TACHYARRHYTHMIAS

The most common drugs ingested which prolong QRS & QT intervals and cause tachyarrhythmias are

• Tricyclic antidepressants *
• Antipsychotic drugs (particularly thioridazine) *
• Antihistamines (particularly terfenadine and astemizole)
• Anticonvulsants (particularly carbamazepine and phenytoin) *
• Antimalarial drugs (chloroquine, quinine) *
• Beta blockers (only propranolol and sotalol) *
• Digoxin *
• Antiarrhythmic drugs *
• Orphenadrine *
• Lithium *
• Chloral hydrate

*These drugs may also cause bradyarrhythmias
These drugs are often referred to as having 'quinidine-like' effects, 'membrane-stabilising' properties or 'nonspecific toxicity'.

In addition

Adrenergic drugs (theophylline, salbutamol, cocaine and amphetamines) cause tachyarrhythmias without causing preceding QRS & QT prolongation.

A number of drugs that lead to tissue hypoxia, acidosis or hypoglycaemia (CO, organochlorines, hypoglycaemic drugs, isoniazid, salicylates etc.) may cause ventricular arrhythmias without marked preceding ECG changes.

Features that help to differentiate between these drugs are

TYPE OF ARRHYTHMIA
LEVEL OF CONSCIOUSNESS PRIOR TO ARRHYTHMIA
SEIZURES
HEART RATE PRIOR TO ARRHYTHMIA
ECG CHANGES PRIOR TO ARRHYTHMIA
PUPIL SIZE & REACTION
RESPONSE TO ANTAGONISTS
ELECTROLYTES/BLOOD GLUCOSE
BLOOD CONCENTRAIONS

It is important to try to make the correct diagnosis, as there are specific treatments.