Effect of arterial hypotension on anaesthetic requirement in dogs

In the dog a reduction of mean arterial pressure to 40-50 mm Hg decreased MAC by 20+/--EM 4% in the 1st hour. No further change resulted during the ensuing 3 h of hypotension (40-50 mm Hg). Arterial and cerebrospinal fluid lactate and pyruvate concentrations were not affected by hypotension, nor did cerebrospinal fluid pH decrease. In a separate group of dogs, we reduced arterial pressure to 40-50, 30-40 and 20-30 mm Hg successively. MAC decreased concomitantly, being 58+/-11% of control value at the lowest pressure. MAC increased following the restoration of normal pressure, but hysteresis was evident. In a third group of dogs, arterial pressure was reduced rapidly to 10-30 mm Hg. MAC decreased to 29+/-5% of control. This decrease was significantly greater than that associated with the three-stage reduction.