The effect of various breath-hold techniques on the cardiorespiratory response to facial immersion in humans.

immediately before facial immersion. There were no differences in blood pressure, cardiac output, heart rate, haematocrit or haemoglobin between each BHT and control conditions (P > 0.05). In conclusion, the duration of apnoea can be extended by manipulating blood gases through repeated prior breath-holds, but changes in cardiac output and red blood cell mass do not appear essential.