Kirklin, 1960; Sloan, Harris, MacKenzie, and Stem, 1962). Although superficially this may be considered a logical approach, the substitution of a state of respiratory alkalosis cannot be expected to correct that of a metabolic acidosis and in fact it will aggravate such a state. We therefore decided to study this problem further in a controlled investigation during experimental bypass procedures in dogs. Part I of this report concerns a comparison of the acid-base ratio before the onset of cardio-pulmonary bypass in dogs which had been subjected to hyperventilation during pre-bypass anaesthesia and that in dogs which had not been subjected to such prebypass hyperventilation. Part II of the report presents the results of studies of the acid-base physiology in the course of six consecutive normothermic bypass operations without hyperventilation during the correction of ventricular septal defects in children.