8.3 Wide-Complex Tachycardias & Ventricular Arrhythmia Mapping Criteria

8.3 Wide-Complex Tachycardias & Ventricular Arrhythmia Mapping Criteria

A Wide-Complex Tachycardia (WCT) is defined electrocardiographically as a cardiac rhythm with a heart rate $> 100\text{ bpm}$ and a QRS duration $\ge 120\text{ ms}$ ($0.12\text{ s}$). In emergency, intensive care, and electrophysiology settings, the differential diagnosis of a wide-complex tachycardia represents one of the most critical clinical challenges. Any wide-complex tachycardia must be treated as Ventricular Tachycardia (VT) until definitively proven otherwise, because misdiagnosing VT as supraventricular tachycardia and administering AV nodal blocking agents (such as verapamil or diltiazem) frequently precipitates catastrophic hemodynamic collapse and ventricular fibrillation.

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Differential Diagnosis & Statistical Distribution

The differential diagnosis of a wide-complex tachycardia comprises four primary entities:

1. Ventricular Tachycardia (VT): Accounts for $80\%\text{ to }85\%$ of all presentations. In patients with documented structural heart disease (prior myocardial infarction, ischemic cardiomyopathy, dilated cardiomyopathy, or hypertrophic cardiomyopathy), the statistical probability that a WCT is VT exceeds $95\%$.
2. Supraventricular Tachycardia with Aberrancy: Accounts for $10\%\text{ to }15\%$ of cases. Occurs when a supraventricular arrhythmia (sinus tachycardia, atrial fibrillation, atrial flutter, AVNRT, or AVRT) conducts antegradely across the AV node but encounters rate-dependent functional bundle branch block (phase 3 aberrancy, most commonly Right Bundle Branch Block due to its longer refractory period) or pre-existing permanent bundle branch block.
3. Pre-Excited Tachycardia (Antidromic AVRT or Bystander Accessory Pathway): Accounts for approximately $1\%\text{ to }5\%$ of cases. Antegrade conduction proceeds down an extranodal bypass tract directly into ventricular myocardium (producing maximal pre-excitation and wide QRS), with retrograde conduction ascending the AV node or a second bypass tract.
4. Ventricular Paced Rhythms & Artifact: Accounts for approximately $1\%\text{ to }2\%$.

Critical Clinical Rule: Hemodynamic stability does NOT distinguish SVT with aberrancy from VT! Over $50\%$ of patients in sustained monomorphic VT present with preserved consciousness and stable blood pressure. Diagnostic decisions must rely exclusively on electrophysiological and electrocardiographic criteria.

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Surface ECG Diagnostic Criteria for Ventricular Tachycardia

1. Atrioventricular (AV) Dissociation: The Pathognomonic Hallmark

Atrioventricular dissociation occurs when the atria and ventricles are depolarized by two completely independent pacemakers, with the ventricular rate exceeding the atrial rate ($V > A$). AV dissociation is present in approximately $50\%\text{ to }60\%$ of VT cases, and when identified, is $100\%$ specific for Ventricular Tachycardia.

• Independent P Waves: P waves march through the rhythm strip at their own intrinsic rate, completely dissociated from the wider, faster QRS complexes. They can be seen deforming ST segments, T waves, and QRS complexes.
• Capture Beats: A normal sinus impulse conducts through the AV node and normal His-Purkinje system during a momentary window when the specialized conduction system and myocardium have recovered excitability. The result is an isolated, completely normal, narrow QRS complex in the midst of the wide-complex tachycardia.
• Fusion Beats (Dressler Beats): Occur when a descending supraventricular wavefront (via the normal His-Purkinje system) and an ascending ventricular ectopic wavefront simultaneously depolarize the ventricles. The resulting QRS complex is a morphological hybrid: narrower and distinctly intermediate in morphology between the baseline normal sinus complex and the wide VT complex.

AV Dissociation in VT:

P Wave:     P             P             P             P             P
            |             |             |             |             |
QRS:      +----+        +----+        +----+        +----+        +----+        +--+
          | VT |        | VT |        |FUS |        | VT |        |CAP |        |VT|
          +----+        +----+        +----+        +----+        +--+          +--+
                                         ^                           ^
                                    Fusion Beat                 Capture Beat

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2. The Brugada 4-Step Precordial Algorithm

The Brugada algorithm evaluates leads $V_1$ through $V_6$ in a stepwise sequence. If any single step is positive, the diagnosis of Ventricular Tachycardia is established. Only if all four steps are negative is SVT with aberrancy diagnosed.

Step 1: Absence of an RS complex in ALL precordial leads (V1-V6)?
        YES ===> VT (100% Specific - Positive or Negative Concordance)
        NO  ===> Proceed to Step 2

Step 2: RS interval > 100 ms in ANY precordial lead?
        YES ===> VT
        NO  ===> Proceed to Step 3

Step 3: Presence of AV Dissociation (Independent P waves, Capture/Fusion)?
        YES ===> VT
        NO  ===> Proceed to Step 4

Step 4: Morphological criteria for VT present in BOTH V1/V2 and V6?
        YES ===> VT
        NO  ===> SVT with Aberrancy

Detailed Breakdown of Brugada Steps

• Step 1: Precordial Concordance: Absence of an $RS$ complex in all leads $V_1\text{-}V_6$. If all precordial leads show entirely positive monophasic $R$ waves (positive concordance, typical of posterior or basal LV origin) or entirely negative monophasic $QS$ complexes (negative concordance, typical of apical or anterior RV origin), VT is diagnosed.
• Step 2: RS Duration $> 100\text{ ms}$: Measured from the initial onset of the $R$ wave to the deepest point (nadir) of the $S$ wave in any precordial lead showing an $RS$ complex. In aberrant SVT, initial activation travels rapidly through specialized conduction tissue ($RS \le 100\text{ ms}$). In VT, initial myocardial activation travels slowly through unspecialized myocardium, widening the $RS$ interval beyond $100\text{ ms}$.
• Step 3: AV Dissociation: Identification of independent P waves, capture beats, or Dressler fusion beats.
• Step 4: Morphologic Criteria:
  • Right Bundle Branch Block (RBBB) Morphology in $V_1$ (predominantly positive in $V_1$):
    • In $V_1$: Monophasic $R$, $qR$, or an $rSR'$ complex with a taller left rabbit ear ($R > r'$). A taller right ear ($r' > R$) suggests typical RBBB aberrancy.
    • In $V_6$: $R/S$ ratio $< 1.0$ (predominantly deep $S$ wave) or a monophasic $QS$ wave.
  • Left Bundle Branch Block (LBBB) Morphology in $V_1$ (predominantly negative in $V_1$):
    • In $V_1$: Broad initial $R$ wave $\ge 30\text{ ms}$, slurred or notched downstroke to the nadir of the $S$ wave $\ge 60\text{ ms}$, or an initial $Q$ wave.
    • In $V_6$: Any $QR$ or $QS$ pattern (in normal LBBB aberrancy, $V_6$ never shows a $Q$ wave; it must be a broad, clean monophasic $R$ wave).

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3. The Vereckei Lead aVR Algorithm

The Vereckei algorithm simplifies diagnosis by focusing exclusively on lead aVR. Because the normal specialized conduction system directs depolarization downward and to the left (away from aVR, which points toward the right shoulder), supraventricular conduction almost always produces a predominantly negative deflection with a sharp, fast initial downstroke in aVR. Ventricular origins, however, produce abnormal, delayed superior activation.

Vereckei aVR Steps (Evaluate sequentially in Lead aVR):

1. Initial R wave present in aVR?                        ===> YES: VT
2. Width of initial r or q wave > 40 ms?                ===> YES: VT
3. Notch on the descending limb of negative QRS?        ===> YES: VT
4. Ventricular Activation-Velocity Ratio (Vi/Vt) <= 1.0? ===> YES: VT
                                                        ===> NO:  SVT with Aberrancy

• Step 1: Initial R Wave: A dominant, unnotched positive $R$ wave in aVR proves that initial ventricular activation is traveling directly toward the right shoulder, diagnostic of VT.
• Step 2: Initial $r$ or $q$ Width $> 40\text{ ms}$: A broad initial deflection indicates slow, non-specialized myocardial conduction.
• Step 3: Notch on Descending Limb: Represents myocardial activation wavebreak around anatomical barriers or myocardial scar.
• Step 4: The $V_i/V_t$ Ratio: Calculated by measuring the vertical voltage excursion (in millivolts) during the initial $40\text{ ms}$ ($V_i$) versus the terminal $40\text{ ms}$ ($V_t$) of the QRS complex in aVR:
  • In SVT with aberrancy, initial activation utilizes the intact His-Purkinje branches, resulting in rapid initial voltage change ($V_i$ is large), whereas terminal conduction through the blocked bundle is slow ($V_t$ is small). Thus, $V_i/V_t > 1.0$.
  • In Ventricular Tachycardia, initial activation travels slowly cell-to-cell through working myocardium ($V_i$ is small), while terminal activation may exit into the Purkinje system ($V_t$ is larger). Thus, $V_i/V_t \le 1.0$, diagnostic of VT.

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Intracardiac Electrogram Criteria for Ventricular Tachycardia

When a patient with wide-complex tachycardia is evaluated in the cardiac electrophysiology laboratory, multipolar diagnostic catheters (HRA, His, CS, RV) provide gold-standard diagnostic validation.

SVT with Aberrancy:                       Ventricular Tachycardia (VT):

Atrium (HRA)  --- A --- A --- A ---       Atrium (HRA)  --- A ------- A ------- A --- (Dissociated)
                  |     |     |                         
His Bundle    --- H --- H --- H ---       His Bundle    --- [No H or Retrograde H] ---
                  |     |     |                         
Ventricle     --- V --- V --- V ---       Ventricle     --- V --- V --- V --- V --- V (V > A)
Interval: Normal or Long HV (>= 35 ms)    Interval: Short or Negative HV (HV <= 0 ms)

1. Ventricular Rate Greater than Atrial Rate ($V > A$):
   • Recorded directly on intracardiac channels. If ventricular electrograms occur at a cycle length of $320\text{ ms}$ while atrial electrograms occur independently at $700\text{ ms}$, VT with AV dissociation is definitively established.
2. The His-Ventricular (HV) Interval:
   • SVT with Aberrancy: The impulse must travel antegradely through the His bundle to reach the ventricles. Therefore, each ventricular electrogram ($V$) is preceded by a distinct His deflection ($H$), with an $HV$ interval that is normal ($35\text{ to }55\text{ ms}$) or prolonged ($> 55\text{ ms}$ due to underlying bundle branch disease), but never shortened or negative.
   • Ventricular Tachycardia: The tachycardia originates in ventricular myocardium. Consequently:
     • The His bundle electrogram is frequently absent (buried within the local ventricular electrogram).
     • The His bundle electrogram follows the onset of ventricular activation ($H$ after $V$), representing retrograde His depolarization.
     • If a His deflection precedes the QRS, the $HV$ interval is abnormally short ($< 35\text{ ms}$) or negative ($HV \le 0\text{ ms}$), proving that ventricular myocardial activation began prior to His bundle activation!

Idiopathic Ventricular Tachycardia Phenotypes

Approximately $10\%\text{ to }15\%$ of ventricular tachycardias occur in patients with structurally normal hearts (absence of ischemic scar, normal left ventricular ejection fraction, and absent delayed enhancement on cardiac MRI). These idiopathic VTs exhibit distinct anatomical origins, cellular mechanisms, and pharmacological responses.

1. Right Ventricular Outflow Tract (RVOT) VT

• Mechanism: Triggered activity mediated by delayed afterdepolarizations (DADs) secondary to intracellular calcium overload and elevated cyclic AMP ($cAMP$). Highly sensitive to catecholamines, exercise, and stress. Terminated by adenosine, beta-blockers, and vagal maneuvers.
• Surface ECG Characteristics:
  • Left Bundle Branch Block (LBBB) Morphology in $V_1$: Dominant negative $QS$ or $rS$ complex in $V_1$.
  • Strongly Positive Inferior Axis: Tall, monophasic $R$ waves in leads II, III, and aVF, reflecting superior-to-inferior wavefront propagation away from the pulmonary valve.
  • Precordial Transition: The precordial transition from negative to positive QRS occurs at or after lead $V_3$ or $V_4$ (transition $> V_3$).

2. Left Ventricular Outflow Tract (LVOT) & Aortic Cusp VT

• Anatomical Origins: Arises from the aortic sinuses of Valsalva (left coronary cusp [LCC], right coronary cusp [RCC], or LCC-RCC junction), aortomitral continuity (AMC), or the superior basal LV epicardium.
• Surface ECG Characteristics:
  • Similar inferior axis (tall R waves in II, III, aVF) to RVOT VT, but exhibits an earlier precordial transition: transition occurs in lead $V_1$ or $V_2$ (early transition $\le V_2$), with prominent, broad $R$ waves in $V_1\text{-}V_2$.
  • LCC Origin: Deep $S$ wave in lead I, multiphasic $R$ in inferior leads.
  • RCC Origin: Positive $R$ wave in lead I, smaller inferior voltages.

3. Idiopathic Left Ventricular Fascicular VT (Belhassen VT)

• Mechanism: Reentry utilizing the specialized Purkinje network and abnormal, slow-conducting subendocardial myocardium within the left ventricle. Crucially, Belhassen VT is verapamil-sensitive (terminated by intravenous calcium channel blockers) and insensitive to adenosine.
• Surface ECG Characteristics:
  • Left Posterior Fascicular VT (~90% of cases): Originates in the arborization of the left posterior fascicle. Exhibits a Right Bundle Branch Block (RBBB) morphology in $V_1$ combined with a superior axis / marked left axis deviation (deep $S$ waves in leads II, III, and aVF; tall $R$ waves in I and aVL).
  • QRS Duration: Characteristically relatively narrow for VT ($110\text{ to }140\text{ ms}$) because the reentrant circuit directly engages the fast Purkinje conduction network.
• Intracardiac Electrogram Signature: During mapping along the mid-to-apical left ventricular septum, a sharp, high-frequency Purkinje potential (P potential) is recorded preceding the local ventricular electrogram ($P - V$ interval of $15\text{ to }40\text{ ms}$). Ablation targeting this presystolic Purkinje potential successfully terminates the arrhythmia.

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Monomorphic VT vs. Polymorphic VT and Ventricular Fibrillation

• Torsades de Pointes: A distinct form of polymorphic VT occurring in the setting of acquired or congenital Long QT Syndrome ($QT_c > 480\text{-}500\text{ ms}$). Characterized by QRS complexes that twist their polarity around the isoelectric line. Typically initiated by a short-long-short cycle length sequence, where a post-extrasystolic pause dramatically lengthens the subsequent QT interval, precipitating early afterdepolarizations (EADs).
• Catecholaminergic Polymorphic Ventricular Tachycardia (CPVT): Genetic channelopathy involving mutations in the cardiac ryanodine receptor ($RyR_2$) or calsequestrin ($CASQ_2$), producing exercise- or stress-induced bidirectional and polymorphic VT due to spontaneous sarcoplasmic reticulum calcium leaks.