In
pneumoperitoneum,
carbon dioxide eliminated in expired gas (
carbon dioxide output) contains both metabolic and absorbed
carbon dioxide from the peritoneal cavity. When elimination of
carbon dioxide is much higher than
carbon dioxide output, storage of tissue
carbon dioxide and arterial
carbon dioxide concentrations change. Finally, the rate of
carbon dioxide eliminated in expired gas is not a match for the real rate of metabolic production and absorbed
carbon dioxide from the peritoneal cavity. During and after insufflation of
carbon dioxide, changes in
carbon dioxide output were elucidated under constant arterial
carbon dioxide pressure (PaCO2), the same as the preinduction level. We studied patients undergoing elective
laparoscopic cholecystectomy.
Carbon dioxide output,
oxygen uptake, respiratory exchange ratio (RER), expired minute ventilation (VE), deadspace to tidal volume ratio (VD/VT ratio) and arterial to end-tidal
carbon dioxide partial pressure difference (PaCO2-PE'CO2) were determined before induction, and during anaesthesia,
pneumoperitoneum and recovery. By controlling ventilatory frequency (f) every 1 min, PaCO2 was adjusted to concentrations before induction. Constant monitoring of end-tidal
carbon dioxide partial pressure (PE'CO2) and intermittent measurement of (PaCO2-PE'CO2) (15-min intervals) were conducted to predict PaCO2).
Carbon dioxide output and
oxygen uptake decreased significantly from mean values of 83.5 (SEM 5.2), 101.6 (5.1) to 68.5 (4.2), 81.1 (4.6) ml min-1 m-2 (ATPS, P < 0.05) with
sevoflurane anaesthesia, and RER did not change. During
carbon dioxide pneumoperitoneum (intra-abdominal pressure 8 mm Hg),
carbon dioxide output increased by 49% (102.4 (5.0) ml min-1 m-2) (P < 0.05) while
oxygen uptake remained stable and RER increased from 0.84 (0.02) to 1.16 (0.03) (P < 0.05). It was necessary to increase VE during
pneumoperitoneum by 1.54 times that during anaesthesia to maintain individual PaCO2 values constant. After removal of
carbon dioxide from the abdominal cavity, the regression equation of excess
carbon dioxide output/BSA best fitted a two-compartment model. The time constants of the rapid and slow compartments were 8.2 and 990 min, respectively. Excess
carbon dioxide output/BSA was still 5.5 ml min-1 m-2, 30 min after
pneumoperitoneum.