Maturation of cardiorespiratory interactions in spontaneous recovery from hypoxic apnea (Autoresuscitation)

The maturation of the process of spontaneous recovery from hypoxic apnea by gasping (autoresus-citation) was characterized in adult, weanling (18-22 d), and infant (5 and 10 d) BALB/c mice. ECG and respiration were recorded and hypoxic apnea was induced with 97% N₂-3% CO₂. Administration of 21 % O₂ at onset of hypoxic apnea resulted in successful autoresuscitation in all 63 animals tested. In all mice, three sequential stages occurred: 1) gasping with marked bradycardia, 2) rapid increase in heart rate (cardiac resuscitation), and 3) increase in respiratory rate (respiratory resuscitation). Despite these similar stages, marked maturational differences were apparent. The times to cardiac and respiratory resuscitation were longer in the pups than in the older mice, whereas the gasp frequency before cardiac resuscitation was lower in the infants. Resuscitation time correlated with gasp interval (1/gasp frequency). In addition, the initial bradycardia at onset of gasping was more pronounced in the pups. A 30- or 60-s delay in oxygen administration after onset of hypoxic apnea caused a marked decrease in the adults' ability to autoresuscitate, without affecting that of the pups. A comparison of gasp frequency in 21% O₂ versus 97% N₂-3% CO₂ showed that the presence of oxygen increased gasp frequency in the pups, but had no effect on gasp frequency in the weanlings or adults. We conclude that autoresuscitation is equally effective in both infant and adult BALB/c mice but only when oxygen is available before the onset of hypoxic gasping. Accordingly, the adult’s ability to autoresuscitate rapidly can be viewed as an adaptation required by a limited tolerance for prolonged hypoxia. In contrast, it appears that the infant mouse's greater tolerance of hypoxia allows a slower autoresuscitation with a lower heart rate and gasp frequency. This conserves energy, thus facilitating prolonged survival and enabling recovery when O₂ availability is delayed.