What is the role of an Electrocardiogram (ECG) in diagnosing Paroxysmal Supraventricular Tachycardia (PSVT)?

ECG Role in PSVT Diagnosis

The ECG is the definitive diagnostic tool for PSVT—diagnosis cannot be confirmed without electrocardiographic documentation of the arrhythmia, making it essential to obtain a 12-lead ECG during symptoms. 1

Why ECG Documentation is Mandatory

The diagnosis of SVT is most often made in the emergency department, but it is common to elicit symptoms suggestive of SVT before initial electrocardiographic documentation. 1 While clinical history can strongly suggest PSVT, the American College of Cardiology emphasizes that electrocardiographic confirmation is required for definitive diagnosis. 1

• Patients with sustained arrhythmia should always be encouraged to have at least one 12-lead ECG taken during the arrhythmia, as this is the only way to definitively establish the diagnosis and differentiate between PSVT subtypes. 1
• Clinical history alone—even with classic features like abrupt onset/termination and response to vagal maneuvers—is insufficient for definitive diagnosis without ECG documentation. 1, 2

Critical ECG Features During Tachycardia

Narrow QRS Complex Tachycardia (QRS <120 ms)

• A narrow QRS complex almost always confirms SVT, with heart rates typically 150-250 beats per minute and regular rhythm. 2, 3
• The P wave is usually hidden within the QRS complex in most PSVT cases, particularly in AVNRT where atria and ventricles depolarize simultaneously. 3

Distinguishing AVNRT from AVRT on ECG

AVNRT (most common form):

• No visible P waves with regular RR interval suggests AVNRT. 2
• Pseudo-R′ wave in lead V1 or pseudo-S wave in inferior leads is pathognomonic for AVNRT, representing P waves hidden at the end of the QRS complex. 1, 2
• The reentrant circuit involves anterograde conduction over a slow AV node pathway followed by retrograde conduction in a fast AV node pathway. 1

AVRT (accessory pathway-mediated):

• P wave visible in the ST segment, separated from QRS by >70 ms, indicates AVRT. 2, 3
• If the reentry circuit includes an accessory pathway, the P wave always follows the QRS, and the R-P interval typically exceeds 70 msec. 3

Baseline ECG: Identifying Pre-Excitation

The presence of pre-excitation on a resting ECG in any patient with a history of paroxysmal regular palpitations is sufficient for the presumptive diagnosis of AVRT and mandates immediate referral to a cardiac electrophysiologist. 1, 4

Three Characteristic Features of Pre-Excitation:

• Short PR interval (reflecting faster conduction through the accessory pathway than the AV node). 4
• Delta wave (slurred, slow upstroke at the beginning of the QRS complex). 4
• Widened QRS complex (from abnormal ventricular activation pattern). 4

All patients with Wolff-Parkinson-White syndrome (pre-excitation combined with arrhythmias) are at potential risk for life-threatening arrhythmias and require immediate electrophysiological evaluation. 1, 4, 2

Critical Pitfalls in ECG Interpretation

• Automatic ECG interpretation systems are unreliable and commonly suggest incorrect diagnoses—always interpret manually. 1, 4, 2
• A 12-lead ECG should be obtained immediately to confirm narrow complex tachycardia, as ventricular tachycardia may masquerade as PSVT if only a single lead is examined. 3
• When QRS is wide (>120 ms), systematic evaluation is required to distinguish VT from SVT with aberrancy—and when in doubt, always treat as VT. 1, 2
• Never administer verapamil or diltiazem for wide-complex tachycardia of unknown origin, as this may lead to hemodynamic compromise or ventricular fibrillation if the rhythm is VT or pre-excited AF. 1

Diagnostic Maneuvers During ECG Recording

Perform vagal maneuvers during continuous ECG recording to observe response:

• Termination of tachycardia suggests AVNRT or AVRT (reentrant tachycardias involving AV nodal tissue). 1, 2
• Transient AV block revealing underlying atrial activity helps identify atrial flutter or atrial tachycardia. 2
• Vagal maneuvers should be performed with the patient in the supine position and will not be effective if the rhythm does not involve the AV node as a requisite component. 1

Ambulatory ECG Monitoring When Symptoms Are Infrequent

• 24-hour Holter recording is useful in patients with frequent (several episodes per week) but transient tachycardias. 1
• Event or wearable loop recorders are more useful than 24-hour recordings in patients with less frequent arrhythmias. 1
• Implantable loop recorders may be helpful in selected cases with rare symptoms (fewer than two episodes per month). 1