Mechanism of failure of recovery from hypoxic apnea by gasping in 17- to 23-day-old mice

The mechanism of failure of autoresuscitation from hypoxic apnea in 17- to 23-day-old (weanling) Swiss Webster related/J mice was investigated by recording electrocardiogram (ECG) and ventilation in adult, weanling, and 11-day-old mice. Hypoxic apnea was induced with 97% N₂-3% CO₂. O₂ (21% or 50% O₂) or 97% N₂-3% CO₂ was given at the onset of apnea. The ECG showed no arrhythmias predictive of failure of autoresuscitation. The first indication of failure was a progressive fall in gasp volume ('run down'). This pattern also occurred in animals given continuous 97% N₂-3% CO₂ and was significantly different from that in mice that survived. Gasping duration in 97% N₂ was longer in weanlings than adults but shorter than in 11 day olds. Respiratory and heart rate recovery were more rapid in adults than in weanlings. Although recovery in high O₂ was more rapid, the survival rate was not increased. The lack of effect of high O₂ on survival and the virtually identical pattern of gasping in mice dying in 97% N₂ and air leads us to conclude that in mice that fail to autoresuscitate little or no O₂ reaches the medullary respiratory centers. We speculate that this may be due to increased vulnerability of cardiac muscle to anoxia in 17- to 23-day-old mice, resulting in early and severe heart failure.