DIFFUSIONAL INHOMOGENEITY - GAS MIXING EFFICIENCY IN THE NEWBORN LUNG

The results of 105 N washouts on 76 newborn babies are presented; the mean lung volume (functional residual capacity, FRC) of the 76 babies measured at more than 1 h of age was 77.5 .+-. 2.9 (SEM [standard error of the mean]; SD = 25.4). The mean gas mixing efficiency was 0.466 .+-. 0.0097 (SEM; SD = 0.096). The reasons for the difference between this figure and the normal for adults of 0.66-0.76 are discussed. The very much greater turnover rate of gas within the lungs is possibly responsible for diffusion inefficiency. There apparently is a 10-fold fall in diffusion inefficiency for every 9.8 s turnover time (the time for .ovrhdot.VE [ventilation] to equal VFRC [volume FRC]). In the baby the turnover time is 5-6 s on the average but with a very wide spread; in the adult it is of the order of 30 s, so that diffusion inefficiency of gas in the lung would be only 0.001 or 0.1%. The significance of this is that the effective alveolar ventilation of the average neonate is less than half the total ventilation (as opposed to 2/3 if deadspace only is considered), becoming a smaller fraction of .ovrhdot.VE on hyperventilation. This would explain discrepancies in the calculation of alveolar CO2 levels from the output of that gas and the non-deadspace ventilation.