Which factor does NOT affect the QT interval?

The QT interval on an electrocardiogram (ECG) represents the time it takes for the heart's ventricles to depolarize and repolarize during a heartbeat. The interval can be influenced by various factors, but cardiac muscle strength does not have a direct impact on its duration.

Hypothermia, electrolyte imbalances, and bradycardia are all known to affect the QT interval. Hypothermia can prolong the QT interval due to the effects of low body temperature on the cardiac conduction system. Electrolyte imbalances, particularly fluctuations in potassium, calcium, and magnesium levels, can significantly alter the repolarization process, thereby affecting the QT interval. Bradycardia, or a slower heart rate, also tends to lengthen the QT interval because it allows more time for the electrical events of depolarization and repolarization to occur.

In contrast, cardiac muscle strength primarily relates to the contractility of the heart and does not have a direct effect on the timing of the electrical conduction throughout the cardiac cycle, specifically in terms of depolarization and repolarization as reflected in the QT interval. Consequently, while several factors can modify the QT interval, the strength of the cardiac muscle itself is not one of those factors.