Total respiratory system, lung, and chest wall mechanics in sedated-paralyzed postoperative morbidly obese patients

Objective: To study the relative contribution of the lung and the chest wall on the total respiratory system mechanics, gas exchange, and work of breathing in sedated-paralyzed normal subjects and morbidly obese patients, in the postoperative period. Setting: Policlinico Hospital, University of Milan, Italy. Methods: In ten normal subjects (normal) and ten morbidly obese patients (obese), we partitioned the total respiratory mechanics (rs) into its lung (L) and chest wall (w) components using the esophageal balloon technique together with airway occlusion technique, during constant flow inflation. We measured, after abdominal surgery, static respiratory system compliance (Cst,rs), lung compliance (Cst,L), chest wall compliance (Cst,w), total lung (Rmax,L) and chest wall (Rmax,w) resistance. Rmax,L includes airway (Rmin,L) and ''additional'' lung resistance (DR,L). DR,L represents the component due to viscoelastic phenomena of the lung tissue and time constant inequalities (pendelluft). Functional residual capacity (FRC) was measured by helium dilution technique, Results: We found that morbidly obese patients compared with normal subjects are characterized by the following: (1) reduced Cst,rs (p<0.01), due to lower Cst,L (55.3 plus or minus 15.3 mLxcm H2O-1 vs 106.6 plus or minus 31.7 mLxcm H2O-1; p<0.01) and Cst,w (112.4 plus or minus 47.4 mLxcm H2O-1 vs 190.7 plus or minus 45.1 mLxcm H2O-1; p<0.01); (2) increased Rmin,L (4.7 plus or minus 3.1 mLxcm H(2)OxL(-1)xs; vs 1,0 plus or minus 0.8 mLxcm H(2)OxL(-1)xs; p<0.01) and DR,L (4.9 plus or minus 2.6 mLxcm H(2)OxL(-1)xs; vs 1.5 plus or minus 0.8 mLxcm H(2)OxL(-1)xs; p< 0.01); (3) reduced FRC (0.665 plus or minus 0.191 L vs 1.691 plus or minus 0.325 L; p<0.01); (4) increased work performed to inflate both the lung (0.91 plus or minus 0.26 J/L vs 0.34 plus or minus 0.08 J/L; p < 0.01) and the chest wall (0.39 plus or minus 0.13 J/L vs 0.18 plus or minus 0.04 J/L; p < 0.01); and (5) a reduced pulmonary oxygenation index (PaO2/PAO(2) ratio). Conclusion: Sedated-paralyzed morbidly obese patients, compared with normal subjects, are characterized by marked derangements in lung and chest wall mechanics and reduced lung volume after abdominal surgery. These alterations may account for impaired arterial oxygenation in the postoperative period.