What are the diagnostic criteria for Supraventricular Tachycardia (SVT) in a patient with a history of seizures?

Diagnostic Criteria for Supraventricular Tachycardia (SVT)

SVT is diagnosed on ECG by a narrow QRS complex (<120 ms) with heart rate typically >100 bpm (often 150-250 bpm), regular rhythm, and specific P wave patterns that distinguish the mechanism—most commonly AVNRT where P waves are hidden in the QRS complex or AVRT where P waves follow the QRS with RP interval >70 ms. 1, 2

Core ECG Diagnostic Features

The diagnosis requires obtaining a 12-lead ECG during tachycardia, as automatic ECG interpretations are unreliable and commonly suggest incorrect diagnoses. 3, 4

Essential ECG Parameters:

• QRS duration <120 ms confirms narrow-complex tachycardia, which is almost always SVT 1, 4
• Heart rate 150-250 bpm with regular RR intervals 2, 5
• P wave analysis is critical for mechanism identification 1

Mechanism-Specific ECG Patterns:

AVNRT (most common form):

• No visible P waves with regular RR interval—atria and ventricles depolarize simultaneously 1, 2
• Pseudo r' wave in lead V1 (positive deflection at end of QRS): 90% specificity, 97% positive predictive value 1
• Pseudo S wave in inferior leads (II, III, aVF): 100% specificity and 100% positive predictive value 1
• RP interval typically <90 ms (short RP pattern) 1

AVRT (accessory pathway):

• P wave visible in ST segment, separated from QRS by >70 ms 1, 2
• P wave always follows QRS complex 2

Clinical Diagnostic Features

History Pattern:

• Abrupt onset and termination of regular palpitations strongly suggests AVNRT or AVRT (paroxysmal SVT) 3, 4
• Termination by vagal maneuvers (Valsalva, carotid massage) further confirms re-entrant tachycardia involving AV nodal tissue 3, 4
• Polyuria after episodes supports sustained SVT due to atrial natriuretic peptide release from atrial contraction against closed AV valve 3, 1
• Syncope occurs in approximately 15% of patients, usually just after initiation or with prolonged pause after termination 3, 1

Physical Examination During Tachycardia:

While physical examination usually does not lead to definitive diagnosis, irregular cannon A waves and/or irregular variation in S1 intensity strongly suggests ventricular origin rather than SVT. 3

Diagnostic Maneuvers

Vagal maneuvers during continuous ECG recording:

• Termination of tachycardia suggests AVNRT or AVRT 1, 4
• Transient AV block revealing underlying atrial activity helps identify atrial flutter or atrial tachycardia 4
• Valsalva maneuver is safer and more efficacious than carotid massage, especially in elderly patients 2

Adenosine administration:

• Terminates AVNRT or AVRT by blocking AV nodal conduction 1, 4
• Helps unmask underlying atrial activity if tachycardia persists 6

Required Baseline Investigations

12-lead ECG during sinus rhythm:

• Pre-excitation (delta waves) in a patient with paroxysmal regular palpitations is sufficient for presumptive diagnosis of AVRT—no further documentation of spontaneous episodes required before specialist referral 3, 1
• Baseline pre-excitation with history of irregular palpitations strongly suggests atrial fibrillation episodes, requiring immediate electrophysiological evaluation due to sudden death risk 3, 4

Echocardiography:

• Should be performed in patients with documented sustained SVT to exclude structural heart disease (valvular aortic stenosis, hypertrophic cardiomyopathy, coronary anomalies) that cannot be detected by physical examination 3, 7

Critical Diagnostic Pitfalls

Never assume hemodynamic stability means SVT—ventricular tachycardia can be well-tolerated, especially in younger patients. 1, 4

Wide-complex tachycardia (QRS >120 ms) may represent SVT with aberrancy (rate-related bundle branch block, pre-existing conduction abnormality, or accessory pathway pre-excitation), but when in doubt, always treat as ventricular tachycardia. 1, 4

Beware of atrial fibrillation with rapid rate appearing regular—irregularity is less easily detected at very fast rates. 4

Pre-excitation (Wolff-Parkinson-White) on baseline ECG mandates immediate electrophysiology referral even without documented tachycardia due to risk of sudden death from rapid conduction during atrial fibrillation. 3, 1, 4

Persistent tachycardia for weeks to months with fast ventricular response can cause tachycardia-mediated cardiomyopathy, which is reversible but requires long-term follow-up. 3, 7

Indications for Immediate Specialist Referral

• Presence of pre-excitation (WPW syndrome) on baseline ECG 3, 4, 7
• Wide complex tachycardia of unknown origin 3, 7
• Severe symptoms (syncope, dyspnea) during palpitations 3, 7
• Drug resistance or intolerance in narrow complex tachycardias 3
• Patients desiring to be free of drug therapy 3

Context for Seizure History

In patients with history of seizures, syncope during SVT episodes (occurring in ~15% of cases) may be misattributed to seizure activity. 3, 1 The key distinguishing features are the abrupt onset of regular palpitations preceding loss of consciousness and polyuria following the episode, which are characteristic of SVT rather than primary seizure disorder. 3, 4