Received January 24, 2008
Revised February 13, 2008
Accepted after revision April 2, 2008
Human, Environmental & Exercise [250]
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Effects of chemoreflexes on hyperthermic hyperventilation and cerebral blood velocity in resting heated humans
Naoto Fujii 1,
Yasushi Honda 1,
Keiji Hayashi 2,
Narihiko Kondo 3,
Shunsaku Koga 4,
Takeshi Nishiyasu 1*
1 University of Tsukuba
2 University of Shizuoka
3 Kobe University
4 Kobe Design University
* To whom correspondence should be addressed. E-mail: nisiyasu{at}taiiku.tsukuba.ac.jp.
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Abstract |
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We tested the hypothesis that hyperthermic hyperventilation in part reflects enhanced chemoreceptor ventilatory O2 drive, and that the resultant hypocapnia attenuates ventilatory responses and/or middle cerebral artery mean blood velocity (MCAVmean) in resting humans. Eleven healthy subjects were passively heated for 50-80 min, causing esophageal temperature (Tes) to increase by 1.6°C. During heating, minute ventilation (VE) increased (P < 0.05), while end-tidal CO2 pressure (PETCO2) and MCAVmean declined. A hyperoxia test in which three breaths of hyperoxic air were inspired was performed once prior to heating and three times during the heating. When we observed hypocapnia (PETCO2 below 40 mmHg), PETCO2 was restored to the eucapnic level by adding 100%CO2 to the inspired air immediately before the last two tests. VE was significantly reduced by hyperoxia, and that reduction gradually increased with increasing Tes. However, the percentage decrease in VE from the normoxic level was small (20-29%) and unchanged during heating. When PETCO2 was restored to eucapnic levels, VE was unchanged, but MCAVmean was partially restored (28.1% at Tes = 37.6°C and 38.1% at Tes = 38.0°C, respectively) to the level seen prior to heating. These findings suggest that although hyperthermia increases chemoreceptor ventilatory O2 drive in resting humans, the relative contribution of the chemoreceptor ventilatory O2 drive to hyperthemic hyperventilation is small (~20%) and unaffected by increasing core temperature. Moreover, hypocapnia induced by hyperthermic hyperventilation reduces cerebral blood flow but not ventilatory responses.
Key Words:
Circulation, Respiratory control, Thermoregulation
