INFLUENCE OF GAS-COMPOSITION ON RECURRENCE OF ATELECTASIS AFTER A REEXPANSION MANEUVER DURING GENERAL-ANESTHESIA

Background: Atelectasis, an important cause of impaired gas exchange d uring general anesthesia, may be eliminated by a vital capacity maneuv er. However, it is not clear whether such a maneuver will have a susta ined effect. The aim of this study was to determine the impact of gas composition on reappearance of atelectasis and impairment of gas excha nge after a vital capacity maneuver. Methods: A consecutive sample of 12 adults with healthy lungs who were scheduled for elective surgery w ere studied. Thirty minutes after induction of anesthesia with fentany l and propofol, the lungs were hyperinflated manually up to an airway pressure of 40 cmH(2)O. Fi(O2) was either kept at 0.4 (group 1, n = 6) or changed to 1.0 (group 2, n = 6) during the recruitment maneuver. A telectasis was assessed by computed tomography. The amount of dense ar eas was measured at end-expiration in a transverse plane at the base o f the lungs. The ventilation-perfusion distributions (VA/Q)) were esti mated with the multiple inert gas elimination technique. The static co mpliance of the total respiratory system (C-rs) was measured with the now interruption technique. Results: In group 1 (FIO2 = 0.4), the recr uitment maneuver virtually eliminated atelectasis for at least 40 min, reduced shunt (VA/Q < 0.005), and increased at the same time the rela tive perfusion to poorly ventilated lung units (0.005 < VA/Q < 0.1; me an values are given). The arterial oxygen tension (Pa-O2) increased fr om 137 mmHg (18.3 kPa) to 163 mmHg (21.7 kPa; before and 40 min after recruitment, respectively; P = 0.028). In contrast to these findings, atelectasis recurred within 5 min after recruitment in group 2 (FIO2 = 1.0). Comparing the values before and 40 min after recruitment, all p arameters of V-A/Q were unchanged. In both groups, C-rs increased from 57.1/55.0 ml . cmH(2)O(-1) (group 1/group 2) before to 70.1/67.4 ml . cmH(2)O(-1) after the recruitment maneuver. C-rs showed a slow decrea se thereafter (40 min after recruitment: 61.4/60.0 ml cmH(2)O(-1)), wi th no difference between the two groups. Conclusions: The composition of inspiratory gas plays an important role in the recurrence of collap se of previously reexpanded atelectatic lung tissue during general ane sthesia in patients with healthy lungs. The reason for the instability of these lung units remains to be established. The change in the amou nt of atelectasis and shunt appears to be independent of the change in the compliance of the respiratory system.