Triiodothyronine improves left ventricular function without oxygen wasting effects after global hypothermic ischemia

Area of Science:

• Cardiology
• Endocrinology
• Physiology

Background:

• Cardiopulmonary bypass induces a "euthyroid sick" state, characterized by reduced serum triiodothyronine (T3) levels.
• The clinical significance of low T3 levels on postoperative cardiovascular hemodynamics remains an area of active investigation.

Purpose of the Study:

• To investigate the acute effects of triiodothyronine on myocardial mechanics and energetics following hypothermic global ischemia.
• To model the clinical condition of cardiopulmonary bypass using an ex-vivo canine heart preparation.

Main Methods:

• Isolated canine hearts underwent 90 minutes of hypothermic (10°C) ischemia and hyperkalemic arrest.
• Hearts were cross-perfused, leading to a spontaneous 65-75% decrease in triiodothyronine levels.
• Triiodothyronine or inactive reverse triiodothyronine was administered as a bolus dose to assess myocardial contractility, energetics, and coronary resistance.

Main Results:

• Acute triiodothyronine administration significantly improved myocardial contractility (preload recruitable stroke work area) by 56% in postischemic hearts (p < 0.001).
• Triiodothyronine treatment decreased coronary vascular resistance and increased coronary flow in postischemic hearts.
• No significant effects were observed in nonischemic hearts or with reverse triiodothyronine administration.

Conclusions:

• The study successfully replicated the low triiodothyronine state observed during cardiopulmonary bypass in an isolated heart model.
• Acute triiodothyronine administration demonstrated a unique inotropic effect specifically on postischemic reperfused myocardium.
• This beneficial effect occurred without an increase in myocardial oxygen consumption, suggesting improved energetic efficiency.