February 7, 2023

The Three Fates of a PAC


Sudden bradycardia alarms were triggered on the cardiac monitor. Is this patient having sinus pause/arrest?

Image 1 - Image case

This is premature atrial complex (PAC) in bigeminal pattern or bigeminy.

Image 2- Inage case annotated

QRS complexes 2,4 and 6 are conducted PAC's with a narrow QRS or conducted normally. The wide QRS complex (R8) is a PAC that is conducted with aberrancy (right bundle branch block morphology - dominant R in V1). The sudden bradycardia after R8 is due non-conducted PACs'. The T waves of the last 3 complexes (R9-R11)  are distorted. If you look closely there are P waves there (red asterisk).

Occasionally PACs pose a challenge to identify. One trick is to look and compare the T waves. In lead II for example, the T waves of complex R1,R3 and R5 are prominent ("bulky) compared to the T waves of R2, R4 and R6. This would mean that the "distortion" of the T wave is due to a PAC increasing the "bulk" of the T wave. Another trick is to look at the distortions in other leads. In this case, lead V1 and aVL clearly show the PAC distortions.  This skill takes time and so you must practice it by looking at several ECG's.

The posted ECG show the 3 fates of a PAC:

1. PAC conducted with a normal QRS

2. PAC conducted with aberrancy (RBBB in this case)

3. Non-conducted PAC

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