To observe the changes of cerebral blood flow resistance(R_CBF) in humans during head-out immersion, whole body (including head and face) immersion at sitting and prone positions, and underwater swimming.

Using telemetering device, we recorded ECG and blood flow velocity in the common carotid artery during the above four different breath-holding diving maneuvers. Resistance index(RI) and pulsatility index (PI) that could reflect changes of R_CBF were then calculated from blood flow velocity parameters.

RI and PI decreased with time during head-out immersion and two whole body immersions, and at the same time the cerebral blood flow would increase significantly due to the increase of blood flow velocity. However, RI and PI did not show marked change during underwater swimming, meanwhile the increase of blood flow velocity in the carotid artery was also insignificant, thereby the cerebral blood flow volume increase would be inconsiderable. This result is considered due to the redistribution of blood flow to four exercising limbs during underwater swimming, Moreover, change of R_CBF during whole body immersion at sitting position was not different from that at prone position. May be it is because that the distribution of blood and body fluid is relatively even in the body underwater and the change of central blood volume is insignificant, hence there will be no marked posture-related circulatory adjustments. Similarly, change of R_CBF during head-out immersion was not different from that during whole body immersion. It is possible that the water temperature was not so low as to stimulate the head and face to cause a change of R_CBF.