Time-related responses to a constant-dose halothane anaesthesia in dorsally recumbent ventilated ponies

Haemodynamic and respiratory responses to halothane were investigated in dorsally recumbent, ventilated ponies during 2 hours. Normocapnia was maintained using intermittent positive pressure ventilation. Compared to the base line values at 30 minutes of constant dose halothane, no significant changes in heart rate, systemic blood pressure, cardiac output, cardiac index, stroke volume and left ventricle work were observed during a 2 hours anaesthesia. Arterial oxygenation increased initially (greater than 300 mm Hg) but tended to decrease non-significantly during the rest of the anaesthesia. Blood temperature decreased significantly during anaesthesia (p less than 0.05 after 30 minutes), probably due to the cold thermodilution injections but also to peripheral losses of body heat. Time-related increases in mean pulmonary artery pressure (p less than 0.05 after 90 minutes), total peripheral and pulmonary resistance (p less than 0.05 after respectively 60 and 90 minutes) were observed. Increases in peripheral and pulmonary resistance probably reflected vasoconstriction of respectively the peripheral and pulmonary vasculature. These changes might be caused by an increase in circulating catecholamines during prolonged anaesthesia. To avoid possible erroneous conclusions the observed time-dependent responses in anaesthetized dorsally recumbent ponies have to be taken in consideration when other influences (e.a. drugs) are investigated during anaesthesia.