Background: There is a possibility that physiological changes in respiratory mechanics caused by aging lead to deterioration of pulmonary gas exchanges and an increase in (A-a)D_<O2> and to a difference between Pa_<CO2> and PET_<CO2> (P(a-ET)_<CO2>) during laparoscopy in the Trendelenburg-lithotomy position (TLP). Methods: The subjects were 51 gynecological patients. Pressure-controlled ventilation was used to maintain PET_<CO2> measured by the side stream method within the range of 4 to 4.67 kPa. During laparoscopy with CO_2 insufflation in the TLP, tidal volume was increased to keep PET_<CO2> within ± 20% of the preinsufflation value. Subjects were divided into three groups by age: a young group (Y: < 45 y.o.), a middle age group (45 y.o. ≤ < 64 y.o.) and an elderly group (E: ≥ 65 y.o.). Results: Before pneumoperitoneum (PPN), significant differences were found between the Y and the E groups in Pa_<O2>, (A-a)D_<O2>, Pa_<CO2> and P(a-ET)_<CO2>. In all groups, peak inspiratory pressure (PIP) and Pa_<CO2> were increased progressively during PPN in the TLP. P(a-ET)_<CO2> increased gradually after starting CO_2 insufflation in the TLP only in the E group. Conclusions: Increase in P(a-ET)_<CO2> was seen during PPN in the TLP in the elderly group. With CO_2 insufflation in the TLP, the setting of mechanical ventilation based on the value of PET_<CO2> measured by the side stream method should be determined with caution in elderly patients.
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