Pharmacology, Toxicology and Addiction MedicineTier 1Approach to a presentation

Associated with adverse drug interactions - arrhythmias

Red flags

QTc >500ms or increase >60ms from baseline - high torsades risk
Polymorphic VT / torsades de pointes
Wide-complex tachycardia after TCA or flecainide overdose
Bradyarrhythmia with hypotension (beta-blocker/CCB/digoxin)
Syncope temporally linked to a new QT-prolonging drug

Differential by mechanism

QT prolongation -> torsades

Antiarrhythmics (sotalol, amiodarone, flecainide), antipsychotics (haloperidol IV, quetiapine), antidepressants (citalopram/escitalopram dose-dependent, TCAs), macrolides, fluoroquinolones, methadone (dose-dependent), ondansetron (high-dose IV)
Risk multiplies with hypokalaemia, hypomagnesaemia, bradycardia, combination QT-prolonging drugs

Sodium channel blockade -> wide QRS, VT

TCA overdose, flecainide, cocaine

Bradyarrhythmia / AV block

Beta-blockers, non-dihydropyridine CCBs, digoxin toxicity, cholinesterase inhibitors (donepezil)

Atrial/ventricular ectopy, AF

Theophylline, sympathomimetics (stimulants, decongestants), digoxin toxicity (any arrhythmia, classically atrial tachycardia with block)

Drug interaction potentiation

CYP3A4 inhibitors (macrolides, azoles) raising levels of QT-prolonging substrates

Focused history

New drug, dose increase, or added interacting drug (esp. CYP inhibitor)
Electrolyte-depleting states - diuretics, vomiting, diarrhoea
Palpitations, syncope, near-syncope temporally linked to dosing
Family history of sudden death / congenital long QT

Focused examination

Pulse rate and regularity
Signs of the causative toxidrome (anticholinergic, cholinergic, sympathomimetic)
Volume status affecting electrolytes

Investigation strategy

ECG - QTc (Bazett), QRS duration, rhythm
UEC, magnesium, calcium, potassium
Drug levels where available (digoxin, lithium)
Continuous cardiac monitoring

Management

1. Stop the causative agent(s) and correct electrolytes

Keep K+ >4.5, Mg2+ >1.0 mmol/L in QT prolongation

2. Torsades de pointes

IV magnesium sulfate first-line regardless of magnesium level
Overdrive pacing or isoprenaline if recurrent/bradycardia-dependent
Defibrillation if pulseless/sustained

3. Sodium channel blockade (wide QRS)

IV sodium bicarbonate - narrows QRS, treats the arrhythmia directly

4. Bradyarrhythmia

Atropine, glucagon (beta-blocker), calcium + insulin-euglycaemia (CCB), DigiFab (digoxin)

5. Review polypharmacy

Deprescribe redundant QT-prolonging agents; avoid combining two

Traps

Citalopram/escitalopram QT effect is dose-dependent and additive with other QT drugs - easily missed in polypharmacy
Standard antiarrhythmics (Class Ia/Ic) can worsen a sodium-channel-blocker-induced arrhythmia - bicarbonate, not more antiarrhythmic
Torsades gets magnesium even with a normal serum level
Digoxin toxicity can present as almost any arrhythmia - atrial tachycardia with AV block is classic

Talk track

1. QT vs QRS drives the algorithm

"Long QT with torsades gets magnesium; a wide QRS gets bicarbonate - different mechanisms, different antidotes."

2. Look at the whole drug list

"Arrhythmia risk is often additive across several 'safe' QT-prolonging drugs plus low potassium."

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