A 68-year-old man undergoes elective CABG for triple-vessel disease. On postoperative day 1, telemetry shows a regular narrow-complex rhythm at 78 bpm. The ECG demonstrates regular QRS complexes with inverted P waves appearing immediately after each QRS complex in leads II, III, and aVF, with upright P waves in aVR. The patient is hemodynamically stable with normal blood pressure and adequate urine output. The rhythm marked **A** in the diagram is most consistent with which of the following regarding its mechanism and immediate management?
A. Atrial flutter with 1:1 conduction; immediate electrical cardioversion is indicated to restore sinus rhythm
B. Junctional tachycardia from digitalis toxicity; digoxin should be discontinued immediately and DigiFab administered
C. Accelerated junctional rhythm arising from AV nodal pacemaker cells; observation with telemetry monitoring is appropriate as this is usually transient and benign post-operatively
D. Sinus tachycardia with first-degree AV block; beta-blockers should be initiated to slow the rate and improve diastolic filling
Explanation
Why option 1 is correct
The ECG findings—regular narrow QRS at 78 bpm with retrograde P waves (inverted in inferior leads, upright in aVR) immediately after the QRS—are pathognomonic for accelerated junctional rhythm (AJR) arising from AV nodal pacemaker cells. By definition, AJR operates at 60–100 bpm, faster than the intrinsic junctional escape rate (40–60 bpm) but slower than junctional tachycardia (>100 bpm). The retrograde P waves indicate that atrial activation is proceeding backward from the junction. In the post-CABG setting, AJR is a common, transient arrhythmia (occurring in the first 24–72 hours) due to surgical trauma to the atria and AV node, and is almost always benign and self-limiting. The patient is hemodynamically stable, so observation with telemetry monitoring is the appropriate management per ACC/AHA/HRS guidelines. Correction of electrolytes, adequate analgesia, and ensuring oxygenation are supportive measures.
Why each distractor is wrong
Option 2 (Atrial flutter with 1:1 conduction): Atrial flutter presents with a sawtooth baseline and a regular ventricular rate that is typically much faster (120–150 bpm with 2:1 block, or 300+ bpm with 1:1 conduction in WPW). The retrograde P waves visible immediately after the QRS are not consistent with atrial flutter morphology. Cardioversion is not indicated for a stable, narrow-complex rhythm at 78 bpm.
Option 3 (Junctional tachycardia from digitalis toxicity): Although digitalis toxicity is a classic cause of AJR, the patient's rate (78 bpm) is in the AJR range, not tachycardia (which would be >100 bpm). There is no mention of digoxin administration, and the post-CABG context makes surgical trauma a far more likely etiology. Premature DigiFab administration without confirmed toxicity is inappropriate and potentially harmful.
Option 4 (Sinus tachycardia with first-degree AV block): Sinus tachycardia would show upright P waves in the inferior leads and PR interval prolongation if first-degree block were present. The inverted P waves in II, III, aVF with upright P in aVR are retrograde, not sinus. Beta-blockers are not indicated in a stable, hemodynamically adequate post-op patient with a near-normal rate.
High-YieldNEET PG
Post-CABG AJR at 60–100 bpm with retrograde P waves is a benign, transient rhythm from surgical trauma; observe and monitor—do not treat unless hemodynamically compromised or rate >100 bpm.
2018 ACC/AHA/HRS Guideline on Bradycardia and Conduction Delay; Harrison 21e Ch 247
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