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

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Abstract
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.