Scenario: This ECG was obtained in a 70-year-old female patient being treated for new onset atrial fibrillation on the telemetry unit. The patient has been given several doses of digitalis for her atrial fibrillation. Currently the patient is asymptomatic.

Interpretation: Sinus rhythm transitioning to accelerated junctional rhythm with incomplete atrioventricular (AV) dissociation.

The first 2 beats are sinus beats at a rate of 72 beats per minute (bpm), with a PR interval of 0.20 seconds. By the third beat, the PR interval begins to shorten (0.16 seconds) and continues to shorten with subsequent beats, indicating that the AV junction is assuming the pacemaker function of the heart. Because the rate is too fast for a junctional rhythm (typically <60 bpm), this rhythm is termed accelerated junctional rhythm. By the sixth beat, the P wave merges into the QRS complex and is then lost within the QRS complex. Eventually, a hint...

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