A 58-year-old man with chronic obstructive pulmonary disease presents with palpitations and dyspnoea. His 12-lead ECG shows a regular narrow-complex tachycardia at 150 bpm. The rhythm strip reveals the pattern marked **A** in the diagram — sawtooth flutter waves visible in leads II, III, and aVF. Which of the following best explains the electrophysiological mechanism underlying this ECG finding?
A. Atrioventricular nodal reentrant tachycardia with retrograde conduction via a slow pathway
B. Macro-reentrant circuit within the right atrium using the cavotricuspid isthmus, with counterclockwise conduction producing negative F-waves in inferior leads
C. Micro-reentrant circuit within the left atrium producing variable-rate irregular ventricular response
Ectopic atrial focus firing at 300 bpm with 2:1 AV conduction block
D.
Explanation
Why option 1 is correct
The sawtooth flutter waves (marked A) in the inferior leads II, III, and aVF are the pathognomonic ECG hallmark of typical atrial flutter. These negative F-waves result from a macro-reentrant circuit within the right atrium that circulates counterclockwise (when viewed from below) around the cavotricuspid isthmus (CTI) — the strip of myocardium between the inferior vena cava and the tricuspid annulus. This is the most common mechanism of atrial flutter and accounts for >90% of cases. The atrial rate is typically 250–350 bpm (classically 300 bpm), and the AV node cannot conduct at this rate, resulting in a fixed 2:1 conduction block that produces the regular ventricular rate of 150 bpm seen in this patient. This mechanism is well-established in Braunwald's Heart Disease and the ESC Guidelines on Supraventricular Tachycardia.
Why each distractor is wrong
Option 2 (Micro-reentrant left atrial circuit): This describes atypical atrial flutter, which arises from non-CTI-dependent circuits (often post-ablation or in scarred atria) and produces variable ECG appearances and irregular ventricular responses. The classic sawtooth pattern in inferior leads is not characteristic of atypical flutter.
Option 3 (Ectopic atrial focus with 2:1 block): Atrial flutter is not due to an ectopic focus but rather a reentrant mechanism. While the 2:1 conduction is correct, the mechanism is macro-reentry, not ectopic automaticity. Ectopic atrial tachycardia would show a single P-wave morphology, not flutter waves.
Option 4 (AVNRT with retrograde conduction): AVNRT is a different supraventricular tachycardia mechanism involving dual AV nodal pathways. It does not produce the characteristic sawtooth flutter waves in inferior leads and typically presents with rates of 140–250 bpm, not the classic 150 bpm of 2:1 flutter.
High-YieldNEET PG
Sawtooth F-waves in inferior leads (II, III, aVF) = typical atrial flutter from right atrial CTI-dependent reentry; regular rate of 150 bpm = 2:1 AV block of 300 bpm atrial rate.
