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Weaning Patients from Mechanical Ventilation Using Gastric pH
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TO THE EDITOR:
In contrast to our finding [1], Mohsenifar and colleagues [2] found respiratory frequency, tidal volume, and the ratio of frequency/tidal volume to be poor predictors of weaning failure. In their important paper on the methodologic standards used in studies of predictive indexes, Wasson and colleagues [3] emphasized the need for investigators to follow the original protocol, but Mohsenifar and associates fell short. First, frequency and tidal volume were measured at 20 to 30 minutes into the weaning trial rather than at the beginning of the trial. Second, they failed to indicate 1) the instrumentation used to measure tidal volume; 2) the sampling period; or 3) the gas being inhaled. Third, they measured frequency and tidal volume during pressure support of “about 7 to 8 cm H2O,” a level judged to overcome resistance of the endotracheal tube and circuit. However, as they reported, this level markedly overcompensates, causing a 39% decrease in work of breathing and a decrease in the ratio of frequency/tidal volume of 14 breaths/min per litre [4]. Thus, it is hardly surprising that the ratio measured during pressure support but using the threshold value derived during spontaneous breathing was inaccurate in predicting weaning failure. Fourth, frequency, tidal volume, and the ratio of frequency/tidal volume were prospectively tested, whereas gastric pH was evaluated in a post hoc manner, which can markedly overestimate the accuracy of a predictive index [3].
Martin J. Tobin, MD
Loyola University of Chicago Stritch School of Medicine; Maywood, IL 60153
The Editors welcome submissions for possible publication in the Letters section. Authors of letters should:
•Include no more than 300 words of text, three authors, and five references
•Type with double-spacing
•Send three copies of the letter, an authors' form signed by all authors, and a cover letter describing any conflicts of interest related to the contents of the letter.
Letters commenting on an Annals article will be considered if they are received within 6 weeks of the time the article was published. Only some of the letters received can be published. Published letters are edited and may be shortened; tables and figures are included only selectively. Authors will be notified that the letter has been received. If the letter is selected for publication, the author will be notified about 3 weeks before the publication date. Unpublished letters cannot be returned.
Annals welcomes electronically submitted letters.
- Copyright ©2004 by the American College of Physicians
