To investigate the changes in cardiovascular responses by different breathing frequencies between a group of seven middle-aged (68±7 years) and five young (21±2 years) healthy subjects during head-out water immersion. Electrocardiograms recorded continuously data at four different breath frequencies, 4, 6, 10 and 15 cycle per minute (4C, 6C, 10C and 15C), at three levels of immersion, head-out navel, breast and shoulder where a given value of end-tidal PCO₂ (PETCO₂) when the subjects spontaneously breathed was regulated in all conditions with fixed tidal volume (VT) of 1.5 L. Beat-by-beat heart rate (HR), stroke volume (SV), cardiac output (Q), systolic and diastolic blood pressure (SBP and DBP), and breath-by-breath VT and PETCO₂ were simultaneously measured during the experiment. During water immersion, HR was decreased significantly in young group, whereas HR was not altered in middle-aged group. By contrast, SV was increased significantly in young group and while these changes did not reach the significant level in middle-aged group. Consequently, Q showed a tendency to increase in middle-aged group during water immersion, not in young group. SBP and DBP were not markedly changed in middle-aged group. These findings suggested that cardiac parasympathetic nerve activity was enhanced and sympathetic nerve activity was suppressed at head-out water immersion in young group, otherwise, in middle-aged group, cardiac autonomic nerve activity would not markedly be changed by head-out water immersion. These responses were unaffected by the different breath frequencies. Four middle-aged subjects developed arrhythmias, e.g., premature ventricular (PVC), and premature atrial contraction (PAC) at higher head-out water immersion. We concluded that increased water pressure during immersion could induce a higher preload into the heart, i.e., increased Q in middle-aged group, subsequently might provoke arrhythmias.

DESCENTE SPORTS SCIENCE Vol.23/THE DESCENTE AND ISHIMOTO MEMORIAL FOUNDATION FOR THE PROMOTION SPORTS SCIENCE