Development of the ventilatory response to hypoxia in Swiss CD-1 mice

We examined developmental changes in breathing pattern and the ventilatory response to hypoxia (7.4% O-2) in unanesthetized Swiss CD-1 mice ranging in age from postnatal day 0 to 42 (P-0-P-42) using head-out plethysmography. The breathing pattern of P-0 mice was unstable. Apneas were frequent at P-0 (occupying 29 +/- 6% of total time) but rare by P-3 (5 +/- 2% of total time). Tidal volume increased in proportion to body mass (similar to 10-13 ml/kg), but increases in respiratory frequency (f) (55 +/- 7, 130 +/- 13, and 207 +/- 20 cycles/min for P-0, P-3, and P-42, respectively) were responsible for developmental increases in minute ventilation (690 +/- 90, 1,530 +/- 250, and 2,170 +/- 430 ml.min(-1).kg(-1) for P-0, P-3, and P-42, respectively). Between P-0 and P-3, increases in f were mediated by reductions in apnea and inspiratory and expiratory times; beyond P-3, increases were due to reductions in expiratory time. Mice of all ages showed a biphasic hypoxic ventilatory response, which differed in two respects from the response typical of most mammals. First, the initial hyperpnea, which was greatest in mature animals, decreased developmentally from a maximum, relative to control, of 2.58 +/- 0.29 in P-0 mice to 1.32 +/- 0.09 in P-42 mice. Second, whereas ventilation typically falls to or below control in most neonatal mammals, ventilation remained elevated relative to control throughout the hypoxic exposure in P-0 (1.73 +/- 0.31), P-3 (1.64 +/- 0.29), and P-9 (1.34 +/- 0.17) mice but not in P-19 or P-42 mice.