Final answer:
A prolonged Q-T interval indicates a lengthened relative refractory period of the heart. It can be caused by various factors such as myocardial ischemia or infarction, electrolyte disorders, sudden decreases in heart rate, acute neurologic events, and certain medications. On the other hand, digitalis and hypercalcemia shorten the Q-T interval.
Step-by-step explanation:
A prolonged Q-T interval on an electrocardiogram (ECG) indicates a lengthened relative refractory period of the heart. This means that the heart muscle takes longer to recover and become ready for another contraction. A QTc of more than 0.40 second in either gender has been associated with a higher risk for ventricular dysrhythmias such as torsades de pointes (TdP).
The Q-T interval represents the time from the start of depolarization to repolarization of the ventricles. It includes the period of ventricular systole. Various factors can cause a prolonged Q-T interval, such as myocardial ischemia or infarction, electrolyte disorders, sudden decreases in heart rate, acute neurologic events, and certain medications (e.g., amiodarone, sotalol).
Digitalis and hypercalcemia, on the other hand, shorten the Q-T interval. Hypercalcemia refers to high levels of calcium ions in the body, which can have impacts on heart rate and contractility. Extremely high levels of calcium may even induce cardiac arrest. Calcium channel blockers, a type of medication, slow down the heart rate by blocking or slowing the inward movement of calcium ions.