Immersion in CO2-rich water containing NaCl diminishes blood pressure fluctuation in anesthetized rats

Remarkably, bathing in hot springs containing high concentrations of carbon dioxide (CO2) influences cardiovascular function more than bathing in fresh water. The CO2-enriched water in hot springs generally contains many salts, whose interactions remain unknown. We separately evaluated the actions of individual factors in CO2-enriched water and confirmed that CO2 and NaCl have combined effects on blood pressure fluctuations in anesthetized rats. Animals equipped with sensor probes to monitor body temperature, skin blood flow and arterial blood pressure were immersed in bathwater (35 degrees C) containing CO2 with NaCl, KCl or sucrose. The effects of these factors on cardiovascular function were evaluated using power-spectral analysis of fluctuations in blood pressure and heart rate. Compared with immersion in tap water, heart rate and skin vascular resistance were reduced during immersion in CO2-enriched water irrespective of the presence of other components. In terms of the very low frequency range (0.02-0.195 Hz), the power of blood pressure fluctuation during immersion was significantly reduced when the CO2-enriched water contained more than 1.5% NaCl but was not influenced by other components of similar osmotic pressure and the same specific gravity. The results indicated that the coexistence of CO2 and sodium ions in bathwater reduce blood pressure fluctuations, and suggested that this combination effect of CO2 and salt contributes to the sedative effect on human cardiovascular functions while bathing in CO2-hot springs.