Effects of carbon dioxide inhalation on physiological responses to cold

Experiments were conducted to assess the thermoregulatory effects of CO2 breathing during cold exposures. Four men and two women (19-35 yrs old), clad in bathing suits, rested for 75 min on three occasions in a thermoneutral environment (29 degrees C) and on three occasions in the cold (5 degrees C). During these exposures the subjects breathed either: 1) air for 75 min, 2) air from 0-30 min, 4% CO2 in air from 30-60 min, and air from 60-75 min, or 3) 4% CO2 from 0-60 min and air from 60-75 min. Oxygen uptake (Vo2) was higher at 5 degrees C than at 29 degrees C due to shivering, but was not significantly effected by CO2 breathing. Rectal temperatures (Tre) were always higher in subjects exposed to 5 degrees C than 29 degrees C, declining with exposure time. Breathing 4% CO2 resulted in an acceleration in the decline of Tre in both environments. Neither Vo2 measurements nor accelerometer recordings of shivering supported previous reports of CO2-induced suppression of shivering. However, enhanced shivering may have occurred after cessation of CO2 breathing. Calculations of partitional heat exchange indicated that the greater decreases in Tre observed during CO2 exposures were due mainly to increased respiratory evaporative heat loss, total convective and conductive heat loss, probably all the result of hyperventilation.