Kao T, Yu BC, Kong CW, Chen CY, Chiang BN. Institute of Medical Engineering, Yang-ming Medical College, Taipei. Pacing Clin Electrophysiol 1988;11(8):1168-75. A noninvasive method employing the technique of signal averaging has been developed for recording sinoatrial (S-A) activity (pre-P wave). Recordings were obtained in man at the time of right heart catheterization. A bipolar esophageal electrode was utilized to record the prominent P waves that were used for triggering in the averaging process. After summing 150 beats, which had coefficients of correlation 0.97 or larger, deflections of small amplitude (less than 40 microV) were obtained preceding the atrial activity. Direct catheter recordings were also taken for comparison. With the new noninvasive method, the sinoatrial conduction time (SACT) estimated for patients with sick sinus syndrome (SSS) was 106 +/- 24 ms (101 +/- 20 ms for the same group measured directly). The SACT for healthy subjects used as the control group was 55 +/- 18 ms. There was good correlation between the pre-atrial activity recorded noninvasively by the esophageal electrode method and invasively from the direct catheter method. The linear correlation coefficient between these two techniques was 0.89 (P less than 0.001) in 17 patients.