The process of cardiorespiratory autoresuscitation in intact newborn rats

To examine the process of spontaneous autoresuscitation and the recovery of the hypoxic ventilatory response (HVR) after prolonged anoxia, we monitored respiratory frequency (f, by body plethysmography) and heart rate (HR, by ECG) in intact newborn rats (n = 12, day 2-4) before, during, and after 100% N2 exposure. The rat before anoxia showed signs of HVR: f changes at acute hypoxia (10% O2) and hyperoxia (100% O2). During anoxia, the spontaneous respiratory movement "gasping" appeared for 21 min (mean). At O2 restoration (with 100% O2), gasping stopped and no respiratory flow was detected for 1 min. One rat failed to autoresuscitate and had heart arrhythmia during the transient apnea, but 11 rats recovered respiration after the HR acceleration. Despite the successful autoresuscitation, the rats did not show HVR at 10 min into the recovery period and the recovery of HVR required more than 30 min. The results indicate that O2 inhalation is useful to trigger autoresuscitation even when the rat has already been in a state of profound hypoxic depression, but the rat becomes transiently insensitive to HVR after autoresuscitation. We estimate that reform of the respiratory control system in newborn rats is not yet firmly established to track HVR early in the recovery phase after prolonged anoxia.