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Rapid regular palpitations for two hours

The case

52-year-old man. Rapid regular palpitations for two hours

No significant past medical history. Takes no regular medication. Presented to the emergency department with a two-hour history of sudden-onset palpitations. He is haemodynamically stable. The initial twelve-lead ECG showed a regular narrow-complex tachycardia at 150 per minute. The registrar administered intravenous adenosine 6 mg as a rapid bolus. A twelve-lead ECG was recorded during the injection.

On arrival

HR
150
Spo2Percent
97
Respiratory rate
20
Temperature (°C)
36.8
Systolic BP (mmHg)
118
Diastolic BP (mmHg)
72

Stage 1 of 1

A 52-year-old man presents with a two-hour history of regular palpitations at 150 per minute. He is haemodynamically stable. The registrar suspects an SVT and gives intravenous adenosine 6 mg. A twelve-lead ECG is recorded during the injection.

The tracing for the quiz “Rapid regular palpitations for two hours”.
Twelve-lead ECG recorded during adenosine administration Open this tracing in the studio
During the pause produced by adenosine, a sawtooth pattern becomes visible in the baseline. What underlying rhythm does this reveal?

The sawtooth baseline visible during the adenosine-induced AV block is the hallmark of atrial flutter. A regular narrow-complex tachycardia at exactly 150 per minute is the classic presentation of flutter with 2:1 conduction, the atrial rate being about 300 per minute. Sinus tachycardia would show discrete P waves rather than a sawtooth pattern. Atrioventricular nodal re-entrant tachycardia would terminate with adenosine rather than merely unmasking atrial activity.

Why did the tachycardia resume after adenosine rather than terminating?

Atrial flutter is a macro-re-entrant circuit confined to the right atrium. Adenosine blocks the AV node transiently and slows or stops ventricular conduction, but the atrial circuit continues uninterrupted because it does not traverse the AV node. By contrast, AV nodal re-entrant tachycardia and orthodromic atrioventricular re-entrant tachycardia both require the AV node as a limb of their circuit, so adenosine typically terminates them. The dose was standard and the tracing shows it clearly reached the AV node.

Adenosine has unmasked atrial flutter. Which drug class is first-line for rate control in this patient?

In a haemodynamically stable patient with atrial flutter, a beta-blocker is first-line for rate control. It slows AV nodal conduction and reduces the ventricular response rate. A class IC agent such as flecainide can paradoxically increase the ventricular rate in flutter by slowing the atrial rate enough to allow 1:1 conduction without adequate AV nodal blockade, and should not be used alone. Atropine would accelerate AV conduction and worsen the tachycardia.

Diagnosis

Atrial flutter with 2:1 conduction unmasked by adenosine

A regular narrow-complex tachycardia at exactly 150 per minute should always raise the suspicion of atrial flutter with 2:1 conduction, because the typical atrial flutter rate is about 300 per minute. Adenosine transiently blocks conduction through the atrioventricular node and unmasks the sawtooth flutter waves in the baseline, but it cannot terminate flutter because the re-entrant circuit is entirely within the atrium and does not depend on the AV node.

The practical lesson is twofold. First, adenosine is diagnostic here rather than therapeutic, and the tachycardia will resume once the drug wears off. Second, rate control with a beta-blocker is the appropriate next step in a stable patient, aiming to slow AV conduction and reduce the ventricular response while a plan for definitive management is made.

References

  1. Page RL, Joglar JA, Caldwell MA, et al. 2015 ACC/AHA/HRS guideline for the management of adult patients with supraventricular tachycardia. Circulation 2016;133(14):e506-e574.
  2. Brugada J, Katritsis DG, Arbelo E, et al. 2019 ESC Guidelines for the management of patients with supraventricular tachycardia. Eur Heart J 2020;41(5):655-720.
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Discussion

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