Influence of the thyroid state on left ventricular tension-velocity relations in the intact, sedated dog

The mechanical properties of left ventricular contraction were described in terms of tension, velocity, length, and time in closed-chest, sedated normal, hypothyroid, and hyperthyroid dogs. Heart rate was controlled at 150 beats/min, and instantaneous contractile element velocity was calculated from left ventricular pressure and its first derivative during isovolumic left ventricular contractions, produced by sudden balloon occlusion of the ascending aorta during diastole. Wall tension was derived from ventricular pressure and volume, the latter being obtained from the pressure-volume relation of the arrested ventricle, and tension-velocity relations were analyzed over a range of ventricular endiastolic volumes. At any level of ventricular volume, the hypothyroid state was associated with a displacement of the tension-velocity relation of the left ventricle downwards and to the left, and the time to peak tension was prolonged (154 msec, normal 139 msec). In the hyperthyroid state, the tension-velocity relation of the left ventricle was displaced upwards and to the right, and the time to peak tension was reduced (80 msec). The changes in the tension-velocity relations indicate that the inotropic state of the left ventricle in the intact dog varies directly with the animal's thyroid state. This influence on myocardial contractility necessarily constitutes an important and integral part of the response of the intact circulation to altered thyroid state.