Diffusional inhomogeneity: gas mixing efficiency in the new-born lung.

1. The results of 105 nitrogen washouts on seventy-six new-born babies are presented; the mean lung volume (functional residual capacity, FRC) of the seventy-six babies measured at more than 1 hr of age was 77.5 +/- 2.9 (S.E. of mean; S.D. = 25.4). 2. The mean gas mixing efficiency was 0.466 +/- 0.0097 (S. E. of mean; S.D. = 0.096). The reasons for the difference between this figure and the normal for adults of 0.66--0.76 are discussed, and it is suggested that the very much greater turnover rate of gas within the lungs is responsible for diffusion inefficiency. 3. Calculations indicate that there is a tenfold fall in diffusion inefficiency for every 9.8 sec turnover time (the time for VE to equal VFRC). In the baby the turnover time is 5--6 sec on average but with a very wide spread; in the adult it is of the order of 30 sec, so that diffusion inefficiency of gas in the lung would be only 0.001 or 0.1%. 4. The significance of this is that the effective alveolar ventilation of the average neonate is less than half the total ventilation (as opposed to two-thirds if deadspace only is considered), becoming a smaller fraction of VE on hyperventilation. This would explain discrepancies in the calculation of alveolar carbon dioxide levels from the output of that gas and the non-deadspace ventilation.