Premature atrial contractions (PACs)

Most common atrial arrhythmia and it can originate at any site in the atria.

P wave morphology varies with the site of origin of the premature atrial contraction.

Have abnormal P wave morphology with a QRS morphology similar to sinus beats, in most cases.

P wave morphology of the PAC differs from the sinus P wave unless the premature complex originates in the high right atrial area adjacent to the sinus node, in which case the PAC may be difficult to distinguish from sinus arrhythmia.

When PACs occur early in the diastolic phase P waves may not be obvious on an EKG as it is often hidden in the preceding T wave, but the P wave may be noted in notched or peaked T waves.

PACs may be so premature that they may not conduct to the ventricles if the AV node is refractory due to the conduction of the preceding sinus impulse.

PACs that are not conducted are blocked PACs, and can be confused with AV block.

Most PACs have a normal have a normal or prolonged PR interval.

The relationship of a PAC to the QRS complex depends on the site of origin of the PAC and the prematurity index.

The PR interval of a PAC is inversely related to its prematurity.

Most PACs are able to be polarize the sinus node, and they can usually reset sinus automaticity causing the subsequent pause following the PACs to be less than compensatory because of the sinus node fires earlier than expected.

Measurement of the P-P interval between the size P-wave preceding a PAC and the P wave following PAC is generally less than twice the basic sinus cycle length.

In some instances a PAC collides with the sinus impulse and fails to reset the sinus node, resulting in a full compensatory pause, similar to a PVC.

With PACs electrical depolarization below the AV node is normal, and the QRS complex is unchanged.

Ab2242ant conduction may occur when the PAC reaches the infranodal tissue during the period when it is still partially refractory.

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