What drug can help diagnose dysrhythmia, specifically supraventricular tachycardia (SVT)?

Adenosine is the Drug of Choice for Diagnosing Dysrhythmias

Adenosine is the preferred diagnostic agent for supraventricular tachycardia (SVT) because it produces transient AV nodal block that unmasks the underlying rhythm mechanism, allowing differentiation between various tachyarrhythmias. 1

Diagnostic Mechanism

Adenosine works diagnostically by creating transient AV block that reveals the underlying atrial activity:

• Terminates AV node-dependent arrhythmias (AVNRT, AVRT) by blocking the reentrant circuit, confirming the diagnosis when the rhythm converts 1
• Unmasks atrial flutter or atrial tachycardia by producing AV block while the atrial arrhythmia continues, making P waves visible on ECG 1
• Has minimal effect on ventricular tachycardia, helping distinguish VT from SVT with aberrancy in wide-complex tachycardias 1

Specific Diagnostic Patterns

The response to adenosine provides critical diagnostic information even when the arrhythmia doesn't terminate:

• Termination with P wave after the last QRS favors AVRT or AVNRT 1
• Termination with a QRS complex suggests atrial tachycardia, which is often adenosine-insensitive 1
• Continuation of tachycardia with AV block is virtually diagnostic of atrial tachycardia or atrial flutter, excludes AVRT, and makes AVNRT very unlikely 1
• Transient AV block with persistent atrial activity helps differentiate focal AT from AVNRT and AVRT 1

Administration for Diagnosis

Always record a continuous 12-lead ECG during adenosine administration to capture the diagnostic response, as the drug's effects last only seconds 1

• Administer as rapid IV bolus via proximal vein followed immediately by saline flush 1
• Initial dose: 6 mg IV push, followed by up to two additional 12 mg boluses if needed 2
• The extremely short half-life (less than 10 seconds) allows rapid repeat dosing if the first dose is ineffective 3, 4

Critical Safety Considerations

Absolute Contraindications for Diagnostic Use

• Avoid in pre-excited atrial fibrillation or WPW with AF/flutter, as adenosine can precipitate ventricular fibrillation by enhancing conduction down the accessory pathway 1, 5
• Avoid in severe bronchial asthma, as adenosine can cause bronchospasm 1

Common but Transient Side Effects

Adenosine frequently causes uncomfortable but brief symptoms that resolve within 1 minute due to its ultra-short half-life:

• Chest discomfort, dyspnea, and flushing occur commonly but are self-limiting 1, 3, 4
• Transient atrial fibrillation develops in 1-15% of patients but usually resolves spontaneously 1
• Nonsustained polymorphic VT or PVCs occur in approximately 50% of cases but are clinically insignificant and self-terminating 6

Drug Interactions Affecting Dosing

• Theophylline antagonizes adenosine's effects, requiring higher doses 1
• Dipyridamole potentiates adenosine, requiring lower doses 1
• Carbamazepine increases risk of heart block when combined with adenosine 1

Advantages Over Alternative Diagnostic Approaches

Adenosine is superior to other diagnostic methods because:

• Rapid onset (within 30 seconds) and ultra-short duration allow immediate diagnosis 3, 4, 7
• Success rate of 78-96% for terminating AV node-dependent SVT, confirming the diagnosis 1
• Safer than verapamil in hemodynamically compromised patients or those with wide-complex tachycardia of uncertain etiology 4, 7
• No risk of prolonged hypotension or cardiac arrest associated with longer-acting AV nodal blockers 3, 4