title: Effect of sauna-based heat acclimation on plasma volume and heart rate variability.
abstract: OBJECTIVE
  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 days baseline training, 10 days training plus sauna, 8 days training). Sauna exposure consisted of 30 min (87 °C, 11 % relative humidity) immediately following normal training. Capillary blood samples were collected while resting seated to assess PV changes. 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 four 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 four exposures. The utility of HR and HRV indices for tracking changes in PV was uncertain. Future studies will clarify mechanisms and performance benefits of post-training sauna bathing.
authors.affiliation: Centre of Excellence for Applied Sport Science Research, Queensland Academy of Sport, Brisbane, Australia, j.stanley@uq.edu.au.
authors.forename: Jamie
authors.initials: J
authors.lastname: Stanley
authors.forename: Aaron
authors.initials: A
authors.lastname: Halliday
authors.forename: Shaun
authors.initials: S
authors.lastname: D'Auria
authors.forename: Martin
authors.initials: M
authors.lastname: Buchheit
authors.forename: Anthony S
authors.initials: AS
authors.lastname: Leicht
mesh_terms: D000222:Adaptation, Physiological; D000328:Adult; D015444:Exercise; D006339:Heart Rate; D006358:Hot Temperature; D006801:Humans; D008297:Male; D010953:Plasma Volume; D029002:Steam Bath / Q000009:adverse effects*