PREDOMINANT ROLE OF PERIPHERAL CHEMORECEPTORS IN THE TERMINATION OF APNEA IN MATURING NEWBORN LAMBS

Apneas are very common and normal in newborns but may become life thre atening if they are not terminated appropriately. The aim of this stud y in newborn lambs was to investigate the influence on apnea terminati on of postnatal maturation, peripheral chemoreceptor function, and hyp oxia. Apneas were induced by passive hyperventilation at varying inspi red Oz fraction levels. The apnea termination threshold Pco(2) (PATT(c o2)) was defined as the arterial P-co2 value at the first breath after the apnea. Three groups of awake intubated lambs were studied: I)inta ct lambs tested at both 1 and 15 days of life, 2) intact i-day-old lam bs with central tissue hypoxia induced by CO inhalation, and 3) i-day- old lambs with carotid body denervation (CBD). In individual lambs and regardless of age and carotid body function, there was a Po-2-Pco(2) response curve that was a determinant for the termination of an apnea. PATT(co2) invariably increased when arterial P-o2 increased, regardle ss of age. During hypoxia and normoxia, PATT(co2) was significantly lo wer in 15-day-old lambs compared with i-day-old lambs. No difference w as seen during hyperoxia. PATT(co2) values were shifted to higher leve ls after carotid body removal. Finally, hypoxia induced by either a lo w inspired Oz fraction or CO inhalation consistently failed to induce a depressive effect on the PATT(co2) even in CBD lambs. Ln conclusion, in awake newborn lambs, the Pco, level for apnea termination changed with postnatal age, and carotid body function was essential in lowerin g PATT(co2), thus protecting the lambs against prolonged apnea. Furthe rmore, hypoxia consistently failed to depress the reinitiation of brea thing after apnea, even in CBD lambs.