EKG of the Week 2018-6-10

This EKG comes courtesy of Dr. Kong.

A 60 y/o male with hx of HTN, presented for palpitations.

V/S: P - 140, BP 180/90, R - 18.

Two EKG's are below. EKG a was the first EKG.

2018 6-10a.png

EKG b was taken after the patient was given diltiazem.

2018 6-10b.png

1.       What rhythm is demonstrated in EKG a?

2.       What other pattern is demonstrated in EKG a?

3.       What happened to the QRS complexes between EKG a and EKG b?



The rhythm is atrial fibrillation with a rapid ventricular response. 

There is a left bundle branch block

The bundle branch block went away. This is known as a rate related bundle branch block.


The first EKG shows rapid a-fib with a left bundle branch block. The patient was treated with diltiazem with resultant control of the heart rate. When that happened the bundle branch block disappeared and the QRS narrowed. This is known as a rate related bundle branch block.

As the heart rate increases, the cardiac cycle shortens. Eventually the next beat arrives when one bundle is still refractory. So it conducts down the other bundle and then across the heart, the same as in a regular bundle branch block. However, when the heart rate slows, the cardiac cycle lengthens and the bundle recovers. When the bundle recovers the bundle branch block disappears.

No specific treatment is needed for a rate-related bundle branch block.