What is the role of EMG (Electromyography) in diagnosing Supraventricular Tachycardia (SVT)?

Role of EMG in Diagnosing Supraventricular Tachycardia (SVT)

Electromyography (EMG) has no role in the diagnosis of supraventricular tachycardia (SVT); instead, electrocardiography (ECG) is the standard diagnostic tool for SVT. 1

Correct Diagnostic Tools for SVT

Electrocardiography (ECG)

• 12-lead ECG is the essential diagnostic tool for SVT, allowing differentiation of tachycardia mechanisms and determination of whether the AV node is an obligate component of the arrhythmia circuit 1
• ECG helps distinguish SVT from ventricular tachycardia by evaluating QRS morphology, particularly important when QRS duration exceeds 120 ms 1
• Continuous ECG recording during diagnostic maneuvers (such as adenosine administration) helps identify the mechanism of arrhythmia 1

Key ECG Features of SVT

• Regular rhythm with heart rates typically between 150-250 beats per minute (average 186 bpm) 2, 3
• Extremely regular R-R intervals after the first 10-20 beats 2, 1
• QRS complex is usually narrow (<120 ms) unless there is aberrant conduction 1
• P wave morphology and relationship to QRS complex helps determine the specific type of SVT 1

Diagnostic Approach to SVT

Initial Assessment

• Record a 12-lead ECG during tachycardia to differentiate SVT mechanisms 1
• Assess for hemodynamic stability, which determines immediate management approach 1
• Evaluate QRS duration to distinguish between narrow and wide complex tachycardias 1, 2

Diagnostic Maneuvers

• Vagal maneuvers (Valsalva or carotid sinus massage) can be both diagnostic and therapeutic 1, 4
  • Response to vagal maneuvers helps identify AV node-dependent SVTs 1, 5
  • Modified Valsalva maneuver shows higher success rates than carotid sinus massage 4
• Adenosine administration serves as both diagnostic and therapeutic intervention 1
  • Terminates AVNRT and AVRT but typically only transiently affects atrial tachycardia or atrial flutter 1
  • Continuous ECG recording during adenosine administration is crucial for diagnosis 1

Definitive Diagnosis

• Electrophysiological (EP) study is the gold standard for precise diagnosis of SVT mechanism 1
• EP study involves placement of multielectrode catheters in the heart to record electrical activity 1
• Pacing and programmed electrical stimulation help define the specific SVT mechanism 1

Common SVT Mechanisms and Their ECG Features

Atrioventricular Nodal Reentrant Tachycardia (AVNRT)

• Most common form of SVT 2, 6
• In typical AVNRT, P waves are often hidden within or appear at the end of QRS complex 1, 7
• Atypical AVNRT shows retrograde P waves with long RP interval 1

Atrioventricular Reentrant Tachycardia (AVRT)

• Involves an accessory pathway between atria and ventricles 3, 7
• P wave follows the QRS with R-P interval typically exceeding 70 ms 3
• Permanent form of junctional reciprocating tachycardia (PJRT) shows long RP interval due to slow retrograde conduction 1

Atrial Tachycardia (AT)

• P wave morphology differs from sinus P wave 1
• Long RP interval is typical as rhythm is driven by the atrium 1

Common Pitfalls in SVT Diagnosis

• Misdiagnosing ventricular tachycardia as SVT when only examining a single ECG lead - always obtain a 12-lead ECG 3, 1
• Failing to distinguish SVT with aberrancy from ventricular tachycardia 1
• Administering verapamil or diltiazem for wide-complex tachycardias of uncertain origin, which can be dangerous if the rhythm is actually VT or pre-excited AF 1
• Overlooking pre-excitation syndromes that may be present during sinus rhythm but not apparent during tachycardia 1, 6

Management Considerations

• Immediate synchronized cardioversion is indicated for hemodynamically unstable patients 1
• For stable patients, vagal maneuvers are first-line, followed by adenosine 1, 6
• EP study with ablation is recommended for definitive diagnosis and treatment of recurrent SVT 1
• Long-term management options include catheter ablation (success rate ~95%) or pharmacological therapy 2, 6