Supraventricular Tachycardia (SVT)

Definition

SVT is a broad term for any tachyarrhythmia that originates at or above the bundle of His (i.e., in the atria, AV node, or AV junctional tissue). The term is often used specifically to mean paroxysmal SVT (PSVT) - a group of regular, narrow-complex tachycardias that arise suddenly and terminate abruptly.

Classification

Type	Mechanism	Frequency
AVNRT (AV Nodal Reentrant Tachycardia)	Reentry within the AV node using dual pathways	~80% of PSVT
AVRT (AV Reentrant Tachycardia)	Reentry using an accessory pathway (e.g., WPW)	~15-20%
Atrial Tachycardia	Ectopic atrial focus or intra-atrial reentry	Remaining cases
Atrial flutter	Macro-reentry in right atrium	Common, usually regular
Atrial fibrillation	Multiple reentrant wavelets	Most common arrhythmia overall
Junctional tachycardia	Automatic or triggered focus in AV junction	Rare in adults

AVNRT - The Prototype

Mechanism: The AV node has two functionally distinct pathways:

Fast pathway: fast conduction, long refractory period
Slow pathway: slow conduction, short refractory period

In typical (slow/fast) AVNRT (85-90% of cases): A premature atrial beat finds the fast pathway refractory, conducts down the slow pathway, and then re-enters via the fast pathway retrogradely, establishing a reentrant circuit. The atria and ventricles are activated simultaneously.

In atypical AVNRT: Fast/slow (fast antegrade, slow retrograde) or slow/slow variants account for the remaining 10-15%.

Fuster and Hurst's The Heart, 15th Ed, p.1127-1128

ECG Features of PSVT

Feature	Finding
Rate	130-300 bpm (typically 170-180 bpm)
Rhythm	Regular
QRS	Narrow (<100 ms) unless aberrancy
P waves	"Buried" in QRS (~70%); retrograde P immediately before/after QRS (~30%)
RP interval	Short RP (RP < PR) in typical AVNRT; Long RP (RP ≥ PR) in atypical AVNRT / AT

Differential diagnosis based on RP interval:

Short RP (RP < PR): Typical AVNRT, AVRT
Long RP (RP ≥ PR): Atypical AVNRT, atrial tachycardia, PJRT (permanent junctional reciprocating tachycardia), sinus tachycardia
Tintinalli's Emergency Medicine, Table 18-16

Clinical Presentation

More common in females, peak in late teens/young adults
Most patients have no structural heart disease
Abrupt onset and offset - patients can often identify when it starts and terminates
Symptoms: palpitations, lightheadedness, dyspnea, near-syncope
Can cause syncope if rates are very high or vasomotor instability occurs
In children: rate >220 bpm (infants) or >180 bpm (older children) suggests SVT over sinus tachycardia
Tintinalli's Emergency Medicine, p.153

Acute Management

Treatment algorithm (hemodynamically stable narrow-complex tachycardia):

SVT Treatment Algorithm - Harrison's Principles of Internal Medicine 22E

Step 1: Hemodynamic instability?

Yes → Synchronized DC cardioversion (QRS-synchronous)
No → proceed to vagal maneuvers

Step 2: Vagal Maneuvers

Valsalva maneuver (first line, patient can self-administer)
Carotid sinus massage (check for carotid bruits first; avoid if prior stroke)
Face immersion in ice water (especially in children)
A 2025 network meta-analysis (PMID 41380061) is evaluating optimal vagal maneuver techniques

Step 3: Adenosine (drug of choice)

6 mg IV rapid bolus → if ineffective, 12 mg IV (repeat once)
Must be followed immediately by NS flush
Nearly 100% effective for AVNRT
Mechanism: transient AV nodal blockade via A1 purinergic receptors
Cautions:
- Cardiac transplant recipients: very sensitive - start at 1 mg
- WPW with pre-excited AF: avoid (can accelerate conduction down accessory pathway)
- Asthma: caution (may trigger bronchospasm)
- Theophylline/caffeine: antagonize effect (higher doses may be needed)
- Dipyridamole: potentiates effect
Side effects: transient chest pain, flushing, dyspnea, sinus bradycardia, may precipitate AF (in up to 15%)

Step 4: AV Nodal Blocking Agents (if adenosine fails or tachycardia recurs)

Verapamil IV (non-DHP CCB): slows both fast and slow pathways
Diltiazem IV
Beta-blockers IV (metoprolol, esmolol)
Note: may cause hypotension; avoid in patients with systolic dysfunction

Step 5: Wide-complex tachycardia (SVT with aberrancy vs VT)

Treat as VT until proven otherwise
If regular, stable, adenosine trial is reasonable (may reveal SVT with aberrancy)
Irregular wide-complex tachycardia: likely pre-excited AF - treat with cardioversion, procainamide, or ibutilide (do NOT use adenosine/verapamil)
Harrison's Principles of Internal Medicine 22E (2025), p.1986
Fuster and Hurst's The Heart, 15th Ed, p.1129-1130

Long-term / Chronic Management

Approach	Details
Catheter ablation	First-line for recurrent/symptomatic PSVT; >95% success for AVNRT slow-pathway ablation; preferred over lifelong drug therapy
Beta-blockers or CCBs (oral)	Rate control; can take "pill-in-pocket" (e.g., verapamil) to terminate acute episodes
Antiarrhythmic drugs	Flecainide, propafenone (class IC) for patients not candidates for ablation
Observation	For infrequent, well-tolerated, self-terminating episodes

Most patients with recurrent PSVT are candidates for catheter ablation, which is curative. - Harrison's Principles of Internal Medicine 22E (2025), p.1986

VT vs SVT with Aberrancy - Key Differentiators

Feature	VT	SVT with Aberrancy
AV dissociation	Present (strongly favors VT)	Absent
Fusion/capture beats	Present	Absent
QRS in tachycardia = QRS in sinus	Unlikely	Likely (same morphology)
Response to adenosine	Transient VA dissociation if 1:1 VA conduction	Terminates or exposes mechanism
Concordance in V1-V6	Positive or negative	Unusual

Braunwald's Heart Disease, Textbook of Cardiovascular Medicine, p.553-554

Special Situations

Situation	Notes
WPW syndrome	Accessory pathway enables pre-excitation; avoid AV nodal blockers (adenosine, verapamil) in pre-excited AF; use procainamide or cardioversion
Pregnancy	Adenosine is safe; electrical cardioversion is safe; avoid beta-blockers and CCBs in first trimester if possible
Children	Adenosine preferred; check Harriet Lane dosing; consider accessory pathways (Ebstein's anomaly, corrected TGA)
Post-cardiac transplant	Exquisite adenosine sensitivity; start at 1 mg

Recent Evidence

A 2025 network meta-analysis (PMID 41380061) compared vagal maneuver techniques for stable SVT, evaluating optimal first-line non-pharmacologic approaches.
A 2024 systematic review/meta-analysis (PMID 39242347) examined troponin elevation in SVT patients - relevant for distinguishing demand ischemia from true ACS in patients presenting with SVT.

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