Shivering and cardiorespiratory responses during normocapnic hypoxia in the pigeon

To study the inhibitory effect of hypoxia on the cold defense mechanism, pigeons were exposed at low ambient temperature (5 degrees C) to various inhaled gas mixtures: normoxia [0.21 fractional concentration of O2 (FIO2)], hypoxia (0.07 FIO2), and normocapnic hypoxia (0.07 FIO2 + 0.045 FICO2). Electromyographic (EMG) activity indicative of shivering thermogenesis was inhibited during hypoxia, and body temperature (Tre) fell by 0.09 degrees C/min. Respiratory frequency (f) and minute ventilation (VE) increased by 143 and 135%, respectively, compared with normoxia, but tidal volume (VT) was not changed. PO2, PCO2, and O2 contents in the arterial and mixed venous blood were decreased and pH was enhanced. During normocapnic hypoxia, shivering EMG was present at approximately 50% of the normoxic intensity; Tre fell by only 0.04 degrees C/min. Arterial and mixed venous PCO2 and pH were the same as during normoxia, but VE increased by 430% because of twofold increases in both f and VT. During normocapnic hypoxia, arterial PO2 and O2 content were higher than during hypoxia alone. We conclude that the persistence of shivering during normocapnic hypoxia is due to maintenance of critical levels of arterial PO2 and O2 content.