During resting rhythm,measurements of conduction through the atrium (PA interval), atrioventricular (AV) node (AH interval), and His-Purkinje system (HV interval) are recorded (Fig. 1). This assessment is followed by atrial pacing, which allows for assessment of sinus node function through the sinus node recovery time (Fig. 2). This is expressed as the longest return cycle after the cessation of atrial pacing and is corrected for the underlying sinus rate. Corrected sinus node recovery times in excess of 525 milliseconds are indicative of abnormal sinus node automaticity. This finding is highly specific for sinus node dysfunction, but this and other techniques are not sensitive predictors and identify only about 50% of cases of proven sinus node disease.