Effects of Short‐term Warm Water Bathing on Cardiac Baroreflex Sensitivity in Healthy Men

Abstract

Bath‐related fatal (sudden cardiac arrest) and non‐fatal accident (dizziness and disturbance of consciousness) have been reported frequently in Japan. Cardiac baroreflex sensitivity (BRS) may relate to these symptoms. However, underlying mechanism(s) is poorly understood. With this information as background, we determined the effects of short‐term warm water immersion on cardiac BRS in healthy men. Twelve apparent healthy men underwent the single sit‐stand test (e.g., 5‐min sitting on the floor of bathtub and then standing up and remained standing for 5 min quietly) twice (e.g., without and with 41 °C warm water at the heart level). Beat‐to‐beat blood pressure was continuously recorded through the protocol (via Finometer). BRS was evaluated by the sequence method from heart rate response to the standing‐induced abrupt drop of mean arterial pressure (MAP). Additionally, after the each test, brachial‐ankle pulse wave velocity (baPWV: an index of arterial stiffness) was measured in the supine position. During the bathing, heart rate and cardiac output (estimated by Modelflow method) were significantly increased but MAP, stroke volume, total peripheral resistance, and tympanic temperature did not change significantly (vs. pre‐bathing). Regarding the single sit‐stand test, there were no significant changes in the baseline (sitting) MAP (from 73±16 to 77±16 mmHg, P=0.322) and the standing‐induced decreases in MAP (from −33±12 to −33±13 mmHg, P=0.970) between pre‐ and post‐bathing, but MAP‐HR slope was blunted after the bathing (from −0.47±0.26 to −0.35±0.25 bpm/mmHg, P=0.024, Figure 1). The baPWV was significantly decreased after the bathing (vs. pre‐bathing (from 12.2±1.2 to 11.8±1.2 m/sec, P=0.002). Change in MAP‐HR slope correlated with corresponding change in baPWV (r=0.610, P=0.036, Figure 2). Our results indicate that: (1) even a short‐term warm water bathing deteriorates cardiac BRS although it did not induce greater orthostatic hypotension, and (2) the change in cardiac BRS with the short‐term bathing is associated with the change in arterial stiffness. Further study to determine the impact of prolonged warm water bathing is needed.Support or Funding InformationThis study was supported by Japan Health & Research Institute Research Grant.Cardiac barorefex sensitivity (BRS) gain and brachial‐ankle pulse wave velocity (baPWV) before and after 5‐min warm‐water bathing. Data are mean and SD. Thin lines indicate individual data.Figure 1Relationship between 5‐min bathing induced changes in baPWV and cardiac BRS gain.Figure 2