Cardiovascular effects of low dose calcium chloride infusions during halothane anaesthesia in dorsally recumbent ventilated ponies.

The effects of calcium chloride administered at low infusion rates on the cardiovascular depression and the blood calcium balance were studied during a constant halothane anaesthesia in dorsally recumbent ventilated ponies. A pronounced cardiopulmonary depression characterized by decreases of all cardiac parameters and lowering of the mean arterial blood pressure was observed after the initial anaesthetic stabilization period of 30 minutes in the ponies. A significant decrease in the total calcium plasma concentration together with a constant ionized and complexed calcium fraction was present after the stabilization period. Calcium chloride administration at different infusion rates (0.1, 0.2 and 0.3 mg/kg/min) induced a dose-dependent increase in mean systemic blood pressure, probably due to the observed increase in total peripheral resistance. A dose-dependent gradual decrease in heart rate, probably mediated by the increased vagal activity, was observed after the calcium infusions. The stroke volume increased also in a dose-dependent way. Cardiac output, arterial blood gases or packed cell volume were not influenced by the exogenous calcium infusions. The observed increases in mean pulmonary artery pressure and total pulmonary resistance were probably time-related responses. Overall, only the effects of the exogenous calcium on the peripheral vasculature, namely a vasoconstriction leading to an increase in blood pressure, were present in this study. Although LV dP/dt max was not measured in this study, minor positive inotropic effects of the exogenous calcium infusions might nevertheless be possible since the observed increase in stroke volume could be an indication of an increase in the ventricular contractility function. The different fractions of the calcium in the plasma (total and ionized & complexed calcium) increased during the exogenous calcium infusions but the proportion of the fractions remained always constant.(ABSTRACT TRUNCATED AT 250 WORDS)