Natural cooling of the brain during outdoor bicycling?

Tympanic membrane temperature (Ttymp) and deep esophageal temperature (Tes) were measured in 8 subjects during normal outdoor bicycling. Metabolic rate (VO2) was determined by the Douglas bag method. Heart rate was sampled continuously. Skin surface temperatures were measured at the forehead, chest and shoulder, and core temperatures in the deep esophagus and at the tympanic membrane using a radio telemetry system. For each outdoor experiment an indoor experiment in a climatic chamber, adjusted to the same air temperature but in still air, was performed. The subjects exercised at the same VO2 as in the outdoor trial on a stationary bicycle ergometer. Measurements were taken with the same equipment as in the outdoor experiments. O2-consumption (l.min-1) and heart rates (beats.min-1) were similar during outdoor and indoor bicycling, averaging 2.38 +/- 0.018 (SE) and 2.26 +/- 0.07, 141 +/- 7 and 147 +/- 8, respectively. During steady state Tes was the same during outdoor and indoor bicycling (37.95 degrees C), while Ttymp was significantly lower during outdoor bicycling. delta (Tes-Ttymp) was 1.25 degrees C during outdoor and 0.5 degrees C during indoor exercise. It is concluded that, if tympanic temperature is lowered by counter-current cooling of its arterial supply, then cooling of the brain may also take place in humans during physical activity under normal outdoor conditions with convective air movements. But the magnitude of a possible brain cooling cannot be deduced from the fall in tympanic temperature.