Normoxic, hyperoxic, and hypoxic ventilation in rats continuously exposed for 60 h to 1 ATA O2

Continuous assessment of ventilation during normobaric hyperoxic exposure may help differentiate between the various effects of hyperoxia on ventilation. Ten rats were exposed continuously to hyperoxia for 60 h with intermittent measurement of ventilation in normoxia, a high-oxygen-mixture (FIO2 > 85%) and hypoxia (FIO2 approximately 1.5%). From the measured parameters of inspiratory and expiratory times (TI and TE) and tidal volume (VT), we calculated breathing frequency (f), minute ventilation (VI), inspiratory drive (VT/TI), TI/(TI+TE) and hypoxic ventilatory drive. Normoxic and high-oxygen-mixture VT increased and f decreased mainly due to increased TI, with no effect on normoxic VI. Hypoxic f and VI decreased as a function of exposure time. The fact that the increase in normoxic VT between 13 and 22 h is followed by an increase in TI between 22 and 30 h, excludes the possibility that sensitization of airway C-fiber receptors causes the increase in VT. There was no correlation between HVD and the inspiratory drive. There would appear to be two stages in O2 toxicity of the ventilatory system: in the first stage, the main effect is on the chemoreflex and mechanoreflex systems, whereas beyond 50 h there are other effects on the lung.