© 2006 American Thoracic Society
Oxygenation as an Indicator for the Optimal Lung Volume in Ventilated Newborn Infants: Useful or Useless?From the Authors:We thank Drs. van Kaam and van Veenendaal for their insightful comments on our article regarding the deflation limb of the pressure–volume relationship during high-frequency oscillatory ventilation (1). We agree that monitoring of changes in total lung volume with respiratory inductive plethysmography (RIP) cannot easily distinguish between changes of volume within stable alveoli versus recruitment/derecruitment of unstable alveoli. It must be acknowledged, however, that only beyond closing pressure, a point identifiable on the deflation limb of most patients with RIP (Figure 1 of our article), will alveolar derecruitment make a significant contribution to lung volume loss. Above the closing pressure, alveolar volume change, rather than decrecruitment, is the predominant mechanism of volume loss (2, 3), and, as such, may be accompanied by very little alteration in oxygenation, as noted in Figure 4C of our article. Our concerns regarding the limitation of oxygenation as an indicator of lung volume relate specifically to this region of the deflation limb between total lung capacity and closing pressure, within which lies a theoretically sound and experimentally validated point of optimal ventilation (4, 5), closely related to, and above, closing pressure. Using oxygenation as an indicator of this optimal ventilation point requires deflation of the lung beyond closing pressure (4), which may be undesirable in the context of significant lung disease. The technical restrictions of SpO2 monitoring, in particular the inability of SpO2 to reflect changes in arterial partial pressure of oxygen once 100% SpO2 is reached, are an additional limitation of SpO2 as an indirect indicator of lung volume. We further agree with Drs. van Kaam and van Veenendaal that a combination of several indicators may be useful in finding the point of optimal lung volume on the deflation limb, and we are actively investigating the possibility that alterations in tidal volume and carbon dioxide clearance can identify this point during high-frequency oscillatory ventilation, without the need to deflate the lung below closing pressure (6).
Royal Children's Hospital, Murdoch Children's Research Institute, Melbourne, Australia FOOTNOTES Conflict of Interest Statement: None of the authors has a financial relationship with a commercial entity that has an interest in the subject of this manuscript. REFERENCES
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