THE EFFECT OF EXTREME HYPOCAPNIA ON THE ANAESTHETIC REQUIREMENT (MAC) OF DOGS BY

The effect on the minimiim alveolar concentration of halothane (MAC) during extretne hypocapnia with oxygen, and with air, was determined in nine dogs. MAC was unaffected by hypocapnia with halothane-oxygen. Changing halothane-oxygen to halothane-air reduced the MAC significantly in only five of nine dogs. The average decrease in this group of nine dogs was H per cent, only slightly more than the possible error of the method. A mild metabolic acidosis remained constant throughout the two or three hours of hypocapnia. Hyperventilation is thought to deepen the level of general anaesthesia, thereby reducing or eliminating the need for potent agents (Qutton-Brock, 1957; Dundee, 1952; Geddes and Gray, 1959). However, recent studies suggest that hypo-capnic hyperventilation (Paco, less than 20 mm Hg) causes cerebral hypoxia primarily by reduc-ing cerebral blood flow (Sugioka and Davis, 1960; Alexander et al., 1968; WoUman et al., 1968; Plum, Posner and Smith, 1968; Siesjo, 1968