Both amitriptyline and moxifloxacin should be discontinued in this patient. Her electrocardiogram (ECG) demonstrates QT-interval prolongation. Many medications may prolong the QT interval, including amitriptyline and moxifloxacin; QT prolongation may be markedly increased in patients taking more than one medication with this effect.

The corrected QT interval (QTc) is most often determined using the Bazett formula (QT interval / √ R-R interval). This patient's ECG at the time of admission shows a QTc interval of 630 ms; a normal QTc is defined as 460 ms or less in women and 440 ms or less in men. A QTc greater than 500 ms is associated with increased risk of torsades de pointes.

The cause of this patient's QT prolongation is likely her pharmacotherapy. Many drugs have been implicated in QT-interval prolongation, including antiarrhythmic agents, antibiotics (including some macrolides and fluoroquinolones), antipsychotic drugs, and antidepressants. A list of QT-prolonging drugs is available at http://crediblemeds.org/. The large degree of QTc prolongation in this patient may be caused by either the presence of two QT-prolonging medications or an underlying ion channel variant that is only evident in the setting of a QT-prolonging medication.

Although both medications may lead to her ECG findings, discontinuing either the moxifloxacin or amitriptyline in isolation is insufficient treatment given the significant degree of QT prolongation on this patient's ECG.

Carvedilol does not prolong the QT interval directly, although it may increase the risk of bradycardia-related arrhythmias in patients with acquired QT-interval prolongation by slowing the heart rate. However, the most important intervention in this patient is to reduce the risk of torsades de pointes by discontinuing the offending QT-prolonging agents.