Triiodothyronine Optimizes Sheep Ventriculoarterial Coupling for Work Efficiency

Triiodothyronine (T3) administration after cardiopulmonary bypass has been shown to significantly improve cardiac performance. The present study was undertaken to elucidate the effects of T3, when administered as an intravenous bolus, on both cardiac energetics and stroke work-oxygen utilization (EW/LVVO2) efficiency. In both unstressed and stressed hearts, energetics were evaluated at baseline and 2 hours after intervention in an in vivo sheep preparation. In the first group (n = 5) sheep received saline vehicle. In the second group (n = 9) sheep received an intravenous bolus of 1.2 µg/kg of T3. In the third group (n = 7) sheep received a 2-hour intravenous infusion of dobutamine at a rate of 5 µg/kg/min. In the unstressed heart, T3 improved cardiac function at no cost in oxygen consumption by decreasing afterload and hence improved EW/LVVO2 efficiency. In contrast, dobutamine improved unstressed cardiac function by increasing contractility at the cost of increased oxygen consumption and thus decreased EW/LVVO2 efficiency. Triiodothyronine optimized ventriculoarterial coupling for efficiency, but dobutamine optimized coupling for maximal work. In the stressed heart, T3 again improved EW/LVVO2 efficiency, but dobutamine had the opposite effect. The bolus administration of T3 improves unstressed cardiac performance through optimization of ventriculoarterial coupling for EW/LVVO2 efficiency, primarily through vasodilation. Triiodothyronine also increases efficiency in the stressed heart. This study supports the use of T3 in cardiac operations to improve cardiac performance with no cost in oxygen consumption characteristic of inotropic agents.