Ventilatory response to hypoxia in unanesthetized newborn kittens

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Abstract
We studied the ventilatory response to hypoxia in 11 unanesthetized newborn kittens (n = 54) between 2 and 36 days of age by use of a flow-through system. During quiet sleep, with a decrease in inspired O2 fraction from 21 to 10%, minute ventilation increased from 0.828 .+-. 0.029 to 1.166 .+-. 0.047 l .cntdot. min-1 .cntdot. kg-1 (P < 0.001) and then decreased to 0.929 .+-. 0.043 by 10 min of hypoxia. The late decrease in ventilation during hypoxia was related to a decrease in tidal volume (P < 0.001). Respiratory frequency increased from 47 .+-. 1 to 56 .+-. 2 breaths/min, and integrated diaphragmatic activity increased from 14.9 .+-. 0.9 to 20.2 .+-. 1.4 arbitrary units; both remained elevated during hypoxia (P < 0.001). Younger kittens (< 10 days) had a greater decrease in ventilation than older kittens. These results suggest that the late decrease in ventilation during hypoxia in the newborn kitten is not central but is due to a peripheral mechanism located in the lungs or respiratory pump and affecting tidal volume primarily. We speculate that either pulmonary bronchoconstriction or mechanical uncoupling of diaphragm and chest wall may be involved.