The effects of inspiratory flow and inflation volume on the mechanical properties of the respiratory system in eight sedated and paralyzed postoperative morbidly obese patients (aged 37.6 +/- 11.8 yr who had never smoked and had normal preoperative seated spirometry) were investigated by using the technique of rapid airway occlusion during constant-flow inflation. With the patients in the supine position, we measured the interrupter resistance (Rint,rs), which in humans probably reflects airway resistance, the "additional" resistance (delta Rrs) due to viscoelastic pressure dissipation and time-constant inequalities, and static respiratory elastance (Est,rs). Intra-abdominal pressure (IAP) was measured by using a bladder catheter, and functional residual capacity was measured by the heliumdilution technique. The results were compared with a previous study on 16 normal anesthetized paralyzed humans. Compared with normal persons, we found that in obese subjects: 1) functional residual capacity was markedly lower (0.645 +/- 0.208 liter) and IAP was higher (24 +/- 2.2 cmH2O); 2) alveolar-arterial oxygenation gradient was increased (178 +/- 59 mmHg); 3) the volume-pressure curve of the respiratory system was curvilinear with an "inflection" point; 4) Est,rs, Rint,rs, and delta Rrs were higher than normal (29.3 +/- 5.04 cmH2O/l, 5.9 +/- 2.4 cmH2O.l-1.s, and 6.4 +/- 1.6 cmH2O.l-1.s, respectively); 5) Rint,rs increased with increasing inspiratory flow, Est,rs did not change, and delta Rrs decreased progressively; and 6) with increasing inflation volume, Rint,rs and Est,rs decreased, whereas delta Rrs rose progressively. Overall, our data suggest that obese subjects during sedation and paralysis are characterized by hypoxemia and marked alterations of the mechanical properties of the respiratory system, largely explained by a reduction in lung volume due to the excessive unopposed IAP.