Stanley, J., Halliday, A., D’Auria, S., Buchheit, M. and Leicht, AS. Effect of sauna-based heat acclimation on plasma volume and heart rate variability. Eur J App Physiol, In press.
Purpose: We investigated the effect of post-exercise sauna bathing on plasma volume (PV) expansion and whether such responses can be tracked by changes in heart rate (HR) based measures.
Methods: Seven, well-trained, male cyclists were monitored for 35 consecutive days (17 d baseline training, 10d training plus sauna, 8d training). Sauna exposure consisted of 30 min (87°C, 11% relative humidity) immediately following normal training. Capillary blood samples were collected to assess PV changes while resting seated. HR (HRwake) and vagal-related HR variability (natural logarithm of square-root mean squared differences of successive R−R intervals, ln rMSSDwake) were assessed daily upon waking. A sub-maximal cycle test (5 min at 125 W) was performed on days 1, 8, 15, 22, 25, 29, and 35 and HR recovery (HRR60s) and ln rMSSDpostex were assessed post-exercise. Effects were examined using magnitude-based inferences.
Results: Compared with baseline, sauna resulted in: 1) peak PV expansion after 4 exposures with a likely large increase [+17.8% (90% confidence limits, 7.4;29.2)]; 2) reduction of HRwake by a trivial-to-moderate amount [−10.2% (−15.9;−4.0)]; 3) trivial-to-small changes for ln rMSSDwake [4.3% (1.9;6.8)] and ln rMSSDpostex [−2.4% (−9.1;4.9)]; and 4) a likely moderate decrease in HRR60s [−15.6% (−30.9;3.0)]. Correlations between individual changes in PV and HR measures were all unclear.
Conclusions: Sauna-bathing following normal training largely expanded PV in well-trained cyclists after just 4 exposures. The utility of HR and HRV indices for tracking changes in PV was however uncertain. Future studies will clarify mechanisms and performance benefits of post-training sauna bathing.
Keywords: heat exposure; blood volume; cardiac parasympathetic activity; post-exercise; cyclists.