Left ventricular pressure-diameter relations in the conscious rabbit.

A model for the study of left ventricular (LV) contractility in a small species was developed using conscious rabbits previously instrumented with a LV catheter and ultrasonic crystals measuring LV internal diameter. Afterload increase produced by methoxamine infusion was associated with a reduction in heart rate from 272(SEM18) to 214(20) beats.min-1, p less than 0.01; and an increase in LV end systolic pressure from 76.3(4.1) to 101(6.5) mm Hg, p less than 0.001, and in end systolic diameter from 8.7(0.7) to 9.9(0.7) mm, p less than 0.01. For matched end systolic pressure and heart rate, the increase in inotropic state produced by noradrenaline infusion significantly shifted the pressure-diameter relation to the left. Similarly, end systolic pressure and diameter were significantly lowered by sodium nitroprusside infusion, with a reflex increase of heart rate; and a decrease in inotropic state produced by verapamil infusion was demonstrated by a significantly larger end systolic diameter for matched end systolic pressure and heart rate. The pressure-diameter relation was not modified by volume loading, which increased end diastolic diameter by 9.6(1.1)% with no evidence of a Bainbridge reflex. The mild depressant effect of diltiazem was not detected by peak dP/dt changes but was demonstrated by the analysis of end systolic pressure-diameter relations. The instrumented rabbit thus appears to be a sensitive model for studying LV function and the cardiovascular effects of drugs.