Apnea threshold and breathing rhythmicity in newborn lambs

This study was designed to determine the effect of the removal of chemical stimuli on breathing rhythmicity in awake newborn lambs; it was also designed to define the chemical threshold below which breathing would stop [arterial PCO2 (PaCO2) apnea threshold]. We used a technique of graded extracorporeal CO2 removal with apneic oxygenation in three groups of animals according to age and carotid body (CB) integrity: < 2 days, CB intact (n = 5); 12 days, CB intact (n = 7); and 12 days, CB denervated (CBD; n = 5). In all animals, whatever their age and CB status, suppression of the chemical drive resulted in sustained apnea. The study, performed at four constant levels of oxygenation (hyperoxia, normoxia, moderate hypoxia, and severe hypoxia), allowed precise determination of the PaCO2 apnea threshold. We found that this PaCO2 apnea threshold depended on the degree of postnatal maturation (it was higher in the younger lambs), the level of arterial oxygenation (it was lowered by hypoxia), and CB status (it was...