Altitude-related variations in heart rate variability among native Sikkimese: A cross-sectional study

Background & objectives Living at high altitudes causes chronic exposure to hypoxia, which triggers various physiological and autonomic adaptations, and many residents show successful acclimatization. Long-term exposure leads to time-dependent alterations in autonomic nervous system function, even in healthy individuals. Heart rate (HR) variability has long been a valuable tool for assessing autonomic activity, yet only a few studies have examined its association with altitude in healthy populations. This study assessed the impact of altitude on cardiac autonomic activity through HR variability analysis in healthy Sikkimese natives. Methods A cross-sectional study was conducted among the Sikkimese population residing at high, intermediate and low altitudes of Sikkim. Two areas from each altitude category were selected. Based on the population of the selected areas, the sample size was distributed using probability proportional to size, sampling. Systematic random sampling was then used to select participants. For each participant, a 5-min ECG was recorded using lead II of a Power Lab system. HR variability analysis was performed to derive time and frequency-domain indices from spectral analysis of successive R-R intervals. Results We found significantly higher values of time domain HR variability indices including standard deviation of all normal-to-normal intervals (SDNN) and root mean square of successive differences between normal heartbeats (RMSSD) among the people residing at higher altitudes in Sikkim (P<0.001). Interpretation & conclusions Residents living at high altitudes exhibit enhanced parasympathetic cardiac activity compared to those residing at lower and intermediate elevations, reflecting a possible adaptive response to chronic hypobaric hypoxia.