a variety of drugs can present prolongation of the qt interval as a side effect. prolongation of this interval is a result of a delay in sodium and calcium channel inactivation; without proper channel inactivation, the threshold potential is reached prematurely and thus arrhythmia tends to result. these drugs, known as pro-arrhythmic agents, include antimicrobials, antipsychotics, methadone, and, ironically, antiarrhythmic agents. the use of such agents is particularly frequent in intensive care units, and special care must be exercised when qt intervals are prolonged in such patients: arrhythmias as a result of prolonged qt intervals include the potentially fatal torsades de pointes, or tdp.