No evidence for hypothalamic cooling during nasal cold air breathing in man

To test the hypothesis that the reduction of ventilatory response to CO2 during nasal breathing of cold air is due to cooling of the hypothalamus, four adult male volunteers breathed either cold air (3 degrees C) or warm air (33 degrees C) through the nose by means of a face mask. Ventilation was increased to a mean of 30 l/min by increasing inspired CO2 concentration, which was adjusted so as to match ventilation in the two conditions. Cold air through the nose reduced the ventilatory response to CO2 so that with cold, higher inspired CO2 concentrations were required to produce the same ventilation as during warm air breathing. Rectal temperature was used as a measure of "core temperature", and tympanic membrane temperature as an indirect index of brain temperature. A hand immersion water calorimeter was used to measure the peripheral vascular response to hypothalamic regulatory changes. There was no measurable change in hand heat loss, rectal, or tympanic membrane temperature. This suggests that the depressant effect of cold air breathing is mediated by skin and mucosal temperature receptors connected to the respiratory centres rather than by direct cooling of the hypothalamus.