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Positive End-Expiratory Pressure and Shunting across Foramen Ovale
- Bibiana Cujec, MD;
- David H. Johnson, MD; and
- Irvin Mayers, MD
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IN RESPONSE:
We agree that blinding echocardiographers to the level of positive PEEP during the saline contrast studies used to diagnose patent foramen ovale would have been optimal. However, some second-generation ventilators had a distinctive sound on expiration after addition of PEEP, which precluded the ability to blind them. However, right-to-left intracardiac shunting was present both with and without PEEP in six of the seven patients in whom patent foramen ovale was diagnosed. One patient had a right-to-left shunt only on one setting of PEEP. So bias is unlikely. Diagnostic bias was minimized by using predetermined criteria for diagnosis of patent foramen ovale. Stollberger and coworkers [1] found that variability in the criteria used to diagnose a right-to-left intracardiac shunt was the main source of interobserver variability in the diagnosis of patent foramen ovale with contrast transesophageal echocardiography.
Suter and associates [2] tested whether changes in compliance could be an index of “best PEEP” (that is, the level at which oxygen transport is maximal). They found that a low initial functional residual capacity was associated with a higher “best PEEP.” We were not interested in “best PEEP” but rather in whether a predetermined end-expired pressure could reduce intrapulmonary shunting and improve gas exchange. We therefore did not measure functional residual capacity but rather dynamic compliance as tidal volume/peak inspiratory pressure − end expiratory pressure. We did not find significant changes in compliance, suggesting that no patient achieved “best PEEP” [2].
Group mean data for arterial partial pressure of oxygen did not differ statistically but tended to decrease with the addition of PEEP in patients with patent foramen ovale. Subtraction of the arterial oxygen content from the pulmonary end-capillary oxygen content decreased the relative standard deviation and made the calculated shunt significantly different. More importantly, the shunt did not decrease as it did in the group of patients without patent foramen ovale.
We agree that PEEP is useful in respiratory failure and do not advocate withholding it in all patients with a patent foramen ovale. Correcting other causes of hypoxemia amenable to PEEP may outweigh the small increase in right-to-left intracardiac shunting. A patent foramen ovale should, however, be considered in patients whose oxygenation is not improved with PEEP.
Bibiana Cujec, MD
David H. Johnson, MD
Irvin Mayers, MD
University of Saskatchewan; Saskatoon, Saskatchewan, Canada 57N OXO
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
