DOES TRAPPED GAS EXIST IN THE LUNGS OF HEALTHY, FULLTERM, NEWBORN INFANTS?

Trapped gas (TG), defined as the difference between thoracic gas volume at end expiration (TGV) and functional residual capacity (FRC), has been reported to be of considerable magnitude in the neonatal period. In a previous investigation we found TG to average 20 ml in 26 full term infants 2 hours after birth, and 17 ml 24 hours later. The study to be reported was performed to assess how much of the TG that could be mobilized from the lungs by hyperventilation. Fifteen fullterm, healthy newborn infants were studied 1 to 4 days after birth by a modified nitrogen wash-out method. A face mask with its outlet connected to an opening in the side of a wide tube was gently applied to the sleeping baby. Air at high flow rate was continuously flushed through the tube which had a pneumotachygraph mounted at the outlet. The probe of a fast nitrogen analyzer was inserted into the mask. During an expiration the gas flow was instantanously shifted to 100% oxygen. After the expired nitrogen concentration reached below 2% the baby was stimulated to cry with the face mask still in place. Flow and nitrogen concentration signals were sampled at high rate by a computer and the lung nitrogen content before the wash-out was calculated. No rise in nitrogen output was detected during hyperventilation after the wash-out in any infant. We conclude, that the difference between TGV and FRC found by other investigators and ourselves in newborn infants most likely does not represent gas trapped in the lungs but is either an artifact due to methodological inaccuracy, or is due to gas localized in other oroans.