Role of the carotid chemoreceptors in the respiratory response of newborn lambs to alternate pairs of breaths of air and a hypoxic gas.

We measured the reflex respiratory response to a 2-breath alternation in fractional inspired oxygen in two groups of lambs. Test runs where F(I),O2 was made to alternate (between 0.21 and 0.14) were compared with control runs in which it was held at 0.21. Within 36 h of birth we cut the carotid sinus nerves bilaterally in one group of lambs (n = 5) whilst in a second group (n = 5) sham operations were carried out. The alternating responses in seven respiratory variables were compared between test and control runs in each group of lambs at postnatal days 5-6 and again at postnatal days 10-11. In sham-operated lambs at days 5-6 there was a significant response in most respiratory variables during test runs and by days 10-11 the response was increased, primarily as a consequence of an increase in the tidal volume component of the response, indicating a postnatal maturation of the responses. The time-scale of this maturation is appropriate to that of peripheral chemoreceptor resetting. There was no significant response to the test in the denervated lambs at 5-6 days but by days 10-11 a small response was measured in some variables. Our results therefore indicate that the reflex respiratory response to alternations in FI,O2 is primarily mediated by the carotid chemoreceptors. The development of a response mediated by the aortic chemoreceptors may account for the small alternating response in the denervated lambs at days 10-11.