Differential Diagnosis
Trifascicular block is important to diagnose because it is difficult to tell based on the surface ECG whether the prolonged PR interval is due to disease in the AV node or due to diffuse distal conduction system disease.
- In the former case, if the block at the AV node level becomes complete, the escape rhythm will originate from the bundle of His, which typically will generate heart rates in the 40s, allowing the individual to survive and complain of symptoms of fatigue or near-syncope to their physician.
- In the latter case, however, because the conduction system disease is diffuse in nature, the escape rhythm may be fascicular or ventricular, which may be at rates that are life-threateningly low.
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