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CRITICAL CARE AND TRAUMA

Positive End-Expiratory Pressure Has Little Effect on Carbon Dioxide Elimination After Cardiac Surgery

Directorate of Anaesthetics, Manchester Royal Infirmary, Central Manchester Hospitals Trust, Manchester, United Kingdom

Address correspondence and reprint requests to Dr. Roger Fletcher, Directorate of Anaesthetics, Manchester Royal Infirmary, Central Manchester Hospitals Trust, Oxford Rd., Manchester M13 9WL, United Kindgom.

We investigated the effects of positive end-expiratory pressure (PEEP) on carbon dioxide (CO₂) elimination in a cross-over study of 14 patients whose lungs were ventilated after cardiac surgery. They initially received either 7.5 cm H₂O PEEP or zero end-expiratory pressure and were then changed over to the other mode. We measured CO₂ minute elimination (CO₂) and "efficiency," a quantification of the shape of CO₂ single-breath test (SBT-CO₂), the plot of expired CO₂ against expired volume. CO₂ and efficiency (and therefore the shape of SBT-CO₂) were not significantly affected by PEEP. These results agree with findings in patients with acute lung injury, but are in contrast with those in an open-chest dog model, in which 7.5 cm H₂O PEEP caused a 19% decrease in CO₂ and significant changes in SBT-CO₂.

Implications: During artificial ventilation, applying a positive pressure in expiration expands the lung and improves the uptake of oxygen, but there is a theoretical risk of reduced carbon dioxide elimination. We applied positive end-expiratory pressure to patients immediately after heart surgery and found that it has no effect on carbon dioxide elimination.

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