Deng ZW, Huang ZZ, Liang YR. Chung Hua Hsin Hsueh Kuan Ping Tsa Chih 1989;17(1):23-5. The intracardiac electrogram recorded in upper, middle and lower right atrium showed a positive, biphasic and negative P wave, respectively. This concept has been widely used in positioning the electrode in transesophageal atrial pacing (TEAP). However, in examination of the P wave figure in the unipolar lead along the esophagus by each two cm distance in 100 normal adults, we found that 32 subjects did not fulfill the above-mentioned criteria partially or completely. In order to study the underlying mechanism of such phenomenon, 190 patients undergone TEAP were examined by simultaneous recording of 2 unipolar esophageal leads (interelectrode distance 2-3 cm) with surface ECG. The patients were classified into 3 groups according to the morphology relation between the upper and lower esophageal P waves: Group A consisting of 155 cases (81.6%) in which the fashion of P wave changes was consistent with the usual criteria; Group B 17 cases (8.9%) was not consistent with the criteria; Group C 18 cases (9.5%) was in a reversed manner. The PEP-PED interval (measured from the peak of P wave in proximal esophageal lead to that in distal esophageal lead) was used as an indicator of left atrium activation sequence in vertical direction. Its value in these three groups was 6.3 +/- 10.1, -6.8 +/- 6.9 and -15.7 +/- 7.3 ms, respectively (P < 0.001), suggesting in the latter two groups the left atrium activation was directed upward.