| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
LETTER TO THE EDITOR
In terms of a further 10- to 20-s time constant, we greatly appreciate the detailed interest in the data and figures of our prior report (1), but there seems to be a misunderstanding of some of the markings on those figures. In our prior study, we had very long data files from the PO2 probe covering 27 ventilator settings in each rabbit (1). To mark the places in the file where the ventilator settings were changed, we made a mark in the PaO2 record manually: one operator turned off the light source at approximately the time the ventilator was to be changed, and then a second operator changed one or more knobs on the ventilator. The timing was of course only approximate: usually the marking occurred several seconds before the ventilator settings were changed, but sometimes it occurred 10 s or more before the change in settings and sometimes during or slightly after the ventilator changes. The most striking feature of the transitions between two steady-state responses after changes in respiratory rate (Fig. 1 in Ref. 1), PEEP (Fig. E1), or plateau pressure (Fig. E2) was that the transitions were nearly immediate, often within one or two breaths. Indeed, if the PaO2 response was on the order of 20 s, either because of intrinsic probe limitations or because of physiological limitations, there would be no possibility to see large PaO2 oscillation amplitudes, even at respiratory rates as low as 10 breaths/min, which is obviously at odds with our experimental findings. We agree that if the PaO2 time response was as slow as 20 s after a step change in recruitment, there would have been little point in trying to study an expiratory time of 0.83 s.
Similarly, although the idea of varying plateau pressures is an interesting one, really very little information can be unambiguously deduced about the behavior of a rapid dynamic phenomenon from measurements of time-averaged PaO2. There are simply too many possibilities for the underlying behavior that could result in the same mean PaO2. For the study of dynamic phenomena like cyclical recruitment of atelectasis, there is no substitute for measurement techniques with adequate temporal resolution.
FOOTNOTES
Address for reprint requests and other correspondence: J. Baumgardner, Dept. of Anesthesiology and Critical Care, Hospital of the Univ. of Pennsylvania, 3400 Spruce St. Philadelphia, PA 19104-5078 (e-mail: baumgarj{at}uphs.upenn.edu)
REFERENCES
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| Visit Other APS Journals Online |
