• A premature atrial beat or contraction (PAC) is generated by an irritable impulse from an atrial automaticity focus that is a bit earlier than the expected sinus impulse. As a result, the expected sinus impulse cannot be conducted because AV junction will enter refractory period once it reaches it.
• The premature atrial beat consists of new P’ waves that appear differently than sinus P waves, as well as normal QRS complex (conduction takes place down the AV-His-Purkinje pathway).
• The next sinus impulse that follows the premature atrial beat will resume normal sinus rhythm.
• In somce cases, AV node might still be in refractory period when the premature impulse reaches it.
• As a result, there is no conduction to the ventricles, hence no QRS complex following the new P’ wave.
• This is also referred to as "blocked PAC" or "dropped PAC".
• Coupling of a premature atrial beat to one or two normal sinus beat(s) are called atrial bigeminy or atrial trigeminy, respectively.
• Each coupling is called “couplet”.
