Short‐term heart rate variability indices at rest and during the first ten minutes of a tilt table test predict test outcome in females with unexplained syncope but not in males

Background In the present study, we tested the hypothesis that amongst patients with unexplained syncope, heart rate variability (HRV) indices during supine rest and the early phase of head‐up tilt would identify subjects likely to have a positive test. Methods We determined mean heart rate, standard deviation of normal‐to‐normal RR intervals (SDNN), low frequency (LF) and high frequency RR spectral powers from stationary RR tachograms obtained from ECG taken during supine rest and between 6–10 minutes of 70° drug‐free head‐up tilt table tests of 33 males and 26 age matched females with unexplained syncope. We grouped patients on the basis of gender and test outcome. Results SDNN and LF power during supine rest and head‐up tilt were significantly lower in females with a positive test compared to females with a negative test ( P < 0.05 for each). In females with syncope, the index that best predicted a positive test outcome was a resting SDNN ≤ 20 ms (sensitivity of 83%, specificity of 75%). In females, a SDNN > 20 ms during supine rest predicted a negative test with an accuracy of 88%. In contrast, none of the HRV indices differed significantly between males with a positive or negative test. Conclusions Our results indicate that in patients with unexplained syncope, short‐term HRV indices at rest and during the first ten minutes of head‐up tilt predict test outcome in females but not in males. Support The authors thank Defense Institute of Physiology and Allied Sciences, Delhi, for financial support.