Cardiovascular physiological effects of balneotherapy: focused on seasonal differences

Hot water bathing has been demonstrated to be an effective way to improve people's cardiovascular health in many studies. This study focused on seasonal physiological changes to provide suggestions on bathing methods based on season for hot spring bathing. Volunteers were recruited to the program of hot spring bathing at 38-40 °C in New Taipei City. Cardiovascular function, blood oxygen, and ear temperature were observed. There were five assessments for each participant during the study process: baseline, bathing for 20 min and 2 cycles *20 (2*20) min, resting for 20 min and 2*20 min after bathing, respectively. Lower blood pressure (p < 0.001), pulse pressure (p < 0.001), left ventricular dP/dt Max (p < 0.001), and cardiac output (p < 0.05) were identified after bathing then rested for 2*20 min in four seasons, compared to baseline by paired T test. However, in multivariate linear regression model, potential risk for bathing in summer was assumed by higher heart rate (+28.4%, p < 0.001), cardiac output (+54.9%, p < 0.001) and left ventricular dP/dt Max (+27.6%, p < 0.05) during bathing at 2*20 min in summer. Potential risk for bathing in winter was postulated by blood pressure lowering (cSBP -10.0%; cDBP -22.1%, p < 0.001) during bathing at 2*20 min in winter. Hot spring bathing is shown to potentially improve cardiovascular function via reducing cardiac workload and vasodilation effects. Prolonged hot spring bathing in summer is not suggested due to significantly increased cardiac stress. In winter, prominent drop of blood pressure should be concerned. We demonstrated the study enrollment, the hot-spring contents and location, and physiological changes of general trends or seasonal variations, which may indicate potential benefits and risks during and after bathing. (Abbreviations: BP, blood pressure; PP, pulse pressure; LV, left ventricular; CO, cardiac output; HR, heart rate; cSBP, central systolic blood pressure; cDBP, central diastolic blood pressure).