Effect of xylazine on indices of myocardial contractility in the dog.

Changes in cardiac contractility were assessed by 2 indices of myocardial contractile performance (dP/dt/Kp max, V max). Both indices demonstrated an initial positive inotropic response to intravenous administration of xylazine (1.1 mg/kg). This positive inotropic effect was followed by a return to or significant decrease from base-line recordings in these contractile indices at faster heart rates or later times following xylazine administration. In addition, an increase in heart rate in control dogs was accompanied by a rise in both dP/dt/Kp max and V max, confirming earlier evidence of the Bowditch staircase effect in the intact dog ventricle.