What are the diagnostic criteria for supraventricular tachycardia (SVT)?

Diagnostic Criteria for Supraventricular Tachycardia (SVT)

SVT is diagnosed on ECG by a narrow QRS complex (<120 ms) with a heart rate typically >100 bpm, and the specific mechanism is determined by analyzing the relationship between P waves and QRS complexes, with key features including the RP interval, P wave morphology, and response to vagal maneuvers. 1, 2

Initial ECG Assessment

QRS Width Classification

• QRS duration <120 ms confirms SVT, distinguishing it from ventricular tachycardia or SVT with aberrant conduction 2, 3
• A 12-lead ECG during tachycardia and during sinus rhythm should be obtained to identify the etiology 1, 2
• Wide-complex tachycardia (QRS >120 ms) may represent SVT with abnormal conduction due to rate-related aberrancy, pre-existing bundle-branch block, or accessory pathway pre-excitation 1

Regularity Assessment

• Regular ventricular rhythm suggests AVNRT, AVRT, or atrial tachycardia with 1:1 conduction 1
• Irregular ventricular rate indicates atrial fibrillation, multifocal atrial tachycardia, or atrial flutter with variable AV conduction 1
• Atrial fibrillation with rapid ventricular response can appear regular at very fast rates and be misdiagnosed as regular SVT 1, 3

Mechanism-Specific Diagnostic Features

Atrioventricular Nodal Reentrant Tachycardia (AVNRT)

• No visible P waves with regular RR interval is most characteristic of AVNRT 3
• Pseudo r' wave in lead V1 (positive deflection at end of QRS) is pathognomonic for AVNRT with 90% specificity and 97% positive predictive value 3, 4
• Pseudo S wave in inferior leads (II, III, aVF) is diagnostic with 100% specificity and 100% positive predictive value 1, 3, 4
• Atrial activation occurs nearly simultaneously with QRS, producing a "short RP" pattern with RP interval typically <90 ms 1, 4
• The P wave is located closer to the prior QRS than the subsequent QRS 1

Atrioventricular Reentrant Tachycardia (AVRT)

• P wave visible in the ST segment, separated from QRS by >70 ms, indicates AVRT 3
• Retrograde P waves are present in 63% of cases, significantly more than in AVNRT 5, 4
• The RP interval is longer than in AVNRT but still represents a "short RP" tachycardia, with P waves in early ST-T segment 1
• P waves are more discernible in AVRT (69%) compared to AVNRT (44%) 4
• Resting ECG showing pre-excitation (delta wave) mandates immediate electrophysiology referral due to sudden death risk 2, 3

Atrial Tachycardia (AT)

• P wave morphology differs from sinus rhythm and is typically seen near the end of or shortly after the T wave 1
• "Long RP" pattern with RP interval typically >187 ms, the longest among SVT types 1, 4
• Shortest P'R interval (mean 125 ms) compared to other SVT mechanisms 4
• If atrial rate exceeds ventricular rate, atrial flutter or atrial tachycardia is present 1
• P waves are discernible in 67% of cases 4

Diagnostic Maneuvers

Vagal Maneuvers

• Perform vagal maneuvers during continuous ECG recording to observe response 3
• Termination of tachycardia suggests AVNRT or AVRT involving AV nodal tissue 3, 6
• Transient AV block revealing underlying atrial activity helps identify atrial flutter or atrial tachycardia 3
• Sudden-onset and abrupt termination strongly suggests AVNRT or AVRT (paroxysmal SVT) 3

Adenosine Administration

• Adenosine can help diagnose AVNRT or AVRT by terminating the tachycardia 3
• May reveal underlying atrial activity in atrial flutter or atrial tachycardia by causing transient AV block 3

Critical Diagnostic Pitfalls

Common Misdiagnoses

• Never assume hemodynamic stability means SVT—ventricular tachycardia can be well-tolerated, especially in younger patients 3
• Automatic ECG interpretations are unreliable and should never be trusted without manual verification 3
• Episodes may be erroneously attributed to anxiety when tachycardia terminates before presentation 7
• Antiarrhythmic drugs and hyperkalemia reduce specificity of QRS width criteria 3

Pre-excitation Warning

• Pre-excitation (Wolff-Parkinson-White) on baseline ECG requires immediate electrophysiology referral even without documented tachycardia 2, 3
• Resting ECG can identify pre-excitation in patients describing prior palpitations 1, 2

Quantitative Diagnostic Parameters

RP' and P'R Intervals

• RP':P'R ratio <0.6 suggests AVNRT, while ratio >2.2 suggests atrial tachycardia 4
• AVNRT has shortest mean RP' interval (86 ms) and longest mean P'R interval (263 ms) 4
• Atrial tachycardia has longest mean RP' interval (187 ms) and shortest mean P'R interval (125 ms) 4

Clinical Context

• Polyuria after episodes supports sustained SVT due to atrial natriuretic peptide release 3
• Syncope occurs in approximately 15% of SVT cases 3
• Sudden-onset, rapid, regular palpitations characterize SVT and allow diagnosis from history alone in most patients 7