Haemodynamic effects of change in position and respiration mode during a standard halothane anaesthesia in ponies

The effects of change in position and respiration modes were studied in 5 experimental ponies during a standard halothane anaesthesia. A marked cardiovascular depression (decrease in mean arterial blood pressure, cardiac output, stroke volume and left ventricle work) occurred in recumbent, spontaneously breathing ponies. No significant differences were found between right and left lateral recumbency. The most pronounced cardiovascular depression was observed in the dorsal position. Sternal recumbency appeared to be slightly beneficial (higher systemic blood pressure and arterial oxygenation). Peripheral resistance tended to decrease in the lateral positions (peripheral vasodilatation) but increased slightly in dorsally recumbent ponies. The mean pulmonary artery pressure decreased significantly in the dorsal position. Pulmonary resistance increased slightly in the laterally and the sternally positioned ponies, suggesting a pulmonary vasoconstriction. On the other hand, pulmonary resistance decreased in dorsal recumbency. Arterial oxygenation decreased progressively during anaesthesia but remained always above standing control values. Hypoventilation (increase in arterial carbon dioxide to +/- 60 mm Hg) was observed in all positions. Controlled intermittent positive pressure ventilation (I.P.P.V.) induced a further decrease of all cardiovascular parameters although no significant differences in cardiac indexes were found regarding the spontaneous breathing protocol. In ventilated ponies the greatest cardiovascular depression was again observed in dorsal recumbency. Peripheral resistance increased slightly in the sternally positioned ponies. A gradual time-dependent increase in pulmonary resistance was observed. Apparently, the arterial oxygenation improved slightly in all positions (especially the sternal posture) when I.P.P.V. was applied.