Effects of five hours of constant 1.2 MAC halothane in sternally recumbent, spontaneously breathing horses

Circulatory and respiratory effects of five h of constant 1.06 per cent alveolar halothane in oxygen were identified in eight healthy horses, which breathed spontaneously, were otherwise unmedicated and positioned in sternal recumbency. Only a few important significant (P<0.05) changes occurred with time. Total peripheral resistance was about 15 per cent lower after two hours of constant dose halothane than after 30 mins of constant dose (P<0.05) and accounted for the significant 10 per cent reduction in mean carotid arterial blood pressure. By 5 h, the reduction in resistance and arterial blood pressure was 20 and 25 per cent respectively. Heart rate increased progressively with time and the increase became significant at 5 h (15 per cent increase). However, the heart rate change was not large enough to alter cardiac output. There were no major time‐related changes in PaO₂ or PaCO₂. Three of four horses recovered from anaesthesia had markedly elevated serum creatine kinase levels and clinical signs of severe post anaesthetic myopathy.