To explore the value of frequency domain indices of heart rate variability (HRV) in the differential diagnosis between pediatric vasovagal syncope and postural tachycardia syndrome (POTS).

Eighty-five patients aged 7-16 years with either vasovagal syncope or POTS were enrolled in the experimental group; 18 healthy children served as controls. Holter electrocardiography was used to detect HRV frequency-domain indices in patients with vasovagal syncope, patients with POTS, and control subjects. The differences in HRV indices were compared between the vasovagal syncope and POTS groups. The receiver operating characteristic (ROC) curve was calculated to analyze the predictive value of HRV for the differential diagnosis between vasovagal syncope and POTS in children. In addition, 37 children aged 7-17 years with either vasovagal syncope or POTS were recruited as an external validation group.

The daytime ultra-low frequency (dULF), nighttime ULF (nULF), daytime very low frequency (dVLF), and nighttime VLF (nVLF) were higher in the vasovagal syncope group compared with the POTS group (P < .01 for dULF, dVLF, and nVLF; P < .05 for nULF). The dULF, nULF, dVLF, and nVLF yielded a sensitivity of 73.3%, 71.1%, 68.9%, and 62.2%, respectively, and a specificity of 72.5%, 62.5%, 60.0%, and 60.0%, respectively, to differentiate vasovagal syncope from POTS. The external validation with clinical diagnostic standard showed that a dULF cutoff value of 36.2 ms² for differentiating POTS from vasovagal syncope yielded a sensitivity of 71.4%, a specificity of 75.0%, and an accuracy of 73.0%.

dULF may be a useful measure for the differential diagnosis between vasovagal syncope and POTS in adolescents.