Case and content by Brandon Fainstad, MD.
Edits and graphics by Yilin Zhang, MD.
70 yo M presents with sepsis secondary to a presumed skin and soft tissue infection. On presentation, he is febrile and tachycardic to 145 bpm.
How do you explain the change from his baseline ECG?
This is a example of rate dependent bundle branch block (BBB). There is an incomplete right BBB on the patient’s baseline ECG that becomes more pronounced at higher heart rates. Diseased His-Purkinjie fibers have prolonged refractory periods. Thus, a baseline ECG may have a completely normal QRS morphology, but as the rate increases and ventricular depolarizations recur before the diseased bundles complete their refractory period a bundle branch block occurs. These typically occur at a critical heart rate1.
This almost always indicates underlying disease in the conduction system, especially if there is any conduction delay at baseline (as there is in this patient)2. This can also be caused by antiarrhythmic drugs.
TAKE HOME POINTS:
- Always compare ECGs to a baseline a ECG if available.
- Rate related bundle branch blocks occur in patients with disease conduction systems. Baseline ECG may have normal QRS morphology but a bundle branch block appears at higher heart rates.
- Denes, P, et al: Electrophysiological observations in patients with rate dependent bundle branch block. Circulation.1975;51:244-250
- Fisher, JD & Aronson, RS. Rate-dependent Bundle Branch Block: Occurrence, Causes, and Clinical Correlation. JACC. 1990;16(1): 240-243.