Ventricular Tachyarrhythmias

Ventricular tachycardia (VT) is generally defined as 3 or more consecutive ventricular complexes at a rate over 100 beats per minute on the ECG; ventricular fibrillation (VF) is a rapid, disorganized rhythm with no recognizable QRS complexes that is almost always fatal without immediate defibrillation.^[1]

VT ranges from a clinically silent, incidental finding to a life-threatening emergency, and management hinges on 3 questions answered in sequence: is the patient pulseless or hemodynamically unstable (an immediate defibrillation/cardioversion emergency, never a medication-first problem); is the QRS morphology monomorphic or polymorphic (polymorphic VT with QT prolongation — torsades de pointes — needs fundamentally different pharmacotherapy from monomorphic VT, and treating it with a standard antiarrhythmic can make it worse); and is there underlying structural heart disease (the strongest predictor of recurrence and of the need for an implantable cardioverter defibrillator). This chapter covers all 3 scenarios, chronic secondary prevention, and drug-induced QT prolongation, since medications are the most common cause of acquired torsades de pointes risk.

Symptom spectrum

Presentation ranges from entirely asymptomatic (most often nonsustained VT found incidentally on a monitor or Holter) to palpitations, dyspnea, chest discomfort, presyncope, or syncope, to cardiac arrest. Symptom severity does not reliably predict prognosis — even marked symptoms from brief nonsustained VT in a structurally normal heart can be prognostically benign, while asymptomatic nonsustained VT in a patient with left ventricular dysfunction can signal a real risk of future sustained VT or VF.^[1]

Any wide-complex tachycardia (QRS 0.12 seconds or more) in a patient over 50, or in a patient with known structural heart disease at any age, should be treated as VT until proven otherwise — most wide-complex tachycardias in any patient of any age are in fact VT, and defaulting to an assumption of supraventricular tachycardia with aberrancy is a common and dangerous error.^[1]

General risk factors for VT/VF

• Structural heart disease — coronary artery disease and prior myocardial infarction are the most common overall cause.
• Cardiomyopathy (ischemic or nonischemic), reduced left ventricular ejection fraction.
• Valvular or hypertensive heart disease.
• Inherited arrhythmia syndromes — long QT syndrome, Brugada syndrome, catecholaminergic polymorphic VT, arrhythmogenic right ventricular cardiomyopathy.
• Family history of sudden death or documented VT/VF, especially under age 65 — consider referral for genetic testing of known heritable disorders.
• Electrolyte abnormalities — hypokalemia, hypocalcemia, hypomagnesemia.
• Active myocardial ischemia.
• Digitalis toxicity.
• Cocaine or other stimulant use.
• Exercise-induced ectopy, particularly when polymorphic — raises concern for catecholaminergic polymorphic VT or ischemia.

Risk factors for drug-induced torsades de pointes specifically

Because medications are the most common cause of acquired QT prolongation, a separate set of risk factors predicts torsades de pointes when a QT-prolonging drug is used — the more of these present, the more caution (or an alternative drug) is warranted.^[2]

Differentiating monomorphic VT, polymorphic VT/torsades de pointes, and SVT with aberrancy

These 3 wide-complex rhythms are commonly confused on initial ECG review, and correct treatment differs for each — getting the differentiation right, or defaulting to treating an uncertain wide-complex tachycardia as VT, is a genuine patient-safety issue.^[1]

History

• Syncope or severe presyncope.
• Angina or heart failure symptoms.
• History suggesting structural heart disease, especially prior MI or heart failure.
• Symptom correlation with exercise or stress.
• Recent ingestion of medications that might cause or contribute to arrhythmias — antiarrhythmics, or any QT-prolonging drug (see Drug Tables).
• Family history of sudden death or documented VT/VF, especially under age 65 — consider referral for genetic testing of known heritable disorders.

Physical examination

Examine for signs of structural heart disease — cardiomyopathy, or valvular or hypertensive heart disease.

Laboratory testing

Check potassium, calcium, and magnesium in every patient presenting with a ventricular arrhythmia — electrolyte abnormalities are a common, correctable contributor.

12-lead ECG

• Signs of prior MI or ongoing ischemia.
• Repolarization abnormalities — short or prolonged QT interval, ECG signs of Brugada syndrome, early repolarization syndrome, T-wave inversions.
• A 12-lead ECG documenting the tachycardia itself is very helpful when available; an ECG captured at tachycardia onset or offset is also very useful.

Echocardiogram

Assess left ventricular size and function and right ventricular size — the degree of LV dysfunction is the single most important prognostic factor and a key driver of ICD candidacy.

Additional testing

• Holter or loop event recorder monitoring — assess the presence and morphology of ventricular ectopy and symptom-rhythm correlation.
• Treadmill exercise testing — assess for exercise-induced VT and ECG or symptomatic evidence of myocardial ischemia.
• When structural heart disease is absent: signal-averaged ECG, cardiac MRI (to assess structure and scar), and provocative drug testing (isoproterenol for suspected long QT syndrome, procainamide for suspected Brugada syndrome) under specialist direction.

Calculating and interpreting the QT interval

The QTc is the rate-corrected QT interval; the most commonly used correction is the Bazett formula, which divides the measured QT interval (in seconds) by the square root of the R-R interval (in seconds). Bazett's formula undercorrects at fast heart rates, so a QTc over 480 ms at a fast heart rate can still reflect normal repolarization.^[3]

When further testing is not indicated

An asymptomatic patient with occasional, isolated premature ventricular complexes and no structural heart disease on a basic history and exam does not routinely need echocardiography, Holter monitoring, or electrophysiology referral — extensive investigation is reserved for sustained VT, symptomatic VT, or ventricular ectopy in the setting of known or suspected structural heart disease.

• Restore a perfusing rhythm as quickly as possible in pulseless VT, VF, or cardiac arrest.
• Terminate hemodynamically unstable VT immediately, without delay for drug therapy.
• Correctly distinguish monomorphic from polymorphic VT/torsades de pointes, since the correct drug therapy for one can worsen the other.
• Identify and correct reversible contributors — electrolyte abnormalities, active ischemia, and offending QT-prolonging drugs.
• Prevent the potentially fatal recurrence of sustained VT or VF.
• Establish appropriate chronic secondary prevention — ICD candidacy assessment, foundational beta-blocker therapy, adjunctive antiarrhythmic drug therapy, and catheter ablation where indicated.

• Immediate CPR and defibrillation remain the primary, life-saving therapy for pulseless VT/VF — no medication replaces this.
• Synchronized cardioversion — a synchronized biphasic shock is usually effective for hemodynamically unstable VT with a pulse.
• Implantable cardioverter defibrillator (ICD) — effective at terminating VT/VF episodes and reduces mortality compared with antiarrhythmic drug therapy alone in patients who meet criteria, though it does not itself prevent arrhythmia recurrence.^[9]
• Catheter ablation — first-line for structurally normal hearts, and an effective first-line alternative to escalating antiarrhythmic drug therapy for selected patients with ischemic cardiomyopathy and VT, generally comparable to amiodarone and superior to other drugs such as sotalol.^[7]
• Stereotactic cardiac radioablation — a largely investigational, not widely available technique reserved as a last resort for VT refractory to other treatment options.
• Temporary transvenous or transcutaneous overdrive pacing — for pause-dependent or bradycardia-associated torsades de pointes, while the underlying cause is corrected.
• Correction of electrolyte abnormalities (potassium, calcium, magnesium) — a core, non-drug intervention in every ventricular arrhythmia presentation.
• Identification and discontinuation of any offending QT-prolonging medication.
• Genetic counselling and first-degree family member screening when a heritable arrhythmia syndrome (long QT syndrome, Brugada syndrome, catecholaminergic polymorphic VT) is suspected or confirmed.

Pregnancy

Manage a pulseless or hemodynamically unstable ventricular arrhythmia in pregnancy with the same urgency and the same defibrillation/cardioversion-first approach as in a non-pregnant patient — maternal resuscitation is the priority, and defibrillation is safe throughout pregnancy. Beta-blockers are the preferred chronic antiarrhythmic in pregnancy. Amiodarone crosses the placenta and can cause fetal or neonatal thyroid dysfunction, so it is reserved for pregnancy only when a life-threatening arrhythmia leaves no safer alternative — this reservation never delays its use during an actual cardiac arrest or hemodynamically unstable VT. An ICD, when indicated, is safe to implant during pregnancy and should not be withheld.

Breastfeeding

A single emergency antiarrhythmic dose given during resuscitation or acute stabilization is not a reason to withhold or delay breastfeeding once the mother has recovered, though interruption for several half-lives is reasonable after a large amiodarone dose given its very long half-life. Beta-blockers (metoprolol, propranolol) are compatible with breastfeeding. Sotalol is excreted into breast milk at higher concentrations than most beta-blockers — monitor the infant for bradycardia or poor feeding if sotalol is continued during breastfeeding.

Pediatrics

Ventricular tachyarrhythmias in children without structural heart disease more often reflect an inherited channelopathy (long QT syndrome, catecholaminergic polymorphic VT) than acquired coronary disease, and pediatric dosing for the antiarrhythmics in this chapter should be directed by pediatric cardiology/electrophysiology specialists rather than extrapolated from adult doses. Beta-blocker therapy and avoidance of QT-prolonging drugs remain foundational in pediatric long QT syndrome specifically.

Renal impairment

Sotalol is renally cleared and requires interval adjustment — every 24 hours for creatinine clearance 30-60 mL/min, every 36-48 hours for creatinine clearance 10-30 mL/min, and individualized dosing below that. Procainamide excretion is also delayed in renal impairment, requiring reduced frequency of administration and closer plasma-level monitoring. Magnesium sulfate should not exceed 20 g in any 48-hour period in significant renal impairment, with close serum magnesium monitoring.

Hepatic impairment

Lidocaine clearance depends on hepatic blood flow and is reduced in hepatic impairment and heart failure — reduce both the bolus and the infusion dose. Amiodarone can itself cause hepatotoxicity and requires baseline and periodic liver enzyme monitoring regardless of baseline hepatic status.

Older adults

Age over 65-67 is itself an independent risk factor for torsades de pointes — use extra caution with any QT-prolonging drug in this population. Start antiarrhythmics at the lower end of the dosing range and titrate more slowly, reflecting the higher likelihood of reduced hepatic, renal, or cardiac function and of interacting medications.

Assessing a patient who needs a QT-prolonging medication

Community and hospital pharmacists are often the first to catch a risky combination of QT-prolonging drugs. Before dispensing or recommending any QT-prolonging medication in a patient with 1 or more torsades de pointes risk factors: confirm the drug is genuinely necessary and cannot be swapped for a non-QT-prolonging alternative; ask about a personal or family history of ventricular arrhythmia, congenital long QT syndrome, syncope, or sudden death, and avoid QT-prolonging drugs altogether in confirmed congenital long QT syndrome; review the patient's other medications and conditions for additional risk factors or impaired drug metabolism; use the lowest effective dose; and counsel the patient to report palpitations, dizziness, or fainting, and to report any illness or new medication — especially a diuretic — that could cause hypokalemia.^[2]

Consult a current, regularly updated QT-drug reference, such as CredibleMeds, when uncertain whether a specific medication carries a meaningful QT-prolongation risk.^[10]

Using 2 or more QT-prolonging medications together does not automatically cause dangerous QT prolongation or arrhythmia, and necessary medications should not be withheld purely out of fear of QT prolongation once the patient has been properly risk-assessed.^[2]

1. VT/VF recurs despite an ICD in place: assess for reversible triggers (recurrent ischemia, electrolyte disturbance, a new interacting or QT-prolonging medication), optimize beta-blocker and any adjunct antiarrhythmic therapy, and refer for catheter ablation.
2. Electrical storm resistant to a beta-blocker plus amiodarone: consider cautiously adding a Class I agent such as mexiletine, or quinidine (available only through the Health Canada Special Access Program), as an entirely empiric last resort, ensure adequate sedation, and pursue urgent ablation or mechanical circulatory support referral.
3. Torsades de pointes recurs despite magnesium: re-check and correct potassium and magnesium, confirm every QT-prolonging drug has actually been stopped, and add isoproterenol or temporary overdrive pacing, particularly if the pattern is pause-dependent or bradycardia-associated.
4. QTc continues to rise on a medication that is still judged necessary: reduce the dose, switch to a non-QT-prolonging alternative if one exists, or discontinue the drug — reassess the overall risk picture rather than continuing unchanged.
5. Suspected inappropriate ICD shocks: arrange device interrogation to distinguish a misclassified supraventricular arrhythmia from true recurrent VT, and adjust device programming or antiarrhythmic therapy accordingly.

• Treating torsades de pointes with a standard Class Ia or Class III antiarrhythmic (procainamide, sotalol, quinidine, amiodarone) — these further prolong the QT interval and can worsen torsades rather than treat it.
• Reaching for procainamide, sotalol, or a beta-blocker as primary therapy for shock-refractory pulseless VF/VT — current AHA guidance considers their benefit uncertain in that specific scenario; amiodarone or lidocaine are the appropriate choices.^[4]
• Giving amiodarone intraosseously when IV access is available — IV administration is preferred, since the IO route may blunt efficacy.^[4]
• Forgetting to identify and stop the offending QT-prolonging drug(s) as part of torsades de pointes management — magnesium alone does not fix a persistent drug-induced trigger.
• Missing hypokalemia or hypomagnesemia correction while treating torsades de pointes.
• Withholding a genuinely necessary QT-prolonging medication purely out of fear, once the patient has been properly risk-assessed.
• Assuming a wide-complex tachycardia in an older patient, or a patient with known structural heart disease, is supraventricular with aberrancy by default — treat as VT until proven otherwise.
• Delaying synchronized cardioversion in unstable VT to try a drug first.
• Relying on antiarrhythmic drug therapy alone for long-term secondary prevention in a patient with left ventricular dysfunction who meets ICD criteria — drug therapy is an adjunct to, not a substitute for, the ICD in this population.
• Missing a genetic testing/electrophysiology referral for a young patient with unexplained VT/VF or a family history of sudden death.

How to use this table: Doses are adult doses and require patient-specific confirmation, including renal/hepatic adjustment. Select any drug name to open its complete Pharmasist Drug Database page.

Immediate therapy for pulseless VT/VF (cardiac arrest)

Given only after CPR and defibrillation have started — never before the first shock. Current AHA guidance considers procainamide, sotalol, and beta-blockers of uncertain benefit specifically for VF/pulseless VT unresponsive to defibrillation, so they are not used in this scenario even though they have established roles elsewhere in this chapter.

Hemodynamically stable monomorphic VT

Not for pulseless arrest, and not for polymorphic VT/torsades de pointes.

Polymorphic VT / torsades de pointes

A distinct entity from monomorphic VT — standard Class Ia/III antiarrhythmics are not used here and can worsen the arrhythmia.

Chronic secondary prevention

Beta-blockers are foundational for essentially all patients on drug therapy; amiodarone, sotalol, and mexiletine are adjuncts, not substitutes for an ICD when ICD criteria are met.

QT-interval-prolonging drug classes

Medications are the most common cause of acquired QT prolongation. This is a reference list of drug classes commonly associated with QT prolongation, not a treatment table — consult a current, regularly updated QT-drug list (e.g., CredibleMeds) for a complete inventory.

Evidence note: Drug inventory, dosing, adverse effects, and comments reflect real Canadian product-labelled dosing (Health Canada product monographs), cross-checked against current AHA/ACC/HRS guidance^[1] and current AHA resuscitation guidance^[4].