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Weaning Patients from Mechanical Ventilation Using Gastric pH
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TO THE EDITOR:
Mohsenifar and colleagues [1] reported that tissue acidosis, as reflected by a very low gastric intramural pH, accurately predicted failure to wean patients from mechanical ventilation.
The authors used the HCO3, PCO2, and pH from arterial blood gas analysis plus gastric PCO2 (PgCO2) and the PCO2 factor in the Henderson-Hasselbach equation to calculate gastric intramural pH [1]. Substitution and rearrangement permit cancellation of the HCO3 and PCO2 factor terms. The simpler revised formula is: gastric intramural pH = arterial pH − log(Pg CO2/PaCO2).
Both before and during weaning, arterial pH and PaCO2 were virtually identical in the weaning success and failure groups [1]. Because gastric PCO2 is the principal determinant of gastric intramural pH, would prediction of weaning outcome be as accurate using Pg CO2 alone? What are the sources of error in measuring gastric PCO2? What effect does hypotension or low cardiac output have on gastric PCO2, independent of weaning?
The mechanism of tissue acidosis could be increased respiratory muscle blood flow at the expense of other organs, especially the splanchnic bed. In animal models of cardiogenic and septic shock, respiratory muscle blood flow ranged from 10% to 20% of cardiac output [2, 3]. Mechanical ventilation lowered respiratory muscle blood flow and made 30% to 50% more blood flow available to the brain and splanchnic bed; however, the patients studied by Mohsenifar and colleagues were normotensive and hemodynamically stable. One therefore would have to seek other explanations for their poor splanchnic perfusion.
Dudley F. Rochester, MD
University of Virginia Health Schiences Center; Charlottesville, VA 22908
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
