© 2008 American Thoracic Society
Whole-Body "Negative-Pressure" Ventilation: Is It Really Different?From the Authors:We thank Drs. Loring and Banzett for their insightful comments. The crux of their commentary is the equivalence of transrespiratory pressure in positive- and negative-pressure ventilation. If the respiratory system is modeled as a balloon, with no heterogeneity in the distribution of inflation, then it is difficult (impossible, in our view) to imagine that Loring and Banzett (and ourselves, and generations of others) could be wrong: positive pressure is equal and opposite to negative pressure of the same magnitude. As we see it, the problem is that—in vivo—these assumptions are not valid; inflation is heterogeneous, and this is in part because of the heterogeneous nature of the thoracic wall. We don't agree that relaxation of the respiratory muscles eliminates all impediments to the transmission of pressure across the chest wall. During dynamic conditions, it is difficult to match the pressure–time profile of positive- and negative-pressure systems; if it were accurately achieved, then concomitant documentation of the pattern of inflation would resolve the issue. In this we agree with Loring and Banzett. We suspect that, in the in vivo context (where it matters), inflation would be seen to differ, but that's conjecture. We focused on static conditions where there is cessation of airflow: here there are no pressure gradients across open passages, but gradients can exist across closed surfaces (e.g., the semirigid chest wall or abdomen), as shown in our article (1). End-expiratory pressure was less efficiently transmitted to the pleural space with negative pressure because, we believe, of chest wall rigidity (1). For a given level of transpulmonary pressure, end-expiratory lung volume was greater when achieved with negative versus positive pressure; this could have been because of the differences in dynamic inflation before static conditions were achieved, or because the measurements of pleural pressure were inaccurate. For the latter to explain the results, the measurement of pleural pressure would have to have been differentially and systematically erroneous for positive versus negative pressure. Leaving aside pressure measurements, we documented that comparable levels of end-expiratory lung (i.e., static) volume achieved by negative pressure resulted in greater oxygenation that was independent of pressure measurements. We certainly did not (and would not) suggest that conventional respiratory mechanics be "relinquished"; rather, we would hope that scientists with the insights of Drs. Loring and Banzett will continue to provide argument and data that will broaden our understanding of how lungs inflate.
Hospital for Sick Children
Hospital for Sick Children
Hospital for Sick Children 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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