The development of stability of respiration in human infants: changes in ventilatory responses to spontaneous sighs.

Serial respiratory recordings using impedance pneumography and barometric plethysmography were made from shortly after birth to 7 months in fifteen normal full-term infants. Each recording was made with the infant asleep and sleep state was estimated from records of electroencephalogram and electro-oculogram made in parallel. The respiratory records obtained during non-rapid eye movement (r.e.m.) sleep were analysed with computer assistance and stretches of the record, approximately 1 min before and up to 2 min after a spontaneous sigh and ensuing apnoeic pause, were processed and presented as sequential values of the fractional deviation of VE, the breath by breath minute volume, from the mean. That part of the sequence which represented the respiratory response to the sigh was then fitted with second order equations representing the critically or underdamped response. The results were presented for each curve in terms of xi, the damping ratio and omega n, the frequency of the undamped respiratory oscillation. Three-quarters of the responses could be so fitted with an error of 20% or less. The residual responses were mainly from infants within a few days of birth. In the youngest infants (4 days or less), the respiratory response to a sigh was highly stable but sluggish: during the period 4-8 days to 3-4 months, the oscillatory period diminished from ca. 25-12 s and respiration was potentially unstable since a small reduction in the damping factor would cause prolonged oscillation while, from 3-4 months, the more mature type of response which was stable with a rapid recovery supervened. The possible mechanisms responsible for this trend are discussed in terms of the factors thought to determine respiratory stability in the adult together with the possible relevance of the results to the normal process of respiratory adaptation at birth and to the respiratory difficulties encountered by some infants in the new-born period and early infancy.