Acclimation to heat interpreted from the analysis of heart-rate variability by the multipole method

The Multipole Method is a recently developed method to describe time series with highly complex time evolution, such as cardiac rhythm. We hypothesized that applying this method in the analysis of heart rate variability (HRV) could provide valuable physiological information that usually cannot be extracted from the commonly used time and frequency domains analyses. The hypothesis was tested during a 12-day heat acclimation process on four young healthy, male subjects. On the first and the last day of acclimation, HRV was investigated by two different measures; the conventional Standard Deviation of Normal to Normal R-R intervals (SDNN), and the Multipole Method. The analyses revealed that stress on the cardiovascular system was still evident after the 12^thday of acclimation, but both methods showed that stress was diminished. Using the Multipole analysis, following acclimation to heat, the autonomic nervous system shows a more effective response. We conclude that the Multipole Method is a very valuable tool for investigating the series of R-R intervals, which are a non-stationary and non-linear complex time series, reflecting a complex regulatory physiological mechanism. Specifically, we showed that stress on the cardiovascular system diminishes following the course of acclimation, but not to its complete relief, which indicates that acclimation to heat is a much longer process than can be judged from the early phenotypic picture.