Left ventricular responses to intravenous administration of clonidine in anaesthetized dogs

We have evaluated left ventricular function in anaesthetized (morphine, pentobarbital), artificially ventilated, normotensive beagle dogs immediately (0 to 30 min) after i.v. bolus injection of clonidine (1, 3, 5, 7, 10 and 15 micrograms/kg of b.w.) using computer assisted catheterization technique. Clonidine decreased heart rate and prolonged pre-ejection, ejection and diastolic periods significantly. Cardiac output, mean systolic ejection rate, left ventricular (LV) dP/dtmax and Vmax decreased (p less than 0.05 to 0.01). LV peak pressure and aortic pressures decreased but the decrease was preceded by an initial rise at doses 5 to 10 micrograms/kg. At dose 15 micrograms/kg the rise persisted through the whole period. LV filling and end-diastolic pressures increased while end-diastolic and stroke volumes decreased. These changes indicate negative chronotropic and inotropic effects and increment in preload with the latter being a situation met in the case of proper LV filling. Tension time index and mean outflow resistance, however, indicate reduction in LV oxygen consumption at doses less than 10 micrograms/kg. It is suggested that enhancement of parasympathetic tone would account for these changes.