As outlined, some drugs that cause bradyarrhythmias may also cause tachyarrhythmias.

Drugs that cause arrhythmias via 'membrane stabilising effects', such as TCAs, will generally only cause bradycardia as a preterminal event and at this time the ECG shows marked QRS and QT prolongation such that the arrhythmia resembles slow ventricular tachycardia (or a sine wave).

On the other hand, digoxin, calcium channel, and beta blocking drugs will often cause heart block with only minor QRS prolongation.

Tachyarrhythmias with preceding QRS/QT changes suggest poisoning by drugs with membrane effects (see list). In some cases, the arrhythmia is a polymorphic VT though in most cases it is a wide complex tachycardia.

The classic “torsade de pointes” is most frequently seen with non-sedating antihistamines.

Supraventricular tachycardia is more suggestive of adrenergic drug (e.g.theophylline) overdose or digoxin.