Effects of whole body heating on dynamic baroreflex regulation of heart rate in humans

The purpose of this project was to identify whether dynamic baroreflex regu lation of heart rate (HR) is altered during whole body heating. In 14 subje cts, dynamic baroreflex regulation of HR was assessed using transfer functi on analysis. In normothermic and heat-stressed conditions, each subject bre athed at a fixed rate (0.25 Hz) while beat-by-beat HR and systolic blood pr essure (SBP) were obtained. Whole body heating significantly increased subl ingual temperature, HR, and forearm skin blood flow. Spectral analysis of H R and SBP revealed that the heat stress significantly reduced HR and SBP va riability within the high-frequency range (0.2-0.3 Hz), reduced SBP variabi lity within the low-frequency range (0.03-0.15 Hz), and increased the ratio of low-to high-frequency HR variability (all P < 0.01). Transfer function gain analysis showed that the heat stress reduced dynamic baroreflex regula tion of HR within the high-frequency range (from 1.04 <plus/minus> 0.06 to 0.54 +/- 0.6 beats.min(-1).mmHg(-1); P < 0.001) without significantly affec ting the gain in the low-frequency range (P = 0.63). These data suggest tha t whole body heating reduced high-frequency dynamic baroreflex regulation o f HR associated with spontaneous changes in blood pressure. Reduced vagal b aroreflex regulation of HR may contribute to reduced orthostatic tolerance known to occur in humans during heat stress.