Control of respiration in newborn babies. IV. Rib cage stability and respiratory regulation.

The recordings from an earlier study regarding the respiratory depth and rate changes induced by exposure to 4% CO2 in air in 13 babies with PM age varying between 32 and 43 weeks were reexamined with regard to the pattern of thoracic abdominal breathing excursion in breathing immediately prior to the CO2 exposure and the type of response induced. The pattern was called "stable" when the thoracic breathing excursions were in phase and congruent with the abdominal ones. When the thoracic excursions in comparison with the abdominal excursions were totally inverted, or incongruous but in phase, or rapidly varying between those two, the pattern was called "unstable". "Unstable" pattern of the breathing prior to the CO2 exposures was followed in an incidence of 60% by the type of response to CO2 which is characterized by a prompt rate increase (the "Type B" response) and only in 16% by the type characterized by an increased breathing amplitude (the "Type A" response). When the excursion pattern of the breathing prior to the CO2 exposures was "stable" "Type A" responses were induced in 59% and "Type B" responses in only 14%. The excursion pattern present when a baby is exposed to 4% CO2 thus seems to affect the type of respiratory depth and rate changes achieved. With increasing postmenstrual age the excursion pattern of the spontaneous breathing is more often "stable" and respiratory depth and rate changes of the "Type B" induced by CO2 less common. The variabilities of the breathing seen preferably in the preterm baby regarding regularity, rate and tidal volumes (as they could be approximated by the registration methods used) were noted most when the excursion pattern was "unstable". The results can be hypothetically interpreted to indicate a dynamic interaction between the thoracic wall and pulmonary mechanoreceptor systems of respiratory regulation. The decreasing variability of the breathing seen with increasing maturation in the baby could be explained by an increasing maturation of the neuromuscular ability to provide stability to the rib cage which would act stabilizing on the pulmonary vagal afferent input to the respiratory center.