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Paroxysmal Supraventricular Tachycardia (PSVT)

Per Harrison's Principles of Internal Medicine (21st Ed.) & Braunwald's Heart Disease

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1. Definition & Terminology

"PSVT refers to a family of tachycardias including AV node reentry, AV reciprocating tachycardia using an accessory pathway, and atrial tachycardia."

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2. Classification of PSVT

• AV nodal-dependent reentry (AVNRT)
• Large atrial reentry (e.g., flutter — excluded from strict PSVT)
• Focal atrial tachycardias (automaticity or micro-reentry)

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3. Mechanisms in Detail

A. AVNRT (Most Common)

• Slow pathway: slow conduction, short refractory period (α pathway)
• Fast pathway: fast conduction, long refractory period (β pathway)

• Anterograde conduction via the slow pathway, retrograde via the fast pathway
• The atria and ventricles are activated nearly simultaneously
• P wave is buried in or just after the QRS (retrograde P, often seen as pseudo-r' in V1 or pseudo-S in inferior leads)

• Anterograde via fast, retrograde via slow
• Long RP interval; P wave appears before the next QRS

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B. AVRT (Accessory Pathway-Mediated)

• Delta wave (slurred QRS upstroke)
• Short PR interval (<120 ms)
• Wide QRS (pseudo-LBBB or RBBB pattern)

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C. Focal Atrial Tachycardia (AT)

• Arises from a single ectopic focus in the atria
• Mechanism: enhanced automaticity, triggered activity, or micro-reentry
• P wave morphology differs from sinus P (key diagnostic clue)
• Long RP tachycardia — P precedes QRS by >50% of the R-R interval
• Common sites: crista terminalis, pulmonary vein ostia, coronary sinus ostium, tricuspid/mitral annulus

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4. Clinical Presentation

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5. ECG Diagnosis

Step-by-step ECG approach to narrow-complex tachycardia:

Narrow QRS tachycardia (QRS <120 ms)
│
├── Regular?
│   ├── YES
│   │   ├── P visible?
│   │   │   ├── NO (or buried in QRS) → AVNRT (typical)
│   │   │   ├── Short RP (<50% RR, P after QRS) → AVNRT or orthodromic AVRT
│   │   │   └── Long RP (>50% RR, P before QRS) → Atypical AVNRT, AT, or PJRT
│   │   └── Sinus tach, flutter with block
│   └── NO → AF, multifocal AT, flutter with variable block

ECG Features by Type:

Wide-complex SVT:

• Antidromic AVRT: fully pre-excited QRS, extremely wide
• AVNRT/AVRT with aberrancy: BBB pattern, rate-related
• Must always consider VT in differential for wide QRS tachycardia

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6. Diagnosis

Initial Workup:

1. 12-lead ECG during tachycardia — most critical
2. 12-lead ECG in sinus — look for pre-excitation (delta wave, short PR)
3. Holter/event monitor — if paroxysmal and not captured in office
4. Echocardiogram — rule out structural disease, especially with AVRT
5. Electrophysiology (EP) study — gold standard for definitive diagnosis and pre-ablation mapping

Vagal Maneuvers as Diagnostic Tool:

• Carotid sinus massage or Valsalva: terminates AVNRT and AVRT (AV nodal-dependent) but only slows atrial flutter/AT (P waves become visible)
• Modified Valsalva (REVERT trial): improves termination rate for AVNRT/AVRT

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7. Acute Management

"Acute management of narrow QRS PSVT is guided by the clinical presentation. Continuous ECG monitoring should be implemented."

Algorithm:

Hemodynamically unstable?
├── YES → Synchronized DC cardioversion (50–200 J biphasic)
└── NO → Vagal maneuvers first
         ├── Successful? → Monitor, no further Rx needed acutely
         └── Failed? → Adenosine IV
                       ├── 6 mg rapid IV push (antecubital or above)
                       ├── If no response → 12 mg × 2 doses
                       └── Failed/contraindicated?
                           ├── Non-DHP CCBs: Verapamil 5–10 mg IV or Diltiazem IV
                           ├── Beta-blockers: Metoprolol IV
                           └── DC cardioversion if drugs fail

Adenosine:

• Mechanism: Hyperpolarizes AV nodal cells via A1 receptor → transient AV block → interrupts AV nodal-dependent reentry
• Dose: 6 mg → 12 mg → 12 mg (rapid IV bolus, followed by saline flush)
• Half-life: <10 seconds (degraded by red blood cells/endothelium)
• Side effects: Chest tightness, flushing, dyspnea, transient asystole (warn patient!)
• Contraindications: 2nd/3rd degree AV block, sick sinus syndrome (without pacemaker), severe asthma, WPW + AF (may accelerate conduction via AP)
• Use with caution: Heart transplant patients (use lower dose, ~3 mg — exquisitely sensitive)
• Theophylline antagonizes adenosine (requires higher doses)

CCBs and Beta-Blockers:

• Verapamil 5–10 mg IV: effective but avoid in wide-complex tachycardia (may cause hemodynamic collapse in VT)
• Diltiazem 0.25 mg/kg IV: alternative
• Metoprolol 5 mg IV q5min × 3: useful if CCBs contraindicated (e.g., LV dysfunction)

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8. Long-Term Management

Catheter Ablation (Definitive Treatment):

• Recurrent symptomatic PSVT patient preference
• Poorly tolerated episodes
• Pre-excitation (WPW) with symptomatic tachycardia or high-risk features (short anterograde refractory period)
• Professional pilots, athletes, or patients unwilling to take long-term medication

Pharmacologic Long-Term Options (if ablation declined):

Important: Avoid AV nodal blocking agents (adenosine, verapamil, digoxin) in WPW + AF — they may paradoxically accelerate ventricular rate via the AP → ventricular fibrillation risk. Use procainamide or ibutilide instead, or cardiovert.

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9. WPW Syndrome — Special Considerations (Braunwald's)

• Shortest pre-excited RR interval during AF <250 ms
• Anterograde effective refractory period of AP <250 ms
• Multiple accessory pathways
• AVRT-induced AF
• Symptomatic tachyarrhythmias

• High-risk occupations
• High-risk electrophysiologic properties (if tested)
• Competitive athletes

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10. Special Populations

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11. Prognosis

• AVNRT and AVRT are generally not life-threatening in structurally normal hearts
• WPW carries a small but real risk of SCD (~0.1%/year in symptomatic patients) — higher with high-risk AP features
• After successful catheter ablation, cure rates exceed 95% with very low recurrence
• Quality of life improves substantially post-ablation, with most patients avoiding lifelong drug therapy

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Summary Table

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