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[CO2 stores in laparoscopic cholecystectomy with CO2 pneumoperitoneum]
H Wurst1, H Schulte-Steinberg, U Finsterer
1Institut für Anaesthesiologie, Ludwig-Maximilians-Universität, München.
Methods:
Two groups of 22 patients each were studied in a prospective, randomised fashion during laparoscopic cholecystectomy (LCh) and CO2 pneumoperitoneum (PP) with regard to end-tidal and arterial PCO2 and pulmonary elimination of CO2 (ECO2, Servoventilator with integrated CO2-analyser 930, Siemens). In group 1 minute ventilation was kept constant, resulting in moderate hypercapnia during PP. paCO2 increased by about 10 mmHg during up to 50 min PP. In group 2 paCO2 was kept constant by a stepwise increase in minute ventilation (Fig. 1, Table 2).
Results:
Compared to values just before PP, ECO2 increased in group 1 rather rapidly up to 20 min of PP and more slowly thereafter, reaching a mean value 35% above control at 45 min PP. In group 2 ECO2 was significantly higher than in group 1 between 15 and 35 min PP. At 45 min PP, however, ECO2 was identical in both observation groups (Fig. 2).
Conclusions:
Assuming a stable metabolic CO2 production rate during the observation period and no differences in CO2 absorption from the PP between the two study groups, differences in ECO2 between groups would be a measure of CO2 stored in group 1 patients during the increase in paCO2 with PP (Fig. 3, Table 3). CO2 storage rapidly increased between 0 and 15 min PP, more or less reached a plateau between 15 and 35 min PP, and ceased at 45 min PP. Storing capacity for CO2 during the first 45 min PP amounted to a mean value of 1.20 ml CO2/kg body weight and mmHg paCO2, which agrees favourably with data from the literature and a computer model from Fahri and Rahn published in 1960 (Fig. 4, Table 4). If during LCh with CO2-PP patients are ventilated with a constant minute ventilation, a moderate increase in paCO2 of about 10 mm Hg can be expected. In this case, during the first 45 min PP a 70-kg patient will retain about 1000 ml CO2 in blood and tissues, which must be eliminated after cessation of PP. If the paCO2 is to be held constant during PP, minute ventilation has to be progressively increased by about 40%.