Positive end-expiratory pressure has little effect on carbon dioxide elimination after cardiac surgery.
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 (Vco2) and "efficiency," a quantification of the shape of CO2 single-breath test (SBT-CO2), the plot of expired CO2 against expired volume. Vco2 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 Vco2 and significant changes in SBT-CO2.


CONCLUSIONS
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.
Smith
R P
RP
Directorate of Anaesthetics, Manchester Royal Infirmary, Central Manchester Hospitals Trust, United Kingdom.
Fletcher
R
R
D000368:Aged; D000758:Anesthesia; D002245:Carbon Dioxide / Q000378:metabolism*; D006348:Cardiac Surgical Procedures; D018592:Cross-Over Studies; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D011175:Positive-Pressure Respiration; D011184:Postoperative Period; D013990:Tidal Volume / Q000502:physiology
D002245:Carbon Dioxide