These calculated measurements are referred to as the corrected QT interval (QTc) and estimate the QT interval at a standard heart rate of 60 beats per minute. Several formulae have been developed to include both the QT interval and the heart rate to determine the risk of ventricular dysrhythmia.
The QT interval alone is not enough to determine the likelihood of developing dysrhythmias as the QT interval is inversely proportional to the heart rate, with the QT interval shortening at faster heart rates and lengthening at slower heart rates. A prolonged QTc interval increases the repolarization period of cardiac myocytes, and a premature ventricular contraction (PVC) that occurs during this repolarization period can cause TdP in the R on T phenomenon, which itself can degenerate into ventricular fibrillation (2). It often occurs in hospitalized patients receiving QT prolonging interventions in the setting of underlying cardiac disease or electrolyte disturbances (1).
TdP is a rare version of polymorphic ventricular tachycardia characterized by a pattern of alternating amplitudes, or “twisting points,” on ECG. QT prolongation puts patients at risk of dysrhythmias such as torsades de pointes (TdP). Other QT-prolonging conditions include hypothermia, myocardial ischemia, increased intracranial pressure, and congenital long QT syndrome.
Several electrolyte disturbances can also prolong the QT interval including hypokalemia, hypomagnesemia, and hypocalcemia. There are many drugs associated with QT prolongation including but not limited to methadone several classes of antimicrobials antifungals antiretrovirals antiemetics antipsychotics and class IA, IC, and III antidysrhythmics. The QT interval corresponds to the time period from ventricular depolarization and contraction to ventricular repolarization and relaxation. One effect many drugs, both pharmaceutical and recreational, have on the heart is to prolong the QT interval. When caring for the poisoned patient, it is essential to rapidly detect dysrhythmias or to determine the risk of the patient developing a dysrhythmia.