Occasionally, a premature P wave may be so early that it does not conduct to the ventricle and is not followed by a QRS complex; this is a non-conducted PAC. The PAC typically conducts into and “resets” the sinus node, with a subsequent pause followed by the next sinus beat. The interval between the sinus P waves that precede and follow the non-conducted PAC, is termed a non-compensatory pause, and is identified by an inability to march the patient’s basic sinus PP interval through the pause.
Non-conducted PAC’s are usually benign and should be differentiated from non-conducted on-time P waves of AV block; the latter P waves are not premature and do not differ in morphology from other sinus beats. The following animation illustrates what we have just described.