What is the principle behind hypothermia during CPB, and how does it protect the myocardium?

• A. Hypothermia increases metabolic demand.
• B. Induced hypothermia reduces metabolic demand, oxygen consumption, and preserves myocardial tissue during periods of ischemia.
• C. Hypothermia has no effect on myocardium.
• D. Hypothermia is used to heat the patient.

Answer: (B)

Cooling the heart during CPB works by lowering its metabolic needs. When the body is cooled, cellular reactions slow, so the myocardium uses less oxygen and produces less metabolic waste. With the coronary blood flow temporarily interrupted during cross-clamping, this reduced oxygen demand means the heart can tolerate the ischemic period longer and sustain tissue viability. The result is better preservation of myocardial function when circulation is restored. Mild to moderate hypothermia is therefore used to protect the myocardium, often alongside cardioplegia to keep the heart in a calm, energy-efficient state.