Altitude influence in the heart rate variability: Recurrence plot analysis

To analyse the responses of the autonomic nervous system (ANS) to different cardiac activity in different athlete’s performances and at different altitudes can provide knowledge about the mechanism of the neural control and regulation linked to sport. In this way, the heart rate variability (HRV), which can be measured as the time between consecutive heart beats, is an accurate and non-invasive measure of the ANS. The effect of ANS on the cardiac activity is done through two branches, sympathetic stimulation and parasympathetic or vagal stimulation path. To probe how the cardiac system adjusts its rhythm depending on the subject physical condition and on the altitude effect, heart rate variability (HRV) was studied in time-domain (TD) and frequency-domain (FD) based processing techniques. Furthermore, in order to describe certain dynamics processes of the cardiac system, some non-linear time-domain signal processing methods have been applied. A total of 29 subjects (13 high trained and 16 low trained athletes) were tested doing incremental exercise in two different altitudes, Girona (sea level) and Masella (mountain altitude, 1800m approximately). HRV was assessed by linear TD indices defined in [Tas96], those estimated using auto correlation function (ACF) and the spectral measures resulting from power spectral density (PSD) analysis, as well as, non-linear TD indices derived from recurrence plots (RP), auto mutual information function (AMIF) and entropy information. The results suggest that there are significant differences of the HRV performances when it is studied at sea level and at mountain altitude, and also differences at ANS responses. Some statistical differences were observed concerning to high and low trained athletes. However, when athletes with better conditions (high VO2max) were compared to those with worse conditions, it could be seen that HRV at sea level presents more complexity behaviour (less regularity and less complexity behaviour (more regularity) at mountain altitude, both for HighVO2max group. The statistical diagnostic analysis has given that indices from non-linear time-domain analysis mainly based on recurrence quantification analysis are the ones that better discriminated the analysed groups when comparing training performances, altitude levels, and athletes with different heath conditions (VO2max)