What are the differential diagnosis and management options for a patient presenting with Supraventricular Tachycardia (SVT)?

Differential Diagnosis of Supraventricular Tachycardia

When evaluating SVT, first determine QRS width (<120 ms vs ≥120 ms) and hemodynamic stability, as these immediately guide your diagnostic approach and treatment decisions. 1

Narrow QRS-Complex Tachycardia (<120 ms)

Primary Differentials

Atrioventricular Nodal Reentrant Tachycardia (AVNRT) - Most common form of SVT 1, 2

• P waves are typically hidden within the QRS complex 1
• May see pseudo-R' wave in lead V1 or pseudo-S wave in inferior leads from P waves deforming the QRS 1
• Regular RR interval with no visible atrial activity 1
• Reentry circuit confined to AV nodal and perinodal atrial tissue 3

Atrioventricular Reciprocating Tachycardia (AVRT) - Second most common 1, 2

• P wave present in ST segment, separated from QRS by >70 ms 1
• Requires anatomically distinct accessory pathway bypassing the AV node 3
• RP interval typically shorter than PR interval (short RP tachycardia) 1

Atrial Tachycardia (AT) 1

• P wave morphology differs from sinus rhythm, usually seen near end of or shortly after T wave 1
• Long RP interval typical (RP > PR) 1
• Rhythm driven by atrium with normal conduction to ventricles 1

Permanent Form of Junctional Reciprocating Tachycardia (PJRT) 1

• Unusual accessory pathway with decremental (slowly conducting) retrograde conduction 1
• Produces delayed atrial activation and long RP interval 1
• Can appear similar to atypical AVNRT or low septal AT on ECG 1

Wide QRS-Complex Tachycardia (≥120 ms)

Critical First Step: Distinguish SVT from Ventricular Tachycardia

If uncertain about diagnosis, treat as ventricular tachycardia - this is a safety-first mandate 1

SVT with Aberrancy Differentials

SVT with Pre-existing Bundle Branch Block 1

• Compare to baseline ECG during sinus rhythm when available 1
• QRS morphology should match baseline BBB pattern 1

SVT with AV Conduction Over Accessory Pathway 1

• Can occur during AT, atrial flutter, AF, AVNRT, or antidromic AVRT 1
• Antidromic AVRT: anterograde conduction over accessory pathway, retrograde over AV node 1
• QRS generally wider (more pre-excited) compared to sinus rhythm 1

SVT with Drug-Induced Widening 1

• Class Ic or Ia antiarrhythmic drugs can widen QRS >0.14 seconds (RBBB) or >0.16 seconds (LBBB) 1
• Also seen with hyperkalemia or severe heart failure 1

Ventricular Tachycardia - Key Diagnostic Features

AV Dissociation 1

• Ventricular rate faster than atrial rate proves VT diagnosis 1
• Only clearly discernible in 30% of VTs 1
• Look for irregular cannon A waves in jugular venous pulse, variable S1 intensity, and variable systolic BP 1

Fusion Beats 1

• Pathognomonic for VT 1
• Represent merger between conducted supraventricular impulses and ventricular depolarization 1

QRS Width Criteria 1

• QRS >0.14 seconds with RBBB pattern favors VT 1
• QRS >0.16 seconds with LBBB pattern favors VT 1
• Not helpful for differentiating VT from SVT with accessory pathway conduction 1

QRS Morphology in Precordial Leads 1

• RS interval >100 ms in any precordial lead highly suggestive of VT 1
• Negative concordance (QS complexes in all precordial leads) diagnostic for VT 1
• Positive concordance does not exclude antidromic AVRT over left posterior accessory pathway 1
• QR complexes indicate myocardial scar, present in ~40% of post-MI VTs 1

Clinical History 1

• Previous MI + first wide-QRS tachycardia after infarct strongly indicates VT 1

Acute Management Algorithm

Hemodynamically Unstable Patients

Immediate synchronized DC cardioversion is first-line for any unstable narrow or wide QRS-complex tachycardia 1, 4, 3

Hemodynamically Stable Narrow QRS-Complex Tachycardia

Step 1: Vagal Maneuvers 1, 2, 5

• Valsalva maneuver: patient bears down against closed glottis for 10-30 seconds, equivalent to 30-40 mmHg intrathoracic pressure 1
• Modified Valsalva maneuver has 43% effectiveness 5
• Carotid sinus massage (after confirming absence of bruit) 1
• Facial immersion in cold water 1
• Perform with patient in supine position 1
• Will not be effective if rhythm does not involve AV node as requisite component 1

Step 2: Adenosine (if vagal maneuvers fail) 1, 2, 3, 5

• 91% effective for acute termination 5
• Preferred agent due to rapid onset and short half-life 1
• Contraindicated in severe asthma 1, 6
• Use with extreme caution when diagnosis unclear: may produce VF in CAD patients with AF and rapid ventricular rate in pre-excited tachycardias 1
• Theophylline reduces effectiveness (requires higher doses) 1
• Dipyridamole potentiates effects 1, 6
• Higher heart block rates with concomitant carbamazepine 1

Step 3: Calcium Channel Blockers or Beta Blockers (if adenosine fails) 1, 2, 5

• Verapamil or diltiazem are longer-acting alternatives 1
• Particularly valuable for patients with frequent ectopic beats triggering recurrence 1
• Never use verapamil or diltiazem for VT or pre-excited AF - may cause hemodynamic collapse or accelerate ventricular rate leading to VF 1, 7

Hemodynamically Stable Wide QRS-Complex Tachycardia

If diagnosis uncertain, treat as VT 1

For confirmed pre-excited AF with accessory pathway 4

• Intravenous procainamide if hemodynamically stable 4
• Synchronized cardioversion if hemodynamically unstable 4
• Avoid AV nodal blocking agents (adenosine, verapamil, diltiazem, beta blockers) 1, 7

Critical Pitfalls to Avoid

Never administer verapamil or diltiazem for wide-complex tachycardia of uncertain etiology 1, 7

• Can cause severe hypotension, hemodynamic collapse, or accelerated ventricular rate in VT or pre-excited AF 1, 7
• Verapamil contraindicated in Wolff-Parkinson-White syndrome with AF/flutter 7

Adenosine risks in specific populations 1, 6

• May precipitate VF in CAD patients with AF 1
• Can cause severe bronchoconstriction in asthma 1, 6
• May accelerate ventricular rate in pre-excited tachycardias 1

Obtain 12-lead ECG before treatment when possible 1

• Single-lead monitoring may miss VT masquerading as SVT 3
• Record ECG during tachycardia for definitive diagnosis 1
• Should not delay emergent therapy in unstable patients 1

Esophageal electrodes can help identify P waves when not visible on surface ECG 1