When Delay Becomes Danger: Sudden Progression of First-Degree to High-Grade Block
1Division of Cardiology, Department of Medicine, Highland Hospital, Oakland, California, USA.
JACC. Case Reports
|May 21, 2026
Summary
Sudden changes from first-degree to high-grade heart block warrant investigation. This may indicate a complete heart block with an isorhythmic junctional escape rhythm, not just a vagal response.
Area of Science:
- Cardiology
- Electrophysiology
Background:
- First-degree atrioventricular block is typically benign.
- High-grade atrioventricular block often signifies conduction system failure, potentially requiring pacemaker implantation.
Purpose of the Study:
- To investigate an unusual case of alternating Mobitz 1 and high-grade atrioventricular blocks.
- To differentiate between various types of heart block based on electrocardiogram findings and clinical presentation.
Main Methods:
- Case presentation of a 63-year-old male with hypertension, prior endocarditis, cellulitis, and fatigue.
- Electrocardiogram analysis revealing fluctuating atrioventricular block patterns.
- Clinical decision-making regarding electrophysiology study versus immediate pacemaker placement.
Main Results:
- The patient exhibited alternating Mobitz 1 and high-grade atrioventricular blocks on electrocardiogram.
- A sudden transition from first-degree to 2:1 block was observed, atypical for a vagal etiology.
- The findings suggested a possible complete heart block with an isorhythmic junctional escape rhythm.
Conclusions:
- Abrupt shifts between first-degree and high-grade atrioventricular blocks necessitate evaluation for 2:1 block with shortened PR interval or complete heart block with isorhythmic junctional escape.
- Noninvasive measures like exercise testing can help identify chronotropic incompetence and infranodal conduction disorders by assessing PR interval shortening.
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