Integration of bronchomotor and ventilatory responses to chemoreceptor stimulation in developing sheep

We analyzed the changes induced by central chemoreceptor stimulation on the lung resistances and phrenic neurogram of anesthetized newborn (3–6 days, n=9) and 9 week old lambs (n=3). Starting from hypocapneic apnea, 5% CO₂ inhalation evoked a reversible increase in total lung resistance in both newborn and 9 week old lambs (median=112%). The resistance increase preceded phrenic breathing and was greater for the peripheral (233%) than for the central airways (57%), independent of age. Increases in lung and airway resistance caused by CO₂ were reversed totally by atropine and only partially by apnea-producing doses of fentanyl. Our results demonstrate that parasympathetic outflow to the sheep airways is already driven by central chemoreceptor inputs during the newborn period. Even at this early age, bronchomotor responses to central chemoreceptor stimulation are more prominent in the peripheral than in the central airways and exhibit a lower threshold for activation and less sensitivity to opioid inhibition than phrenic responses.