© 2006 American Thoracic Society
Oxygenation as an Indicator for the Optimal Lung Volume in Ventilated Newborn Infants: Useful or Useless?To the Editor:We read with great interest the recent article by Tingay and colleagues, showing that the deflation limb of the pressure–volume relationship of the newborn lung can be successfully mapped during high-frequency oscillatory ventilation using respiratory inductive plethysmography (RIP) (1). The authors also report that changes in lung volume during the initial part of the deflation limb were not accompanied by changes in oxygen saturation (SpO2), and they state that this observation highlights the imprecision of SpO2 as a proxy indicator of optimal lung volume. We would like to propose a different explanation for this observation. Mathematical models and animal studies have shown that lung recruitment using relatively high airway pressures is an essential step in reaching the optimal lung volume (2, 3). Following this recruitment maneuver, airway pressures should be reduced as soon as possible, trying to determine the pressure level sufficient to stabilize the recruited alveoli but avoiding alveolar overdistension. This process of alveolar recruitment and stabilization is accompanied by changes in lung volume, as also elegantly shown in the study by Tingay and colleagues. However, these changes in lung volume are based on changes in alveolar distension and/or alveolar (de)recruitment (2). As RIP only measures changes in the total lung volume, it will not be able to distinguish between these two determinants of lung volume. Oxygenation, however, will mainly detect lung volume changes caused by alveolar (de)recruitment, as this affects the intrapulmonary shunt fraction and thus oxygenation. We therefore suggest that the relatively stable SpO2 during the first part of the deflation limb in the study by Tingay and colleagues is caused by the fact that the relatively small loss in lung volume measured by RIP was mainly due to a reduction in alveolar distension. As the normalized RIP volume decreases below 50%, oxygenation also deteriorates, indicating alveolar derecruitment, which is potentially injurious to the lung. In summary, we agree with the authors that oxygenation measured by SpO2 is probably not an accurate indicator of changes in total lung volume, but its ability to distinguish between lung volume changes caused by alveolar distension and (de)recruitment certainly seems to be of additional value when determining the optimal lung volume. Although the study by Tingay and colleagues has greatly advanced our knowledge on lung recruitment in newborn infants, it also shows that we will probably need a combination of several indicators to optimize lung volume.
Emma Children's Hospital AMC, Amsterdam, The Netherlands FOOTNOTES Conflict of Interest Statement: Neither author has a financial relationship with a commercial entity that has an interest in the subject of this manuscript. REFERENCES
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