What happens if you give adenosine to a patient with Atrial Fibrillation (AFib) when it's mistaken for Supraventricular Tachycardia (SVT)?

What Happens When Adenosine is Given to AFib Mistaken for SVT

Adenosine will not terminate atrial fibrillation but will transiently slow the ventricular rate by blocking AV nodal conduction, potentially unmasking the underlying irregular atrial rhythm and clarifying the diagnosis—however, it carries a 12% risk of precipitating or worsening atrial fibrillation and rare but serious risks of ventricular fibrillation, particularly in patients with accessory pathways. 1, 2

Immediate Clinical Effects

Diagnostic Value Through Transient AV Block

• Adenosine blocks AV nodal conduction for seconds, temporarily slowing or halting ventricular response, which unmasks the underlying irregular atrial activity characteristic of AFib 1
• This diagnostic effect helps distinguish AFib from regular narrow-complex SVT (AVNRT/AVRT), where adenosine would typically terminate the arrhythmia 1
• The rhythm will continue after adenosine's brief effect wears off (half-life <10 seconds), confirming the diagnosis of an atrial arrhythmia rather than AV node-dependent tachycardia 1

Common Adverse Outcomes

• In real-world hospital practice, adenosine is inappropriately administered for AFib in approximately 32-40% of cases, primarily due to misdiagnosis on ECG 3
• The drug causes transient but uncomfortable side effects in ~30% of patients including chest discomfort, dyspnea, and flushing, which resolve in <1 minute 1, 4
• Adenosine may cause transient hypotension, particularly problematic in patients who are already hemodynamically compromised 4

Serious Proarrhythmic Risks

Precipitation or Worsening of Atrial Fibrillation

• Adenosine precipitates new atrial fibrillation or worsens existing AFib in 12% of patients when given during tachycardia 2
• The mechanism involves adenosine-induced atrial premature complexes occurring in a "long-short" sequence (ratio of premature beat interval to preceding cycle length of 0.37 vs 0.49 in those without AFib) 2
• Adenosine shortens atrial action potential duration in a dose- and rate-dependent manner (up to 38% shortening at 12mg during rapid rates), creating substrate for AFib 5
• This induced AFib typically begins 1.5-3 minutes after adenosine administration and may last from 15 seconds to 6 hours, though most cases convert spontaneously 4

Life-Threatening Ventricular Arrhythmias

• In patients with unrecognized Wolff-Parkinson-White syndrome presenting with pre-excited AFib, adenosine can precipitate ventricular fibrillation requiring immediate defibrillation 1
• The ACC/AHA guidelines explicitly warn that "adenosine may precipitate AF that may then conduct rapidly to the ventricle and even cause ventricular fibrillation" 1
• Case reports document ventricular fibrillation occurring even in structurally normal hearts without accessory pathways, though this is extremely rare 6
• The overall proarrhythmia rate (including asystole and polymorphic VT) is approximately 2% 3

Critical Safety Considerations

Mandatory Precautions

• A defibrillator must be immediately available whenever adenosine is administered, specifically because of the risk of precipitating rapidly conducted AFib or ventricular fibrillation 1
• Continuous ECG recording during adenosine administration is essential for both diagnostic purposes and safety monitoring 1
• The FDA label mandates that "resuscitative measures should be available prior to adenosine injection administration" 4

When Adenosine Should Never Be Used

• Wide-complex tachycardias of uncertain etiology (may be pre-excited AFib) 1
• Known or suspected Wolff-Parkinson-White syndrome with AFib 1
• Irregular wide-complex tachycardia (likely pre-excited AFib requiring cardioversion or procainamide/ibutilide) 1, 7
• Patients with severe asthma (can cause severe bronchospasm) 1, 4

The Misdiagnosis Problem

Why This Error Occurs

• Studies show that 31% of internal medicine house officers misdiagnose rapid AFib as paroxysmal SVT on 12-lead ECG 3
• The primary reason for inappropriate adenosine use in AFib is failure to recognize the irregular rhythm on ECG, not misunderstanding of adenosine's mechanism 3
• AFib with rapid ventricular response can appear deceptively regular at very fast rates, leading to diagnostic confusion 3

Key Distinguishing Features

• Always assess for rhythm irregularity before giving adenosine—irregular narrow-complex tachycardia is AFib or multifocal atrial tachycardia, not SVT 1
• Look for absence of discrete P waves and presence of fibrillatory waves between QRS complexes 3
• If diagnostic uncertainty exists in a stable patient, adenosine can be used diagnostically but only with defibrillator immediately available 1

Clinical Bottom Line

While adenosine given to AFib mistaken for SVT will not cause harm in most cases and may actually help clarify the diagnosis, it exposes patients to unnecessary risks including a 12% chance of worsening atrial fibrillation, potential for life-threatening ventricular arrhythmias in unrecognized pre-excitation, and a 2% overall proarrhythmia rate 3, 2. The key to avoiding this scenario is careful ECG interpretation to identify rhythm irregularity before administering adenosine, and ensuring immediate defibrillation capability whenever adenosine is used 1, 3.