How to diagnose supraventricular tachycardia (SVT) and ventricular tachycardia (VT) clinically?

Clinical Diagnosis of SVT and VT

When faced with a tachycardia, immediately assess QRS width on ECG: narrow QRS (<120 ms) is almost always SVT, while wide QRS (>120 ms) requires systematic evaluation to distinguish VT from SVT with aberrancy—and when in doubt, always treat as VT. 1

Initial Clinical Assessment

History Taking

• Onset pattern: Abrupt onset and termination strongly suggests AVNRT or AVRT (paroxysmal SVT), while gradual acceleration indicates sinus tachycardia 1
• Regularity: Ask if palpitations feel regular or irregular—irregular suggests atrial fibrillation, multifocal atrial tachycardia, or premature beats 1
• Response to vagal maneuvers: Termination with Valsalva or carotid massage suggests re-entrant tachycardia involving AV nodal tissue (AVNRT or AVRT) 1
• Associated symptoms: Polyuria after episodes supports sustained SVT due to atrial natriuretic peptide release; syncope occurs in ~15% of SVT cases 1
• Prior myocardial infarction: In adults with previous MI, wide-complex tachycardia is VT until proven otherwise 1, 2

Physical Examination During Tachycardia

• Irregular cannon A waves in jugular venous pulse indicate AV dissociation, strongly suggesting VT 1
• Variable intensity of S1 and fluctuating systolic blood pressure also indicate AV dissociation and VT 1
• Hemodynamic stability: Stable vital signs do NOT distinguish SVT from VT—VT can be well-tolerated 1

ECG-Based Diagnosis

Narrow QRS Complex Tachycardia (<120 ms)

A narrow QRS almost always confirms SVT 1

P Wave Analysis

• No visible P waves with regular RR: Most likely AVNRT 1
• Pseudo-R wave in V1 or pseudo-S wave in inferior leads: Pathognomonic for AVNRT (P waves hidden in QRS) 1
• P wave in ST segment, separated from QRS by >70 ms: AVRT is most likely 1
• RP interval longer than PR interval: Consider atypical AVNRT, permanent junctional reciprocating tachycardia, or atrial tachycardia 1
• Atrial rate exceeds ventricular rate: Atrial flutter or atrial tachycardia 1

Wide QRS Complex Tachycardia (>120 ms)

Critical principle: If you cannot definitively prove SVT, treat as VT—giving verapamil or diltiazem for VT can cause hemodynamic collapse 1, 3

Definitive VT Criteria (Pathognomonic)

• AV dissociation with ventricular rate faster than atrial rate proves VT (visible in only 30% of cases) 1
• Fusion complexes (merger of conducted supraventricular and ventricular beats) are pathognomonic for VT 1
• Capture beats demonstrate VT 1

Highly Suggestive VT Criteria

• QRS width >140 ms with RBBB pattern or >160 ms with LBBB pattern favors VT 1
• RS interval >100 ms in any precordial lead is highly suggestive of VT 1
• Negative concordance (all QS complexes in precordial leads) is diagnostic for VT 1
• QR complexes indicate myocardial scar and VT (present in ~40% of post-MI VT) 1

Advanced ECG Algorithms

• Brugada criteria: Examines QRS morphology in precordial leads 1, 3
• Vereckei algorithm: Based on QRS complex examination in lead aVR 1, 3
• These algorithms can achieve >90% accuracy in distinguishing VT from SVT 4, 5

Diagnostic Maneuvers

Vagal Maneuvers

• Perform during continuous ECG recording to observe response 3
• Termination of tachycardia: Suggests AVNRT or AVRT 1
• Transient AV block revealing underlying atrial activity: Helps identify atrial flutter or atrial tachycardia 1
• No effect: Suggests VT or atrial tachycardia 1

Adenosine Administration

• Give during 12-lead ECG recording to capture diagnostic information 1, 3
• Termination: Indicates AVNRT or AVRT 1
• Transient AV block without termination: Reveals underlying atrial activity in flutter or atrial tachycardia 1

Critical Pitfalls to Avoid

• Never assume hemodynamic stability means SVT—VT can be well-tolerated, especially in younger patients 1
• Beware of atrial fibrillation with rapid rate appearing regular—irregularity is less easily detected at very fast rates 1, 3
• Pre-excitation (WPW) on baseline ECG mandates immediate electrophysiology referral due to sudden death risk 1, 3
• Automatic ECG interpretations are unreliable—always interpret manually 1
• Antiarrhythmic drugs and hyperkalemia reduce specificity of QRS width criteria 1

When to Refer

Immediate electrophysiology referral indicated for: 1, 3

• Pre-excitation on resting ECG (even without documented tachycardia)
• Wide-complex tachycardia of unknown origin
• Syncope or severe dyspnea during palpitations
• Any suspicion of Wolff-Parkinson-White syndrome