The rhythm strip shows a heart rate of 150 bpm with no discernible P waves — is it atrial flutter or SVT? The distinction drives two completely different interventions.
Atrial flutter and supraventricular tachycardia (SVT) are both rapid atrial-origin rhythms, but they differ in mechanism, ECG appearance, and management. Atrial flutter is a reentrant circuit in the right atrium firing at approximately 300 bpm. The AV node typically conducts every second impulse, producing a ventricular rate near 150 bpm with a classic sawtooth pattern of flutter waves (F waves) best seen in leads II and V1. Conduction ratios can vary — 3:1, 4:1 — yielding rates of 100 or 75 bpm. SVT (most commonly AVNRT) involves a reentrant loop through or near the AV node, producing a regular narrow-complex tachycardia at 150–250 bpm with P waves often buried in the QRS or absent. SVT onset and termination are abrupt, which is why patients describe sudden palpitations that start and stop like a switch. First-line intervention for stable SVT is vagal maneuvers (bearing down, carotid massage on one side only, ice to the face), followed by rapid IV push adenosine 6 mg, then 12 mg, then 12 mg again if needed — pushed fast through a proximal IV with an immediate saline flush. Atrial flutter does not respond to vagal maneuvers or adenosine in the same way; rate control uses calcium channel blockers or beta-blockers, and cardioversion or ablation may be needed.
Key Distinctions
Don't confuse atrial flutter's sawtooth F waves with the chaotic fibrillatory baseline of atrial fibrillation — flutter is organized, fibrillation is not. Students mistake any fast narrow-complex rhythm for SVT, but a ventricular rate locked at 150 bpm should trigger suspicion for 2:1 flutter hiding sawtooth waves under the T wave. Adenosine is diagnostic and therapeutic for SVT but primarily only unmasking for flutter — it briefly slows conduction to reveal F waves, then the flutter returns.
Clinical Pearl
Adenosine is a reset button for SVT but only a window shade for flutter — it briefly pulls back the curtain to show the sawtooth, then the shade snaps back up.