Ventricular Tachycardia with Pulse

01Pathophysiology

Ventricular tachycardia (VT) originates from electrical impulses arising below the bundle of His within ventricular myocardium. The most common underlying mechanism is re-entry, which occurs when an electrical impulse continuously loops around a region of scarred myocardium. This scar tissue, most often resulting from a prior myocardial infarction, creates zones of slow conduction adjacent to normal tissue, establishing the anatomical substrate for a re-entrant circuit. This is why VT is overwhelmingly associated with structural heart disease, and why a history of MI is the single strongest risk factor.

The second mechanism is enhanced automaticity, where abnormal ventricular foci develop spontaneous depolarization rates that exceed the sinoatrial node. This is often triggered by catecholamine surges, ischemia, or metabolic derangements. The third mechanism is triggered activity, which arises from afterdepolarizations. Early afterdepolarizations (EADs) occur during phase 2 or 3 of the action potential and are linked to prolonged QT intervals and Torsades de Pointes. Delayed afterdepolarizations (DADs) occur after full repolarization and are classically associated with digitalis toxicity and intracellular calcium overload.

Because the impulse originates in the ventricle rather than traveling through the normal His-Purkinje system, ventricular depolarization is slow and disorganized, producing the hallmark wide QRS complex (>120 ms) on ECG. The atria and ventricles beat independently (AV dissociation), explaining the cannon A waves seen on physical exam: the atrium occasionally contracts against a closed tricuspid valve during ventricular systole.

The reason VT with a pulse is clinically distinct from pulseless VT is that the ventricle still generates enough organized contraction to maintain cardiac output. However, the rapid rate severely reduces ventricular filling time and can compromise hemodynamics at any moment, making this a time-sensitive diagnosis.

02Classification and Clinical Manifestation

The distinction between monomorphic and polymorphic VT is critical for management decisions. Monomorphic VT suggests a stable re-entrant circuit and responds to standard antiarrhythmics. Polymorphic VT with prolonged QT (Torsades) requires a fundamentally different approach: magnesium and overdrive pacing rather than standard antiarrhythmics, which can worsen it.

03Diagnostic Workup

The best initial test is a 12-lead ECG. The defining features that favor VT over SVT with aberrant conduction are:

1. AV dissociation: P waves march independently of QRS complexes. This is the single most reliable criterion for VT on ECG, though it can be difficult to see when the ventricular rate is fast.

2. Capture beats: A narrow, normal QRS complex appearing intermittently, indicating that a sinus impulse occasionally "captures" the ventricle through the normal conduction system.

3. Fusion beats: A hybrid QRS morphology resulting from simultaneous ventricular activation by the sinus impulse and the ventricular focus.

4. Positive or negative concordance: All precordial leads (V1 through V6) show QRS deflections pointing in the same direction, either all positive or all negative. This finding strongly favors VT.

5. Very wide QRS: A QRS duration >160 ms with RBBB pattern or >140 ms with LBBB pattern increases the likelihood of VT.

The Brugada criteria provide a systematic stepwise algorithm. If the answer to any of the following is "yes," the rhythm is VT: (1) absence of an RS complex in all precordial leads, (2) R-to-S interval >100 ms in any precordial lead, (3) AV dissociation, (4) morphology criteria in V1-V2 and V6 not consistent with typical RBBB or LBBB aberrancy.

Laboratory studies should be sent simultaneously. Electrolyte correction is both diagnostic and therapeutic: hypokalemia and hypomagnesemia lower the threshold for VT and must be corrected. Troponin is obtained to evaluate for acute ischemia as a reversible trigger.

Echocardiography is essential for assessing the structural substrate. A reduced ejection fraction ($EF\le 35\mathrm{\%}EF\; \backslash leq\; 35\backslash \%$) not only identifies the underlying pathology but also determines candidacy for ICD implantation.

The most accurate test is an electrophysiology (EP) study, where programmed ventricular stimulation can reproduce the arrhythmia, map the circuit, and guide catheter ablation. This is not performed acutely but is the gold standard for definitive characterization.

A high-yield exam principle: when you cannot distinguish between VT and SVT with aberrancy, always treat as VT. This is safer because treating SVT with aberrancy as VT does no harm, but treating VT as SVT (for example, giving verapamil) can cause cardiovascular collapse.

04Management and Treatment

Step 1: Assess hemodynamic stability. This is the single most important decision point. Ask: does the patient have signs of hemodynamic compromise (hypotension with SBP <90 mmHg, altered mental status, severe dyspnea from pulmonary edema, or ongoing ischemic chest pain)? If yes, the patient is unstable, and the next best step is immediate synchronized cardioversion starting at 100J biphasic. Synchronized cardioversion delivers the shock on the R wave to avoid hitting the T wave, which could degenerate VT into ventricular fibrillation. Provide procedural sedation with a short-acting agent (midazolam 1 to 2 mg IV or etomidate 0.3 mg/kg IV) if the patient is still conscious, but do not delay cardioversion for sedation if the patient is crashing.

Step 2: If the patient is hemodynamically stable, pharmacologic treatment is appropriate. For stable monomorphic VT, the recommended first-line agent is IV amiodarone: a 150 mg bolus over 10 minutes, followed by a continuous infusion of 1 mg/min for 6 hours, then 0.5 mg/min for the next 18 hours. Amiodarone is a class III antiarrhythmic that blocks potassium channels (prolonging repolarization) but also has sodium channel, calcium channel, and beta-blocking properties. Its broad mechanism makes it effective but also accounts for its side effect profile: hypotension (from the IV solvent), bradycardia, QT prolongation, thyroid dysfunction, pulmonary fibrosis, and hepatotoxicity with chronic use.

IV procainamide is an alternative, particularly favored in some guidelines for stable monomorphic VT. It is infused at 20 to 50 mg/min and must be stopped when any of four endpoints are reached: arrhythmia termination, QRS widening by more than 50% from baseline, hypotension, or the maximum cumulative dose of 17 mg/kg. Procainamide requires continuous blood pressure monitoring. It is contraindicated in patients with prolonged QT because it further prolongs the QT interval and can provoke Torsades.

Step 3: Torsades de Pointes (polymorphic VT with prolonged QT) requires a completely different strategy. Standard antiarrhythmics like amiodarone and procainamide worsen it because they further prolong the QT interval. The first-line treatment is IV magnesium sulfate 2 g over 15 minutes, regardless of the serum magnesium level. If the arrhythmia persists or recurs, overdrive pacing via a transvenous pacemaker at a rate of 90 to 110 bpm is the definitive acute therapy. Increasing the heart rate shortens the QT interval and suppresses the "short-long-short" trigger sequence. If pacing is unavailable, IV isoproterenol (2 to 10 mcg/min, titrated to heart rate) can be used as a temporizing measure. All QT-prolonging drugs must be discontinued immediately. Correct hypokalemia to a target of $K^{+}>4.0K^+\; >\; 4.0$ mEq/L.

Step 4: Long-term management. Any patient who survives an episode of sustained VT not due to a fully reversible cause (such as acute MI within 48 hours, isolated drug toxicity, or isolated electrolyte abnormality) should receive an implantable cardioverter-defibrillator (ICD) for secondary prevention. The ICD continuously monitors for VT/VF and delivers an internal shock when detected. Beta-blockers (metoprolol or carvedilol) are added to reduce sympathetic triggers and decrease the frequency of ICD shocks. Oral amiodarone or sotalol may be added as adjunctive antiarrhythmic therapy in patients with recurrent episodes. Catheter ablation is indicated for patients with recurrent VT causing frequent ICD discharges (sometimes called "VT storm").

Key contraindication to remember: Never give verapamil or diltiazem (IV calcium channel blockers) for a wide complex tachycardia presumed to be VT. These agents cause peripheral vasodilation and negative inotropy without terminating the VT circuit, leading to severe hypotension and potential cardiac arrest.

05Differential Diagnosis and Distractors

06Traps and High-Yield Pearls

The most common way students lose points on VT questions involves misidentifying it as SVT with aberrancy and selecting an inappropriate treatment, most dangerously IV verapamil. On the exam, when you see a wide complex tachycardia in a patient with a history of heart disease (especially prior MI or heart failure), the diagnosis is VT until proven otherwise. The test-writers are assessing whether you know the cardinal rule: all wide complex tachycardias should be treated as VT unless there is clear evidence to the contrary. This principle is safe because amiodarone and synchronized cardioversion are appropriate for both VT and SVT with aberrancy, while verapamil given to a patient in VT can be lethal.

The second major trap is confusing Torsades de Pointes with standard monomorphic VT and giving amiodarone or procainamide. Both agents prolong the QT interval and will worsen Torsades. The exam will present a patient on a QT-prolonging medication (commonly a fluoroquinolone, antipsychotic, or class III antiarrhythmic) who develops a polymorphic wide complex tachycardia with a "twisting" QRS axis. The correct answer is IV magnesium, not amiodarone.

A third tested concept is the distinction between synchronized cardioversion and defibrillation. VT with a pulse gets synchronized cardioversion (shock delivered on the R wave). Pulseless VT gets unsynchronized defibrillation (random delivery). Using unsynchronized shock on a patient with VT who still has a pulse risks delivering energy during the vulnerable period of the T wave, potentially triggering VF.

Finally, know the long-term disposition. Any question asking about the "next step after stabilization" for a patient who survived sustained VT will test whether you know to recommend an ICD for secondary prevention. Antiarrhythmic drugs alone are not a substitute for device therapy in this population. The exam vignette may describe a patient who is now stable and in sinus rhythm post-cardioversion; the answer is ICD evaluation, not discharge on oral amiodarone alone.