© 2007 American Thoracic Society
From the Authors:We thank Dr. Sindelar and colleagues for their letter regarding our recently published article (1). We agree that it is very difficult to measure the pressure change in dependent alveoli during partial liquid ventilation (PLV). Dr. Sindelar and colleagues indicate that since pulmonary stretch receptor (PSR) activity is similar during gas ventilation (GV) and PLV (at 30 ml/kg fill volume) in 2.3-kg cats that no excessive lung stretch occurs during PLV (2). As a result, they reason that alveolar pressure with the same gas-ventilating pressures do not differ in PLV from those measured in GV. However, as noted by Dr. Sindelar's group (2, 3) and Norsted and coworkers (4), both PaCO2 level and breathing frequency also affect PSR response. In lung lavage–injured cats, the PaCO2 during PLV that resulted in PSR inhibition was higher during GV than during PLV (6.3 ± 1.7 [about 48 mm Hg] vs. 5.2 ± 0.6 [about 39 mm Hg] kPa, p < 0.027) (2). Since the lower the PaCO2 the greater the likelihood for PSR inhibition (4), and the greater the pulmonary stretch the greater the likelihood for PSR inhibition (2), it seems reasonable to conclude that if PSR inhibition occurred at a higher PaCO2 during PLV than GV then pulmonary stretch must have been higher during PLV than GV. That is, PSR activity was inhibited during PLV at a higher pressure than during GV since PaCO2 was high during PLV. In addition, since the model studied by Sindelar and coworkers was a small lung lavage–injured cat (2.3 ± 1.0 kg), it is unlikely that the impact of perfluorocarbons on dependent lung would be the same as in an adult human. The difference in the height of the lungs (about 3 to 4 cm vs. 15 to 20 cm) would result in marked differences in the effect of the perfluorocarbon on dependent alveoli. Considering the quantity of perfluorocarbon instilled in adult patients (30 ml/kg or about 2,250 ml, 75 kg adult) versus the studied cats (30 ml/kg or about 69 ml, 2.3 kg) and a perfluorocarbon density of 1.9, it is difficult to conceive that the most gravity-dependent alveolar pressure during both inspiration and expiration would be similar during GV and PLV at the same airway gas pressure. It is important to remember that, during PLV, gas ventilation is provided on top of the perfluorocarbon-filled lungs. It is true that with gas ventilation the perfluorocarbon spreads over a larger lung area. However, there is still a column of fluid on top of dependent alveoli that is pressurized by the gas ventilation. Overall, what is clearly needed is additional research into the use of perfluorocarbon-assisted ventilation and the mechanisms associated with both its beneficial and detrimental effects.
Massachusetts General Hospital Harvard Medical School, Boston, Massachusetts
St. Michael's Hospital, University of Toronto, Toronto, Ontario, Canada FOOTNOTES Conflict of Interest Statement: R.M.K. received a $65,000 grant from Alliance Pharmaceuticals in 1999 to support studies of partial liquid ventilation in animal models. A.S.S. has been a paid consultant to Maquet in the field of mechanical ventilation (> $10,000/yr). He chaired a Data Safety Monitoring Board (DSMB) for Leo Pharma in relation to a surfactant trial (> $10,000/yr), and was on the Alliance Advisory Board for the trial presented in this publication but received no financial compensation for this. REFERENCES
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