Cardiovascular and blood gas responses to inhaled anaesthetics in normoxic and hypoxic dogs

Changes in haemodynamics and blood gases were investigated before and after administration of 0.5, 1 and 1.5 MAC of halothane, enflurane and isoflurane in respectively 7, 7 and 9 dogs ventilated alternatively with a fraction of inspired O2 in N2 (FiO2) of 0.4 and with brief periods (10 min) of FiO2 of 0.1. Anaesthesia was induced with pentobarbital and the animals were paralysed with pancuronium. Acute hypoxic challenges with FiO2 of 0.1 consistently decreased arterial PO2 to 3.5-4.5 kPa and increased pulmonary vascular resistances by 60-100%. At identical inspired concentrations, as expressed in MAC units, all three inhaled anaesthetics induced a broadly comparable dose-related decrease in systemic blood pressures, due to a depression in cardiac performance as well as a reduction in systemic vascular resistances. Enflurane was the most potent myocardial depressor and isoflurane the most potent vasodilator, halothane being intermediate. Oxygen deprivation was associated with some enhancement of the cardiovascular depressant effects of the inhaled anaesthetics but, in spite of this, matching of O2 transport to tissue O2 demand appeared to be improved, probably in relation to a concomitant reduction in metabolic rate. Only isoflurane inhibited the hypoxic pulmonary pressor response, and this was associated with a slight deterioration in arterial oxygenation in both normoxic and hypoxic conditions.