Drugs that cause QT prolongation
Class IA antiarrthymic drugs work by blocking sodium and potassium channels. Blocking sodium channels tend to shorten the action potential duration, while blocking potassium channels prolongs the action potential. When the drug concentration is at a low to normal concentration, the potassium channel blocking activity takes precedence over the sodium channel blocking activity
Because of the predominance of the potassium blocking activity, TdP is seen more frequently with therapeutic levels of quinidine. Sodium blocking activity is dominant with subtherapeutic levels, which does not lead to QT prolongation and TdP.
Class III antiarrhythmic drugs are potassium channel blockers that cause QT prolongation and are associated with TdP.
Amiodarone works in many ways. It blocks sodium, potassium, and calcium channels, as well as alpha- and beta-adrenergic receptors. Because of its multiple actions, amiodarone causes QT prolongation but TdP is rarely observed.
Ibutilide differs from other class III antiarrhythmic agents in that it activates the slow, delayed inward sodium channels rather than inhibiting outward potassium channels.
Sotalol has beta-blocking activity. Approximately 2 to 7 percent of patients taking at least 320 mg/day experience proarrhythmia, most often in the form of TdP. The risks and effects are dose-dependent.
Psychotropic medications have been shown to lengthen the QT interval and induce TdP, especially when given intravenously or in higher concentrations.
Haloperidol functions by blocking the KCNH2 channel, the same pathway that other drug-inducing LQTS block. Patients taking haloperidol are at a higher risk if they also have electrolyte abnormalities (such as hypokalaemia and/or hypomagnesemia), congenital LQTS, cardiac abnormalities, hypothyroidism, or if they are concurrently taking other medications known to lengthen the QT interval.
Overdoses on quetiapine cause QT prolongation in patients with cardiac risks.
Mild QT prolongation can be caused by risperidone but there are no specific drug warnings associated with this.
An EKG is recommended before patients are prescribed SSRI agents citalopram and escitalopram if the prescribed dose is above 40 mg or 20 mg per day, respectively.
Antibiotics like ofloxacillin
When taken independently, erythromycin has been shown to cause both QT prolongation and TdP. Erythromycin works inhibiting the CYP3A protein. Patients who have low CYP3A activity and are also concurrently taking other medications such as disopyramide, which can lead to QT prolongation and TdP.