Components and mechanisms of thermal hyperpnea.
The pattern of breathing during a hyperthermia-induced hyperventilation varies across different species. Thermal tachypnea is a first phase panting response adopted during hyperthermia when tidal volume is minimized and the frequency of breathing is maximized. Blood-gas tensions and pH are maintained during this hyperventilation, and the associated heat loss helps the animal regulate its body temperature. A second pattern of breathing adopted in hyperthermia is thermal hyperpnea; this response is the focus of this review. This form of hyperventilation is evident after an increase in core temperature and it is apparent in humans. Increases of tidal volume as well as frequency of breathing are evident during this response that results in a respiratory alkalosis. The cause of thermal hyperpnea is not resolved; evidence of the potential mechanisms underlying this response support that modulators of the response act in either a multiplicative or additive manner with body temperatures. The details of the designs and methodologies of the studies supporting or refuting these two views are discussed. A physiological rationale for thermal hyperpnea is presented in which it is suggested this response serves a heat-loss role and contributes to selective brain cooling in hyperthermic humans. Ongoing research in this area is focused on resolving the mechanisms underlying thermal hyperpnea and its contribution to cranial thermoregulation. The direct application of this research is for the care of febrile and hyperthermic patients.
Laboratory for Exercise and Environmental Physiology, 8888 University Dr., School of Kinesiology, Simon Fraser Univ., Burnaby, British Columbia, Canada V5A 1S6. matt@sfu.ca
Matthew D
MD
White
D000818:Animals; D001831:Body Temperature / Q000502:physiology; D001833:Body Temperature Regulation / Q000502:physiology; D002628:Chemoreceptor Cells / Q000502:physiology; D005334:Fever / Q000150:complications* / Q000503:physiopathology*; D006801:Humans; D006863:Hydrogen-Ion Concentration; D006935:Hypercapnia / Q000503:physiopathology; D006985:Hyperventilation / Q000209:etiology* / Q000503:physiopathology*; D000860:Hypoxia / Q000503:physiopathology; D012123:Pulmonary Ventilation / Q000502:physiology; D015656:Respiratory Mechanics / Q000502:physiology; D013990:Tidal Volume / Q000502:physiology