Wide Complex Tachycardia: VT vs SVT with Aberrancy

The clinical stakes are lopsided. If we treat SVT as VT (for example, giving IV amiodarone to a patient who actually has SVT with aberrancy), the patient tolerates the drug without harm. The rhythm may slow or convert, and no damage is done.

If we treat VT as SVT, the consequences can be fatal. Verapamil, a calcium channel blocker commonly used for SVT, causes profound vasodilation and negative inotropy. In a patient with VT and a compromised left ventricle, verapamil can precipitate cardiovascular collapse and death. This has been documented repeatedly in the literature.

This asymmetry of risk is the entire reason behind the "always assume VT" rule. The cost of being wrong in one direction is negligible. The cost of being wrong in the other direction is a cardiac arrest.

AV dissociation is the single most specific criterion for VT. When the ventricles are driven by a focus below the AV node, the atria and ventricles beat independently. The sinus node continues to fire P waves at its own rate while the ventricular focus drives the QRS at a different, usually faster rate.

On the surface ECG, we see independent P waves marching through the QRS complexes at their own rhythm. The P-wave rate and the QRS rate have no fixed relationship. This proves the atria and ventricles are electrically disconnected, which can only happen when the ventricular rhythm originates below the AV node.

At fast ventricular rates, spotting P waves buried within wide QRS complexes and T waves is difficult. The best leads for this task are usually II and V1, where P waves tend to be most visible. A long rhythm strip helps.

Capture Beats and Fusion Beats

Two closely related findings confirm AV dissociation. A capture beat occurs when one of those independent sinus P waves happens to conduct through the AV node at exactly the right moment, "capturing" the ventricles and producing a single narrow QRS complex in the middle of the wide complex tachycardia. It is a sinus beat that snuck through during VT.

A fusion beat occurs when the sinus impulse and the ventricular focus depolarize the ventricles simultaneously. The resulting QRS is a hybrid: narrower than the tachycardia complex but wider than sinus rhythm. Its morphology is intermediate between the two. Both findings are pathognomonic for VT, because they prove that the atria and ventricles are operating independently.

When AV dissociation is not visible (and it often is not at fast rates), we turn to the shape of the QRS complex in lead V1. The logic is straightforward: a true bundle branch block, whether RBBB or LBBB, produces a specific, predictable QRS morphology. VT, which originates from ventricular muscle and spreads via slow cell-to-cell conduction, does not replicate these clean patterns.

RBBB-Pattern Wide Complex Tachycardia

In true RBBB (SVT with aberrancy), the initial septal depolarization and left ventricular activation proceed normally. Only the right ventricle depolarizes late. This produces the classic triphasic rsR' pattern in V1: a small r, a small s, then a tall R'.

In VT with an RBBB-like pattern, the complex in V1 looks different. A monophasic R wave (a single broad upstroke with no initial r-s), a qR complex, or a broad R with a notched downstroke all favor VT. These patterns reflect abnormal activation spreading through diseased myocardium rather than through the His-Purkinje system.

LBBB-Pattern Wide Complex Tachycardia

In true LBBB, the septum is depolarized from right to left. V1 sits over the right ventricle and records a narrow r wave followed by a quick, clean plunge to the S-wave nadir. The initial r wave is thin (under 30 ms), and the downstroke is smooth.

In VT mimicking LBBB, the initial r wave in V1 is wide (greater than 30 ms). There is often a notch or slur on the downstroke of the S wave, reflecting slow, stuttering conduction through damaged tissue. The interval from QRS onset to the S-wave nadir exceeds 60 ms. Any of these three findings favors VT over SVT with true LBBB.

In 1991, Brugada and colleagues published a stepwise algorithm for differentiating VT from SVT with aberrancy. It applies four criteria in sequence. If any step is positive, the diagnosis is VT. Only if all four steps are negative does the algorithm conclude SVT with aberrancy.

Concordance refers to a pattern where all six precordial leads (V1 through V6) show QRS complexes deflecting in the same direction. In positive concordance, every precordial lead has a dominant R wave. In negative concordance, every lead has a dominant S wave.

Normal bundle branch block, whether RBBB or LBBB, always produces a transition zone somewhere across the precordium: leads on one side are predominantly positive, and leads on the other side are predominantly negative. Concordance eliminates this transition entirely.

A rhythm that produces uniformly positive or uniformly negative complexes across V1 to V6 is originating from the ventricular apex (positive concordance) or the ventricular base (negative concordance), spreading in a single direction through myocardium. No SVT conducted through the His-Purkinje system can generate this pattern. Concordance strongly favors VT.

One exception: positive concordance can occur in antidromic AVRT using a posterior accessory pathway. We will address this below.

The frontal plane axis provides another powerful clue. A northwest axis, sometimes called extreme axis deviation, means the QRS is negative in both lead I and lead aVF. The electrical vector points upward and to the right, a direction that no normal or aberrant conduction pattern produces.

In LBBB, the axis typically remains between −30 and +90 degrees. In RBBB, the axis shifts rightward. Neither pattern puts the axis into the northwest quadrant (between −90 and ±180 degrees).

A wide complex tachycardia with a northwest axis is nearly pathognomonic for VT. It indicates a ventricular origin where the wavefront travels from apex toward base and from left toward right, a vector incompatible with any supraventricular mechanism.

Several clinical scenarios produce wide complex tachycardias that look like VT on the surface ECG but are actually supraventricular in origin.

Pre-existing Bundle Branch Block

A patient with a baseline LBBB or RBBB will have wide QRS complexes during any tachycardia, including sinus tachycardia, SVT, or atrial flutter. The width of the QRS reflects the fixed conduction defect, not a ventricular origin. Comparing the tachycardia QRS to a baseline ECG in sinus rhythm is often the fastest way to resolve the question. If the morphology is identical to the known BBB, SVT is far more likely.

Antidromic AVRT

In patients with a Wolff-Parkinson-White (WPW) accessory pathway, antidromic AVRT conducts antegrade over the bypass tract and retrograde through the AV node. The entire ventricle is activated from the accessory pathway insertion site, producing a maximally pre-excited, very wide QRS. This can perfectly mimic VT. The clinical context (young patient, no structural heart disease, known WPW on prior ECGs) is critical.

Hyperkalemia

Severe hyperkalemia (potassium above 6.5 to 7.0 mEq/L) slows conduction velocity throughout the myocardium, widening the QRS even during sinus rhythm. A patient with hyperkalemia and sinus tachycardia can present with what appears to be a wide complex tachycardia. The T waves are typically tall and peaked, and the clinical context (renal failure, dialysis patient) usually points to the correct diagnosis.

In all of these scenarios, the baseline ECG is the most valuable reference. Every wide complex tachycardia workup should include a comparison to the patient's most recent ECG in sinus rhythm.