| Rate | Normal or accelerated |
| P wave | Premature, positive and abnormal shape because they originate from an ectopic pacemaker |
| PR Interval | Normal or longer |
| QRS | Normal, 0.10 sec or less |
| Rhythm | Irregular - PAC's occur early in the cycle and they usually do not have a complete compensatory pause. |
Premature atrial contractions (PACs) are the most common type of cardiac arrhythmias, often occuring in healthy individuals without heart disease. Patients with PACs may complain of palpitations or a sense of skipped beat. While PACs are usually benign, they can precede, atrial flutter, atrial fibrillation, or supraventricular tachycardia.
A PAC is an atrial contraction initiated by an action potential originating in an atrial cardiomyocyte, but outside of the usual sinoatrial node (SA node). There are a many factors that can cause a cardiomyocyte to depolarize before the SA node can generate its next action potential. The cause of PACs is often idiopathic, but stress or stimulants such as caffeine, tobacco, and alcohol can increase the frequency of PACs
The P-waves of PACs are typically a different shape from those generated in the SA node. This is because the duration and path to the (+) electrode will be different from those generated by the SA node.
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P-waves are premature, positive and abnormal shape because they originate from an ectopic pacemaker.