Supraventricular Tachycardia (SVT)

Definition & Scope

SVT is a broad term for any tachyarrhythmia originating above the bundle of His. In common clinical usage, "SVT" or "PSVT" refers specifically to tachycardias that are dependent on the AV node for initiation and maintenance - primarily AVNRT, AVRT, and junctional tachycardia. Technically the umbrella also includes atrial fibrillation, atrial flutter, sinus tachycardia, and atrial tachycardia, but those are usually discussed separately.

Classification of Paroxysmal SVT

Type	Mechanism	Key Feature
AVNRT	Reentry within AV node / perinodal atrium	Most common (~60% of cases); retrograde P near or within QRS
AVRT (Orthodromic)	Reentry via accessory pathway (antegrade AV node, retrograde AP)	Narrow QRS; common in WPW
AVRT (Antidromic)	Reentry via AP antegrade, AV node retrograde	Wide QRS; uncommon
Atrial Tachycardia (AT)	Automaticity or microreentry in atrium	P wave before QRS, independent of AV node
Atrial Flutter (AFL)	Macroreentry around tricuspid valve	"Sawtooth" P waves; atrial rate 250-350 bpm; often 2:1 block -> 150 bpm
MAT	Multiple ectopic foci	≥3 distinct P-wave morphologies; seen in COPD, CHF
Junctional (JET)	Automaticity in AV node	Rare in adults; AV dissociation

Washington Manual, p. 234

Epidemiology

Prevalence: ~2.25/1000; incidence 35/100,000 person-years
SVT most commonly presents between ages 12-30 in structurally normal hearts
Women are twice as likely to develop SVT as men
AVRT tends to present earlier (first 2 decades); AVNRT and AT present later in life
Washington Manual, p. 235

AVNRT (Most Common Type)

Mechanism: Dual AV nodal pathways - a slow pathway (right inferior extension along tricuspid annulus) and a fast pathway. The reentry wavefront travels up the slow pathway and exits via the fast pathway. Atrial and ventricular activation occur nearly simultaneously.

ECG findings:

Narrow QRS tachycardia, rate 150-250 bpm
P wave hidden within or just at the end of the QRS
Pseudo-r' in lead V1 and pseudo-S waves in leads II, III, aVF (retrograde P)
No clear RP interval

Symptoms: Palpitations, neck pounding (cannon A waves from atrial contraction against closed tricuspid valve), dizziness. Post-tachycardia diuresis from natriuretic peptide release.

Harrison's 22E, p. 1983

ECG Approach to Wide vs. Narrow Complex

Regular narrow-complex tachycardia -> think AVNRT, AVRT, or AFL (2:1)
Irregular narrow-complex -> AF, MAT, AFL with variable block
Wide-complex tachycardia -> treat as VT until proven otherwise
- Regular + stable: trial of IV adenosine is reasonable
- Very irregular wide-complex: likely preexcited AF/flutter -> cardioversion, IV procainamide, or ibutilide

Acute Management

Treatment algorithm for hemodynamically stable narrow-complex tachycardia (PSVT):

SVT Treatment Algorithm - Harrison's 22E

Step-by-step approach:

Hemodynamic instability? -> Immediate synchronized DC cardioversion
Stable -> Vagal maneuvers (Valsalva, carotid sinus massage if no carotid bruits/prior stroke)
- A 2025 network meta-analysis (PMID: 41380061) evaluated optimal vagal maneuvers in stable SVT
Vagal maneuvers fail -> IV adenosine
- Terminates the vast majority of PSVT by transiently blocking AV conduction
- Side effects: chest pain, dyspnea, flushing, anxiety (very brief)
- Cautions: contraindicated post-cardiac transplant; use cautiously in WPW (can precipitate AF with rapid ventricular response); can aggravate bronchospasm
- Precipitates AF in up to 15% of patients
If adenosine fails -> IV non-DHP CCB (verapamil or diltiazem) or IV beta-blocker
- Effective but may cause hypotension; longer duration of action than adenosine
Recurrent episodes -> antiarrhythmic therapy (beta-blockers, CCBs, flecainide, propafenone)
Recurrent/incessant or drug failure -> catheter ablation

Harrison's 22E, p. 1986

Long-Term Management

AVNRT: Catheter ablation of the slow AV nodal pathway - curative in >95% of patients. Risk of AV block requiring pacemaker is <1%.
AVRT/WPW: Accessory pathway ablation
Medical alternatives: Oral beta-blockers, verapamil, diltiazem; flecainide/propafenone for recurrent episodes
Patients can be taught to self-terminate with Valsalva maneuver

Special Considerations

WPW Syndrome:

Accessory pathway conducts anterogradely -> short PR (<0.12s), delta wave, wide QRS during sinus rhythm
Orthodromic AVRT: narrow complex, clinically indistinguishable from AVNRT - treat with vagal maneuvers and adenosine
Avoid AV nodal blocking agents (adenosine, CCBs, digoxin, beta-blockers) in preexcited AF/flutter (antidromic) - these can accelerate conduction down the AP causing VF

Pediatric SVT:

Rate >220 bpm in infants or >180 bpm in children suggests SVT over sinus tachycardia
AVNRT (reentrant) is the classic pediatric SVT
Associated with Ebstein's anomaly, corrected transposition of great arteries

Wide-complex SVT vs. VT (key distinguishing features):

Any wide-complex tachycardia of uncertain origin -> treat as VT
AV dissociation, fusion beats, capture beats all strongly favor VT
RBBB or LBBB morphology with preceding known bundle branch block may indicate SVT with aberrancy

Key Drug Summary

Drug	Route	Dose	Notes
Adenosine	IV rapid push	6 mg -> 12 mg -> 12 mg	Flush with NS; very short half-life (~10s)
Verapamil	IV	2.5-5 mg slow push	Avoid in wide-complex/uncertain tachycardia
Diltiazem	IV	0.25 mg/kg over 2 min
Metoprolol	IV	5 mg q5min x3
Procainamide	IV	15-17 mg/kg loading	For preexcited AF/flutter or wide-complex

Sources: Harrison's Principles of Internal Medicine 22E (2025), Washington Manual of Medical Therapeutics, Tintinalli's Emergency Medicine, Rosen's Emergency Medicine; 2019 ESC SVT Guidelines

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