The effect of lung inflation on the control of respiratory frequency in the neonate

1. We have measured the relationship between tidal volume (V(T)) and the duration of inspiration (T(i)) and expiration (T(e)) for individual breaths (30 in each steady state).2. Ten pre-term and ten term infants were studied during steady state while breathing 21% O(2), then 21% O(2) plus 2 and 4% CO(2).3. In all infants, the average T(i) at the various chemical drives was remarkably constant, and did not decrease as the tidal volume increased. However, at any given level of respiratory drive, there was a slightly positive correlation of V(T) with T(i) and T(e) in 95% of the cases.4. In four pre-term and two term infants, T(e) increased with increasing respiratory drive. In these infants, therefore, instantaneous respiratory frequency (1/(T(i) + T(e))) actually decreased as lung volume increased.5. We suggest that T(i) is independent of V(T) within the range of volumes studied (up to 2 times the resting V(T)) and that changes in instantaneous respiratory frequency (1/(T(i) + T(e))) result from changes in T(e).