• A single contraction occurs earlier than the next expected sinus contraction.
• After the PAC, sinus rhythm usually resumes.

Rate: Depends on rate of underlying rhythm

Rhythm: Irregular whenever a PAC occurs

P Waves: Present; in the PAC, may have a different shape

PR Interval: Varies in the PAC; otherwise normal (0.12–0.20 sec)

QRS: Normal (0.06–0.10 sec)

Clinical Tip: In patients with heart disease, frequent PACs may precede paroxysmal supraventricular tachycardia (PSVT), atrial fibrillation (A-fib), or atrial flutter (A-flutter).