RESPIRATORY SYSTEM MECHANICS IN SEDATED, PARALYZED, MORBIDLY OBESE PATIENTS

The effects of inspiratory flow and inflation volume on the mechanical properties of the respiratory system in eight sedated and paralyzed p ostoperative morbidly obese patients (aged 37.6 +/- 11.8 yr who had ne ver smoked and had normal preoperative seated spirometry) were investi gated 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 reflec ts 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 cap acity was measured by the helium-dilution technique. The results were compared with a previous study on 16 normal anesthetized paralyzed hum ans. Compared with normal persons, we found that in obese subjects: 1) functional residual capacity was markedly lower (0.645 +/- 0.208 lite r) and IAP was higher (24 +/- 2.2 cmH(2)O); 2) alveolar-arterial oxyge nation gradient was increased (178 +/- 59 mmHg); 3) the volume-pressur e curve of the respiratory system was curvilinear with an ''inflection '' point; 4) Est,rs, Rint,rs, and Delta Rrs were higher than normal (2 9.3 +/- 5.04 cmH(2)O/l, 5.9 +/- 2.4 cmH(2)O . l(-1). s, and 6.4 +/- 1. 6 cmH(2)O . l(-1). s, respectively); 5) Rint,rs increased with increas ing inspiratory flow, Est,rs did not change, and Delta Rrs decreased p rogressively; 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 charact erized by hypoxemia and marked alterations of the mechanical propertie s of the respiratory system, largely explained by a reduction in lung volume due to the excessive unopposed IAP.