Alveolar data in healthy, awake neonates during spontaneous ventilation: A preliminary investigation

A computerized method for the measurement of alveolar ventilation (V A ) and the mean alveolar partial pressures of CO 2 and O 2 is described and tested in healthy, awake fullterm (FT) newborns and preterm (PT) infants (postnatal age 40 days). This study emphasizes the technical pitfalls generally encountered when dealing with very small infants. A sensitive pneumotachograph with a small dead space volume and a low‐flow sampling system for the gas analyzer are minimum requirements. Under these technical conditions, the major problem is the scanning time of the mass spectrometer (50 Hz), which fixes the time constant, as well as the digitization sampling rate of the gas signals. This rate was not sufficient when the respiratory rate was above 90 min −1 . As critical visual evaluation of each breath is required, fully automatic access to mean alveolar data is not possible. The mean dead‐space volume in both FT and PT infants was 2 ml. kg‐1, with a V D /V T ratio of 0.3. The mean alveolar point was 2/3 V T , as found in adults. When expressed as ml. min −1 . kg −1 , V A appeared to be higher than in other studies, but related to the metabolic requirements. (V CO 2 = 7.8 ± 0.2 ml. min −1 . kg −1 and V O 2 = 8.0 ± 0.2 ml. min −1 . kg −1 ), in accordance with the state of vigilance of the neonates. Indeed, the PA CO 2 values, which depend on the ratio V CO 2 /V A , are in agreement with the values of Pa CO 2 generally considered normal in healthy newborns over 1 week of age. The values obtained for small infants indicated a highly significant statistical difference between the end‐tidal and the corresponding mean alveolar values of P CO 2 and P O 2 . It appears that the use of Pet CO 2 and Pet O 2 for the alveolar partial pressures produces only a rough approximation of the real values.