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.