Airway closure in each lung of anesthetized human subjects.

Airway closure and functional residual capacity (FRC) were assessed for each lung separately in the anesthetized subject by means of a double-lumen tracheal catheter. Airway closure was studied by argon-bolus and nitrogen-washout techniques, and FRC was calculated from single-breath nitrogen washout. Recordings were done with subjects in the supine and lateral postures. In the supine position, closing capacity (CC) exceeded FRC in each lung. Airway closure occurred synchronously in the two lungs. Argon CC was 0.05-0.1 liter larger than nitrogen CC of either lung. Minor gas trapping occurred during the vital capacity (VC) maneuver, so that inspired VC exceeded expired VC by 3%. In the left lateral posture, CC remained unaltered in either lung, whereas FRC was markedly increased in the nondependent and reduced in the dependent lung. Airway closure occurred asynchronously in the two lungs, and its distribution was discontinuous between them. Onset of airway closure in the dependent lung caused an early (60% VC) upstroke on the overall tracer gas recording (sampling of mixed expirate at the mouth), whereas onset of airway closure in the nondependent lung caused an additional upstroke at 10% VC. Gas trapping was more marked in the dependent lung than in the supine position, but some gas was released (expired VC greater than inspired VC) n the nondependent lung.