The most common drugs ingested which prolong QRS and QT intervals and cause bradyarrhythmias are:

• Calcium channel blockers (particularly verapamil and diltiazem)
• Beta blockers (particularly propranolol * and sotalol *) #
• Digoxin #
• Tricyclic antidepressants # *
• Antipsychotic drugs (particularly thioridazine) # *
• Anticonvulsants (particularly carbamazepine and phenytoin) # *
• Dextropropoxyphene *
• Antimalarial drugs (chloroquine, quinine) # *
• Antiarrhythmic drugs # *
• Orphenadrine # *
• Lithium # *

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

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
• DRUG CONCENTRATIONS

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