Premature Atrial Complexes

Premature Atrial Complexes

ECG Recognition:

• The ECG feature of a premature atrial complex (PAC) is prematurity and altered P wave morphology. 
• The closer its origin to the SA node, the more it will look like the sinus P wave. A PAC originating in the low atrium will have a low amplitude or inverted P wave in II, III and aVF. 
• A PAC will have a PRI more than 0.12 s compared to premature junctional complex (PJC) which will be 0.12 s or less. 
• PAC can manifest as a complex that appears after every other normal beat (bigeminal pattern); after 2 normal beat or the PAC is the 3rd beat (trigeminy; etc.) 
• However, some PJC's will have delayed retrograde conduction. So, it suggested to call complexes with inverted P wave morphologies AV junctional rhythms.

There are 3 fates of a PAC: conducted with normal QRS morphology, conducted with aberrancy and a PAC that is not conducted. Depending on the degree of prematurity, a PAC will be conducted to the ventricles with normal of long PRI. A PAC that falls during the absolute refractory period of the AV node will not conduct (non-conducted PAC). A PAC that conducts through the AV node but finds the right bundle branch refractory will conduct with a RBBB morphology (conducted with aberrancy). Refractory period is the time when an excitable tissue is not responsive to an incoming stimulus.

• PAC conducted with normal QRS duration
• PAC conducted with aberrancy
• Non-conducted PAC

Sinus rhythm with PAC in bigeminy conducted normally, conducted with aberrancy and blocked or non-conducted PAC. Every other beat is PAC or in bigeminal pattern. The first 3 PAC's which created QRS # 2,4 and6 are conducted with a normal QRS morphology. The 4th PAC which is which created QRS #8 is conducted with a right bundle branch block (RBBB) configuration. The last 3 PAC's which are after QRS #s 9,10 and 11 are not conducted.  

Sinus rhythm with PAC in trigeminy that is blocked or non-conducted. This is a challenging ECG. The non-conducted P wave is better seen in V1 as a small distortion of the T wave on the second QRS (red arrows). To determine that this is a P wave on top of a T wave, compare the shape of previous T wave. The second T wave is more prominent. Thus, we can prove that this is indeed a P wave that is premature that is not conducted.

Sinus rhythm with PAC in trigeminy (non-conducted and conducted with  normal QRS). This is another challenging ECG. The PAC's are better seen in V1. The PAC with red arrows are not conducted while the PAC with blue arrows are conducted with a normal QRS but with a prolonged PR interval. The reason of the prolonged PRI on the conducted PAC is because the AV node is in a relative refractory period. It will allow AV conduction but it will delay the AV conduction because it is still recovering from the previous sinus impulse.

Home