Type of Arrhythmia
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).
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.