Premature Atrial Complex
Today's article is about Premature Atrial Complex (PAC).
The aim of this post is to refresh basic concepts that may help you to identify the different types of PACs. I will explain it in a simple way and provide classic examples.
Premature atrial complex occurs when an ectopic focus within the atria, generates an action potential before the next scheduled sinus beat and is often characterized by an abnormal and premature P wave.
In general, PAC can be:
- Conducted with normal PR, short PR, or prolonged PR.
- Aberrantly conducted: The PAC is conducted with wide QRS (RBBB or rarely LBBB pattern).
- Blocked or non-conducted: The PAC is not followed by a QRS. Usually merged with the preceding T wave and is often mistaken for sinus pause.
PACs may occur in a pattern of:
- Bigeminy: every other beat.
- Trigeminy: every third beat.
- Quadrigeminy: every fourth beat.
- Couplet: two consecutive PACs.
The origin of premature atrial complex lies outside the sinoatrial node (SAN), hence the term ectopic. The distinctive features of a PAC include prematurity and a change in P wave configuration (upright, negative, or biphasic). Typically, the ensuing pause after a PAC is non-compensatory. The proximity of the origin of the premature atrial complex to the SAN influences the extent to which the PAC may resemble the sinus P wave.
If a PAC falls within the effective refractory period of the atrioventricular (AV) node, it may fail to conduct, resulting in a non-conducted premature atrial complex.
When an ectopic P wave depolarizes and resets the SAN, the ensuing pause is often, but not always, equal to the the basic sinus cycle. The sum of the coupling interval and the PAC to the next P wave interval is typically less than two basic sinus cycles, and the term non-compensatory is applied. Occasionally, a PAC may suppress the sinus node rather than simply resetting it, resulting to a pause longer than the basic sinus cycle. This is also true when the entrance time of the PAC to the sinus node and the exit time from the SAN are included in the post-ectopic interval. Under such circumstances, the pause may equal two sinus cycles equivalent to a full compensatory pause. It has been estimated that around 45% of PACs are followed by a full compensatory pause, rendering the presence or absence of compensatory pause in differentiating PACs from PVCs unreliable.
A pause resulting from the failure of the dominant sinus rhythm may be terminated by an atrial escape, with the escape P wave differing in appearance from the sinus P wave. If a PAC reaches the bundle branch system during its effective refractory period, it may result in right bundle branch block (RBBB) aberrant conduction because of a longer reractory period of the right bundle compared to that of the left bundle. On rare occasions, when the sinus rate accelerates or the PAC follows an abruptly shorter R-P interval (from R wave to PAC), the refractory period of the left bundle may exceed that of the right, resulting in LBBB aberrant condcution (crossover of refractory periods).
Please, don't point arrows at the QRS as if they were the PACs. This is the most common mistake. Above there is a beautiful EKG with sinus rhythm and conducted PACs with aberrancy in the setting of prolonged QT interval.
I think it's time to apply what we've learned. In regards to the above EKG: Is this a PAC with aberrancy or a PVC?
Here is a sample of a beautiful EKG with several types of PACs. Courtesy of @jeffrey_vinocur
Another good sample. Courtesy of @MaruanCarlos
Another beauty. Courtesy of @narrowQRS
Here is a nice sample of PACs with different degrees of aberrancy. Courtesy of @SergioPinski
Interpolated PACs. Courtesy of @PPodrid
On very rare occasions, an atrial premature impulse is interpolated and does not disturb the normal sinus rhythm. This happens when the tissue surrounding the SAN is abnormal and prevents the atrial premature impulse from reaching and discharging the SAN (entrance block).
Another sample with more than one atrial premature complex.
Possible causes:
- Normal findings
- Anxiety
- Stress
- Organic heart disease
- Hypokalemia
- Hypomagnesemia
- Smoking
- Abuse of caffeine or alcohol
- Others
WELL, that's it for today's article. I hope it has been useful to you. If you have a friend or colleague who has asked you about this topic, send them this article. See you at the next one.
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