The influence of suxamethonium on cardiovascular and respiratory function in the anaesthetized horse.

1. In horses anaesthetized with halothane the intravenous administration of suxamethonium chloride, at a dose level of 0.2 mg/kg, produced a short-lived period of hypoventilation, which was associated with increases in arterial blood PCO(2) levels and in plasma concentrations of bicarbonate, sodium and potassium ions, and reductions in arterial blood pH and PO(2) values.2. The respiratory depressant action of suxamethonium chloride 0.2 mg/kg was accompanied by increases in blood pressure and heart rate. Doses of suxamethonium chloride 0.4 mg/kg produced similar but quantitatively greater changes in cardiovascular and respiratory function. These effects were not accompanied by cardiac arrhythmias, with the exception of one animal, in which an unusually prolonged period of apnoea occurred.3. The cardiovascular effects of suxamethonium during halothane anaesthesia were diminished but not abolished when the respiratory depressant action of suxamethonium was prevented by applying positive pressure ventilation.4. The cardiovascular effects of suxamethonium in horses anaesthetized with halothane were partially antagonized by propranolol and completely antagonized by hexamethonium. It is suggested that the cardiovascular effects of suxamethonium are mediated by two distinct mechanisms: reflexly mediated increases in heart rate and sympathetic vasoconstrictor tone due to the respiratory depression, and a direct stimulant action of suxamethonium on peripheral, autonomic ganglia.5. Much less pronounced changes in cardiovascular function, but not in respiratory function, were recorded when suxamethonium was administered to horses anaesthetized with ether.6. A slight degree of tachyphylaxis to the cardiovascular and respiratory effects of suxamethonium was recorded in horses anaesthetized with halothane.7. Some atypical effects of suxamethonium on respiration are described.