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Interpretation of PV Curves

Although acknowledged for my comments on the paper by Dr. Kornecki and colleagues (1), I do not agree with the authors' conclusions. It is well established that in the infant, the dynamic "FRC" is much higher than the true FRC defined as the static passive balance of forces between the lung and the chest wall (2). In vivo, a number of factors contribute to this auto-PEEP; ex vivo, only one of these seem to apply, the elastic recoil of the lung, which is a major factor in maintaining airway patency. The very low elastic recoil of the infant therefore maintains a higher lung volume. Although this has never been measured, it is beautifully illustrated in Kornecki and coworkers' Figures 2 and 6. The adult PV curves start at FRC and end at FRC. In the infant, Kornecki and coworkers assume that they start at FRC, but the deflation limb stops abruptly at 5 cm H₂O, trapping 40% of total lung capacity. It follows that during mechanical ventilation, the tidal volume will be superimposed on an auto-PEEP of 5 cm H₂O, thus minimizing parenchymal damage. This buffer is absent in the adult, and injury is caused by the crumpling and uncrumpling of alveoli at low lung volume. It is notable that the juveniles have a small amount of auto-PEEP and lesser degrees of lung injury. I believe this to be the most important finding of Kornecki and coworkers. The authors do not mention this abrupt truncation of the PV curve in infants, let alone make any attempt to explain it.

A. Charles Bryan

Hospital for Sick Children (retired), Toronto, Ontario, Canada

FOOTNOTES

Conflict of Interest Statement: A.C.B. does not have a financial relationship with a commercial entity that has an interest in the subject of this manuscript.

Dr. Bryan died on August 1, 2005.

REFERENCES

1. Kornecki A, Tsuchida S, Ondiveeran HK, Engelberts D, Frndova H, Tanswell AK, Post M, McKerlie C, Belik J, Fox-Robichaud A, et al. Lung development and susceptibility to ventilator-induced lung injury. Am J Respir Crit Care Med 2005;171:743–752.[Abstract/Free Full Text]
2. Bryan AC, Wohl ME. Respiratory mechanics in children. In: Fishman AP, section editor. Handbook of physiology. Section 3: The respiratory system, Volume III, edited by Mcklem PT, Mead J. Mechanics of breathing. Bethesda, MD: American Physiological Society; 1986. pp 179–191.

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