Effect of CO2 pneumoperitoneum on ventilation-perfusion relationships during laparoscopic cholecystectomy

Background: Previous studies have shown that pneumoperitoneum transiently reduces venous admixture as assessed by a calculation based on the shunt formula, and increases arterial oxygen tension (PaO2) in patients without heart or lung disease. The aim of the present study was to further explore the relationship between ventilation-perfusion ((V) over dot (A)/(Q) over dot) before and during pneumoperitoneum by using the multiple inert gas technique. Methods: Nine patients without heart or lung disease (ASA I), with a mean age of 42 years, scheduled for laparoscopic cholecystectomy were included. After premedication and induction of anaesthesia, radial artery and pulmonary artery catheters were introduced percutaneously. The (V) over dot (A)/(Q) over dot relationships were evaluated by the multiple inert gas elimination technique before and during pneurnoperitoneum to obtain a direct measure of the pulmonary shunt. Results: Induction of pneumoperitoneum decreased the pulmonary shunt from 5.8 (4.5) to 4.1 (3.2)% (P<0.05) and increased PaO2 from 21.7 (5.9) to 24.7 (4.8) kPa (P<0.01). During surgery, the shunt increased from 3.2 (2.8) to 5.2 (3.4)% to the same level as before pneumoperitoneum induction. No area with low (V) over dot (A)/(Q) over dot was seen. Dead space ventilation amounted to 20.0 (1.2)% in the supine position and did not change during the investigation. Conclusions: In patients without heart or lung disease, pneumoperitoneum at an intra-abdominal pressure level of 11-13 mmHg- causes a transient reduction of the pulmonary shunt. The mechanisms underlying the present finding remain to be elucidated.