CENTRAL CHEMICAL-REGULATION OF RESPIRATION IN TERM NEWBORN

The role of the medullary H+-sensitive chemoreceptors on the drive of breathing was studied in 10 unanesthetized newborn animals (8 lambs and 2 kids). The experiment consisted of sequential measurements of ventilation (V̇) during a progressive change in the arterial pH (pHa) and in the pH of the cisternal cerebrospinal fluid (pHcsf), induced by intravenous infusion of hydrochloric acid (HCl) followed after an 8-h steady state of acidosis by rapid bicarbonate [HCO3-]infusion. It is shown that a rapid change in [HCO3-]csf occurs during the infusion of HCl or NaHCO3. As a consequence both CSF and arterial pH change in the same direction and large changes in pHcsf (from 7.331 to 7.227) were observed. Such CSF acidosis did not contribute to further increase V̇ beyond the level by hyperventilation induced by the initial fall of pHa. The ventilatory response to the decrease in pHa was found to fall off with moderate to severe acidosis (pHa < 7.20). In conclusion, this study demonstrates an instability of the pHcsf during neonatal metabolic acidosis and it suggests an immaturity of both the H+-sensitive medullary and peripheral chemoreceptors in the 8-day-old newborns.