Grouped Beating That Looks Like Wenckebach
A 72-year-old woman with hypertension and paroxysmal atrial fibrillation is on telemetry after a hip replacement. The technician observes grouped beating: groups of three narrow QRS complexes followed by a pause, repeating. The technician initially interprets the rhythm as Mobitz I second-degree AV block because the PR interval appears to lengthen before each pause. On closer inspection, the T wave before each pause has a different shape, and the PP intervals are 760, 760, 620, 760, 760, and 620 ms. The interval from the P wave before the early P to the P wave after the pause is 1,390 ms, and the post-extrasystolic pause between QRS complexes is 1,140 ms. The patient is comfortable, with blood pressure 132/78 mmHg, and denies dizziness. Her potassium is 4.0 mmol/L, and she takes bisoprolol 5 mg daily and apixaban.
1. Explain why this is not Mobitz I AV block and identify the actual mechanism of the grouped beating.
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Why it is not Mobitz I:
- In classic Mobitz I, the PP interval is constant and the PR interval lengthens progressively until a P wave is non-conducted. Here the PP intervals are not constant: 760, 760, 620, 760, 760, 620 ms.
- The early P wave at 620 ms is premature. It falls near the preceding T wave, which explains why the T wave looks different before each pause.
- The apparent PR lengthening is caused by the prematurity of the P wave, not by progressive AV nodal delay. A premature P wave arrives when the AV node is still partially refractory, so it conducts more slowly or not at all.
Actual mechanism: sinus rhythm with frequent premature atrial complexes, some of which are non-conducted (blocked PACs). The blocked PAC creates the pause, and the next sinus beat follows after the sinus node resets. The result is grouped beating that mimics Wenckebach.
Key distinguishing steps: measure PP intervals rather than RR intervals, inspect the T wave before each pause for a hidden P wave, and compare the pause to the basic PP interval.
2. Calculate the expected pause and explain the concept of a non-compensatory pause in this context.
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Basic PP interval: 760 ms.
Fully compensatory pause: if the PAC did not reset the sinus node, the interval from the P wave before the PAC to the P wave after the pause would equal 2 x 760 = 1,520 ms.
Actual interval: 1,390 ms, which is less than 1,520 ms. That is a non-compensatory pause, meaning the premature atrial impulse entered the sinus node and reset its timing. The post-extrasystolic pause between QRS complexes is 1,140 ms, also less than 2 x 760 = 1,520 ms, consistent with reset.
Why this matters:
- A non-compensatory pause supports an atrial origin of the premature beat.
- In Mobitz I, the pause is caused by a dropped beat, not by an early P wave, and the PP interval across the pause is a multiple of the basic PP interval.
- The calculation is useful only after the premature P wave has been identified; without that, the pause length alone can be misleading.
3. What would you report, and what is the clinical significance of this finding in a patient on a beta-blocker?
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Report: 'Sinus rhythm with frequent premature atrial complexes, including non-conducted PACs producing grouped beating; not second-degree AV block.' Include the PP intervals, the coupling interval of the premature P wave, the pause duration, the patient's symptoms, and the current beta-blocker.
Clinical significance:
- Non-conducted PACs can mimic Mobitz I or marked bradycardia and may lead to inappropriate holds of AV-nodal blocking drugs or unnecessary pacing evaluation.
- PACs are common after surgery and with pain, stress, anemia, hypokalemia, hypomagnesemia, or atrial enlargement. In this patient with paroxysmal atrial fibrillation, frequent PACs may also be a trigger for AF, so continued monitoring is reasonable.
- The beta-blocker slows AV conduction and can make a premature P wave more likely to block, exaggerating the appearance of Wenckebach. That does not make the rhythm Mobitz I.
Actions: report to the nurse, document the corrected interpretation, suggest electrolyte and magnesium review, and continue monitoring for AF or symptoms. No urgent escalation is needed while the patient is comfortable and hemodynamically stable.