[Visceral resorption of intra-abdominal insufflated carbon dioxide in swine]

Total intraperitoneal carbon dioxide (CO2) resorption from CO2-pneumoperitoneum increases in relation to intraabdominal pressure (IAP) up to an upper limit of 10 to 15 mmHg. The purpose of this prospective study was to evaluate the visceral fraction of CO2 resorption in comparison to total intraperitoneal CO2 resorption in pigs to address possible reasons for this upper limit.

16 pigs were chronically instrumented. Via midline laparotomy, a transit-time ultrasound flow probe was placed around the portal vein for continuous recording of the portal venous blood flow and a catheter was inserted into the portal vein via lienal vein. After complete recovery (7-10 days), animals were anesthetized with propofol and fentanyl and a pulmonary artery, a hepatic venous, an arterial, and an intraabdominal insufflation catheter were inserted. Mechanical ventilation (O2/air; FiO2 = 0.4) was adjusted to maintain endtidal CO2 at 34 to 36 mmHg using an Engstrøm Elvira ventilator. After an equilibration period of 3h, CO2, (n = 8) or air (n = 8) was insufflated. IAP was increased in steps of 4 mmHg and maintained constant at each respective IAP-level for 20 min. Blood gas analyses were assessed from portal venous, hepatic venous, central venous, and arterial probes at each IAP-level. Total intraperitoneal CO2 resorption was calculated from parameters derived from indirect calorimetry, the portal venous fraction from blood gas values and the portal venous blood flow following Fick's principle. Data were analyzed using Friedman's test.

Total CO2 resorption increased continuously with rising IAP. Highest values were measured at IAP = 16 mmHg with 84 (74-93) ml/min. A further increase of IAP resulted in a significant decrease of total CO2 resorption. The visceral fraction of intraperitoneal CO2 resorption increased up to 28 (17-36) ml/min at IAP = 12 mmHg. Portal venous blood flow was also elevated or unchanged up to this IAP. At IAP = 20 mmHg or IAP = 24 mmHg portal venous blood flow decreased (79% of baseline) and in consequence portal venous calculated fraction of intraperitoneal carbon dioxide resorption decreased to 14 (8-20) ml/min. 20 min after desufflation, intraabdominal CO2 resorption was completed. With air insufflation, all parameters of CO2 balance were unchanged.

The IAP dependent increase in CO2 resorption is limited due to an IAP related occlusion of the peritoneal capillaries and the limited expansion of peritoneal diffusion area. In this model, it was possible to show that visceral fraction is about one third of the total intraperitoneal carbon dioxide resorption and that this fraction depends on portal venous blood flow. Thus, a decrease in total CO2 resorption may indicate a reduction in portal venous blood flow.