End-expiratory lung volume remains stable during N₂ MBW in healthy sleeping infants

We have previously shown that functional residual capacity (FRC) and lung clearance index were significantly greater in sleeping healthy infants when measured by N₂ (nitrogen) washout using 100% O₂ (oxygen) versus 4% SF₆ (sulfur hexafluoride) washout using air. Following 100% O₂ exposure, tidal volumes decreased by over 30%, while end-expiratory lung volume (EELV, i.e., FRC) rose markedly based on ultrasonic flow meter assessments. In the present study to investigate the mechanism behind the observed changes, N₂ MBW was performed in 10 separate healthy full-term spontaneously sleeping infants, mean (range) 26 (18-31) weeks, with simultaneous EELV monitoring (respiratory inductance plethysmography, RIP) and oxygen uptake (V´O₂ ) assessment during prephase air breathing, during N₂ washout by exposure to 100% O₂ , and subsequently during air breathing. While flow meter signals suggested a rise in ELLV by mean (SD) 26 (9) ml over the washout period, RIP signals demonstrated no EELV change. V'O₂ /FRC ratio during air breathing was mean (SD) 0.43 (0.08)/min, approximately seven times higher than that calculated from adult data. We propose that our previously reported flow meter-based overestimation of EELV was in fact a physiological artifact caused by rapid and marked movement of O₂ across the alveolar capillary membrane into the blood and tissue during 100% O₂ exposure, without concomitant transfer of N₂ to the same degree in the opposite direction. This may be driven by the high observed O₂ consumption and resulting cardiac output encountered in infancy. Furthermore, the low resting lung volume in infancy may make this error in lung volume determination by N₂ washout relatively large.