Atrial premature contractions are also due to enhanced automaticity and are usually distinguished by a premature P wave that differs in configuration from the sinus P wave. Depending on where the focus occurs, the PR interval may be the same, longer, or shorter than the normal PR interval. If the atrial focus occurs in the low atrium, the PR interval will be shorter and the P wave will be inverted in the inferior leads as depolarization occurs in a retrograde fashion. The QRS complex should remain normal unless the atrial premature contraction occurs during the relative or effective refractory period of the previous beat. During the effective refractory period, the atrial premature contraction will be blocked and only the P wave can be identified unless it is obscured by the previous T wave. The pause that occurs is less than compensatory owing to resetting of the sinus node. When the atrial premature contraction occurs during the relative effective refractory period, the QRS will be wide and usually of right bundle block pattern as this area may not yet have recovered.
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