Premature Atrial Contractions (PACs), Animation.

Premature Atrial Contractions (PACs), Animation.


Premature atrial contractions, PACs, are premature
heartbeats originating in one of the upper chambers of the heart, the atria. PACs are common among patients with lung disorders,
such as chronic obstructive pulmonary disease, COPD, but they also often occur in healthy
people. PACs may be caused or worsened by caffeine,
alcohol use, and certain medications. Apart from occasional palpitations, PACs are
generally asymptomatic and do not require treatment in otherwise healthy people. A PAC happens when the atria are activated
by an ectopic site in an atrium, instead of the SA node. Because atrial depolarization is initiated
outside the SA node, the associated P wave has an unusual, non-sinus morphology. An early atrial activation may cause the P
wave to merge with the preceding T wave producing a peaked fusion wave. In situations where the ectopic site is located
near the AV node, the atria are depolarized mainly by retrograde conduction and the resulting
P wave is inverted; the PR interval representing the time the signal reaches the AV node, is
slightly shorter. An ectopic atrial activation can usually enter
the SA node, depolarize it and reset its timing, causing a so-called non-compensatory pause. On an ECG, this is seen as changes in the
PP intervals that contain the ectopic beats. This feature can be used to differentiate
PACs from ectopic beats of ventricular origin, PVCs. PVCs typically do NOT conduct back to the
atria, SA node firing is NOT affected, and PP interval remains unchanged. The downstream ventricular conduction of a
PAC can be normal, aberrant or absent. In most cases, conduction through the AV node
and ventricles is not affected, resulting in a normal narrow QRS complex. Occasionally however, ventricular conduction
may be aberrant, causing a widened QRS complex, usually with right bundle branch block morphology,
which may look similar to a premature beat of ventricular origin. In this case, differentiation is made based
on the presence of a preceding P wave and a non-compensatory pause in PAC. A non-conducted PAC is one that arrives too
early to the AV node, at the time when the AV node is still in refractory period and
thus cannot be activated.