How can ventricular tachycardia (VT) be distinguished from supraventricular tachycardia (SVT)?

Distinguishing Ventricular Tachycardia from Supraventricular Tachycardia

When faced with a wide-complex tachycardia (QRS ≥120 ms), treat it as ventricular tachycardia (VT) unless you can definitively prove it is supraventricular tachycardia (SVT), because misdiagnosing VT as SVT and giving AV nodal blockers like verapamil or diltiazem can cause hemodynamic collapse and death. 1, 2

Initial Assessment: QRS Width

• Narrow QRS (<120 ms): This is SVT until proven otherwise 1, 3
• Wide QRS (≥120 ms): This is VT until proven otherwise—approximately 80% of wide-complex tachycardias in hospitalized patients are VT 1, 2

Pathognomonic Features for VT (If Present, Diagnosis is Certain)

AV Dissociation

• Independent P waves marching through at a slower rate than the QRS complexes prove VT 1, 4, 2
• Look for this on physical exam: irregular cannon A waves in the jugular venous pulse, variable intensity of S1, and beat-to-beat blood pressure variation 1
• AV dissociation is visible in only 30% of VT cases on surface ECG, but when present it is diagnostic 1

Fusion and Capture Beats

• Fusion beats (hybrid QRS morphology from simultaneous supraventricular and ventricular activation) are pathognomonic for VT 1, 4, 2
• Capture beats (occasional narrow QRS complexes during wide-complex tachycardia) prove VT 4

High-Specificity ECG Criteria for VT

Precordial Lead Analysis

• Absence of RS complex in all precordial leads (V1-V6)—meaning all leads show either pure R waves or pure QS/QRS patterns without an R-to-S transition—strongly indicates VT 1, 2
• RS interval >100 ms (measured from R wave onset to S wave nadir) in any precordial lead suggests VT 1, 4
• Concordance: All precordial leads uniformly positive or uniformly negative is diagnostic for VT (negative concordance is 100% specific) 1, 4

Lead aVR Criteria

• Initial R wave in aVR or initial R or Q wave >40 ms in aVR suggests VT 4

QRS Width Thresholds

• QRS >140 ms with RBBB morphology or >160 ms with LBBB morphology favors VT 1
• However, these criteria fail in patients taking class Ic or Ia antiarrhythmics, those with hyperkalemia, or those with pre-existing bundle branch block 1

Morphology-Specific Criteria

• QR complexes (indicating myocardial scar) are present in ~40% of post-MI VT cases 1, 4
• R-wave peak time ≥50 ms in lead II suggests VT 4

Clinical Context That Favors VT

• Prior myocardial infarction with new wide-complex tachycardia strongly indicates VT 1, 4
• Structural heart disease or reduced ejection fraction makes VT far more likely 2
• Age >35 years increases VT probability in wide-complex tachycardia 5

Features Suggesting SVT with Aberrancy

For Narrow-Complex Tachycardia (SVT)

• Pseudo r' in V1 (small positive deflection at end of QRS) is 90% specific for AVNRT 1, 3
• Pseudo S waves in inferior leads (II, III, aVF) are 100% specific for AVNRT 1, 3
• RP interval <90 ms (P wave buried in or immediately after QRS) indicates typical AVNRT or AVRT 1
• RP interval >PR suggests atypical AVNRT, atrial tachycardia, or PJRT 1

For Wide-Complex Tachycardia

• Identical QRS morphology during tachycardia compared to baseline sinus rhythm with known bundle branch block suggests SVT with aberrancy 1, 2
• Termination with adenosine or vagal maneuvers supports SVT (but adenosine should NOT be used if the diagnosis is uncertain or if the rhythm is irregular/polymorphic) 1, 3

Diagnostic Maneuvers

Adenosine Administration

• For regular, monomorphic wide-complex tachycardia in stable patients where the diagnosis is uncertain, adenosine 6-12 mg IV rapid push can be both diagnostic and therapeutic 1
• Termination suggests AVNRT or AVRT 1, 3
• Transient AV block revealing underlying atrial activity (flutter waves, fibrillation) confirms SVT 1
• NEVER give adenosine for irregular or polymorphic wide-complex tachycardia 1

Vagal Maneuvers

• Perform Valsalva, carotid massage, or facial immersion during continuous 12-lead ECG recording 1
• Termination indicates AVNRT or AVRT 1, 3
• Transient slowing that reveals atrial activity confirms SVT 1

Critical Pitfalls to Avoid

• Hemodynamic stability does NOT distinguish VT from SVT—VT can be well-tolerated, especially in younger patients 1, 3
• Never rely on QRS width alone in patients taking antiarrhythmics (class Ia, Ic), those with hyperkalemia, or those with severe heart failure 1
• MCL1 cannot substitute for V1—it shows different morphology in 40% of VT cases and is diagnostically inferior 5
• Tachycardias >190 bpm often lack clear diagnostic features on surface ECG 5
• Pre-excitation (WPW) on baseline ECG requires immediate electrophysiology referral even without documented tachycardia 3

Algorithmic Approach

1. Obtain 12-lead ECG during tachycardia and compare to baseline sinus rhythm ECG 1, 3, 2

2. Assess hemodynamic stability—if unstable, perform immediate synchronized cardioversion regardless of rhythm 1, 2

3. Measure QRS duration:

   • <120 ms → SVT
   • ≥120 ms → Proceed to step 4

4. Search for pathognomonic VT features:

   • AV dissociation? → VT
   • Fusion/capture beats? → VT
   • If present, treat as VT

5. Apply high-specificity criteria:

   • Absence of RS in all precordial leads? → VT
   • RS interval >100 ms? → VT
   • Concordance? → VT
   • QRS >140 ms (RBBB) or >160 ms (LBBB)? → Likely VT

6. Compare to baseline ECG:

   • Identical QRS morphology to known BBB? → Consider SVT with aberrancy
   • Different morphology + prior MI? → VT

7. If diagnosis remains uncertain: Treat as VT with amiodarone 150 mg IV over 10 minutes or proceed to synchronized cardioversion 1, 2

8. Consider adenosine only if: stable patient, regular rhythm, monomorphic QRS, and no contraindications 1