The ECG Rhythms content is currently under development and is being updated by medical professionals.
Premature atrial contractions (PACs) are ectopic heartbeats that originate in the atria at a location other than the sinoatrial (SA) node. These premature beats occur when an ectopic focus in the atrium generates an electrical impulse earlier than the next expected impulse from the SA node.
Analogy: “An unexpected musical note played slightly ahead of the regular rhythm.”
On an ECG, a PAC is characterized by a premature P wave that appears earlier than the next expected sinus P wave. This premature P wave may have a different morphology (shape) compared to the normal sinus P waves because it arises from an ectopic location within the atria. In some instances, the premature P wave may be hidden within the preceding T wave. The QRS complex that follows a PAC is usually narrow, as the electrical impulse typically travels through the normal ventricular conduction system. However, if the premature impulse reaches the ventricles when one of the bundle branches is still partially refractory, it can result in aberrant ventricular conduction, leading to a wider QRS complex (often resembling a right bundle branch block). Following a PAC, there is typically a noncompensatory pause, meaning the interval from the sinus P wave before the PAC to the sinus P wave after the PAC is shorter than two normal P-P intervals.
Premature atrial contractions often cause few or no symptoms. However, some individuals may perceive them as palpitations, a fluttering sensation in the chest, or a feeling that their heart has skipped a beat. PACs are generally benign, especially when they occur infrequently in individuals without underlying heart conditions. They can be triggered by various factors such as stress, caffeine, alcohol, and certain medications. PACs are more common in older adults and in individuals with conditions like chronic obstructive pulmonary disorder (COPD), congestive heart failure, diabetes, or high blood pressure.
“Like a musician momentarily rushing a beat, the atria fire off an early contraction.”