Influence of Gas Composition on Recurrence of Atelectasis after a Reexpansion Maneuver during General Anesthesia

Background: Atelectasis, an important cause of impaired gas exchange during general anesthesia, may be eliminated by a vital capacity maneuver. However, it is not clear whether such a maneuver will have a sustained effect. The aim of this study was to determine the impact of gas composition on reappearance of atelectasis and impairment of gas exchange after a vital capacity maneuver.

Methods: A consecutive sample of 12 adults with healthy lungs who were scheduled for elective surgery were studied. Thirty minutes after induction of anesthesia with fentanyl and propofol, the lungs were hyperinflated manually up to an airway pressure of 40 cmH2 O. FI sub O2 was either kept at 0.4 (group 1, n = 6) or changed to 1.0 (group 2, n = 6) during the recruitment maneuver. Atelectasis was assessed by computed tomography. The amount of dense areas was measured at end-expiration in a transverse plane at the base of the lungs. The ventilation-perfusion distributions (V with dot A/Q with dot) were estimated with the multiple inert gas elimination technique. The static compliance of the total respiratory system (Crs) was measured with the flow interruption technique.

Results: In group 1 (FIO2 = 0.4), the recruitment maneuver virtually eliminated atelectasis for at least 40 min, reduced shunt (V with dot A/Q with dot < 0.005), and increased at the same time the relative perfusion to poorly ventilated lung units (0.005 < V with dot A/Q with dot < 0.1; mean values are given). The arterial oxygen tension (PaO2) increased from 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 parameters of V with dot A/Q with dot were unchanged. In both groups, Crs increased from 57.1/55.0 ml *symbol* cmH2 O sup -1 (group 1/group 2) before to 70.1/67.4 ml *symbol* cmH2 O sup -1 after the recruitment maneuver. Crs showed as low decrease thereafter (40 min after recruitment: 61.4/60.0 ml *symbol* cmH2 O sup -1), with no difference between the two groups.