Negative pressure breathing and the control of skin blood flow during exercise in a hot environment

Factors which modify the relationship between body temperature and skin blood flow during exercise in heat were studied. Direct measurement of right atrial pressure during exercise in heat showed that the leveling off of forearm blood flow took place when blood temperature exceeded 38 degrees C and central venous pressure was lower than 6.3 mm Hg. Continuous negative pressure breathing increased the forearm and chest blood flow and the esophageal temperature at which leveling off was observed shifted from 37.7 degrees C to 38.0 degrees C. When the leveling off temperature was compared between subjects with high and low blood volume, the subject with low blood volume showed the leveling off of forearm blood flow at a temperature of 37.6 degrees C, while it was 38.0 degrees C in the subject with high blood volume. All these results suggest the involvement of cardiopulmonary mechanoreceptors, while further studies are required to clarify the mechanism which the leveling off of skin blood flow is observed at 38 degrees C of body temperature.