Slide 1: What is the most concerning EKG finding? 80M with h/o CAD on the general medicine service reports a burning epigastric sensation after eating.

Slide 2: There’s a left bundle branch block (LBBB)! Down in V1, wide QRS, up in lateral leads. The patient is given antacid…

Slide 3: You repeat the ECG…a chart review reveals a baseline ECG that looks identical to this second ECG. What is going on?

Slide 4: It’s a rate dependent LBBB. What causes a rate dependent block? When the refractory period of bundle cells is longer than the R-R interval. Reminder the R-R interval decreases at HR increases.

At normal heart rates, rate dependent blocks may suggest degenerative conduction disease most commonly due to chronic hypertension. Graphic showing excitable and refractory periods.

Slide 5: Transition point, approximately 80 bpm. HR greater than 80, abnormal QRS, R-R interval less than LBB refractory period. HR less than 80, normal QRS, R-R interval greater than LBB refractor period.

1st EXG tracing, HR = 90 bpm. Wide QRS. 2nd ECG tracing, HR = 72 bpm. Narrow QRS.

Slide 6: Takeaways. Aberrant ventricular conduction (such as LBBB) is not always permanent…it may depend on the HR!

Always consider prior EKG history and clinical context.

Rate-dependent blocks in a non-tachycardic patient should raise suspicion for early conduction disease.

Add it to you ddx for wide QRS: RBBB, LBBB, intraventricular conduction delay, rate-dependent block, ventricular rhythm (VT, VF, AIVR), paced rhythm, pre-excitation (WPW), electrolyte disturbances, antiarrhythmic agents.