Evolution of the Heart Rate Variability complexity during Kangchenjunga climbing

At high altitude there is a reduced oxygen pressure in the atmosphere which results in physiological changes. Heart Rate Variability (HRV) is a technique to quantify the autonomic nervous system (ANS) regulation of the heart rate, allowing a noninvasive assessment of the ANS in extreme environments. The aim of this work was to assess the evolution of the HRV complexity during Kangchenjunga (8.586 m) climbing. Five spanish climbers recorded their RR-interval time series every day during the expedition. We divided the data in different stages: Spain Baseline, Kathmandu Baseline, Acclimation Trekking, Kathmandu After Acclimation, Base Camp 1, Base Camp 2, Summit, Base Camp 3, and Kathmandu after expedition. At the submission time we had acces only to complete recordings from one climber. We assessed the complexity of HRV using sample entropy (SampEn) and normalized compression distance (NCD), which exploits linear and nonlinear relations in the data and allows the comparison of sequences of different sizes. We estimated the dissimilarity of every stage in the climb against the first stage. From the beginning and during acclimation dissimilarity (NCD) increased and then decreased once the climbers were acclimated. Dissimilarity jumped up in Base Camp stage and then decreased from that point until the end of the expedition. Both indices showed an initial change in the complexity until the Summit and from the Summit both showed a tendency to recover the complexity of the HRV. Results showed that NCD is able to measure the changes in complexity with a little more detail and smoother than SampEn.