TRIIODOTHYRONINE OPTIMIZES SHEEP VENTRICULOARTERIAL COUPLING FOR WORKEFFICIENCY

Background. Triiodothyronine (T-3) administration after cardiopulmonar y bypass has been shown to significantly improve cardiac performance. The present study was undertaken to elucidate the effects of T-3, when administered as an intravenous bolus, on both cardiac energetics and stroke work-oxygen utilization (EW/LWO(2)) efficiency. Methods. In bot h unstressed and stressed hearts, energetics were evaluated at baselin e and 2 hours after intervention in an in vivo sheep preparation. In t he first group (n = 5) sheep received saline vehicle. In the second gr oup (n = 9) sheep received an intravenous bolus of 1.2 mu g/kg of T-3. In the third group (n = 7) sheep received a 2-hour intravenous infusi on of dobutamine at a rate of 5 mu g/kg/min. Results. In the unstresse d heart, T-3 improved cardiac function at no cost in oxygen consumptio n by decreasing afterload and hence improved EW/LVVO(2) efficiency. In contrast, dobutamine improved unstressed cardiac function by increasi ng contractility at the cost of increased oxygen consumption and thus decreased EW/LVVO(2) efficiency. Triiodothyronine optimized ventriculo arterial coupling for efficiency, but dobutamine optimized coupling fo r maximal work. In the stressed heart, T-3 again improved EW/LVVO(2) e fficiency, but dobutamine had the opposite effect. Conclusions. The bo lus administration of T, improves unstressed cardiac performance throu gh optimization of ventriculoarterial coupling for EW/LVVO(2) efficien cy, primarily through vasodilation. Triiodothyronine also increases ef ficiency in the stressed heart. This study supports the use of T, in c ardiac operations to improve cardiac performance with no cost in oxyge n consumption characteristic of inotropic agents.