An interpolated PAC

Although most PACs reset the sinus node producing an incomplete compensatory pause, this PAC, indicated by the black arrow, is interpolated, i.e., sandwiched between two sinus beats. Note that the subsequent sinus P wave conducts with prolonged PR interval due to the relative refractoriness of the AV junction left by the PAC. Auscultation of the heart during this single PAC event would reveal three rapid beats in a row, suggesting a brief tachycardia.