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1 From the Istituto di Anestesia e Rianimazione, Universita' di Milano and Servizio di Anestesia e Rianimazione, Ospedale Maggiore IRCCS, Milan, Italy
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±15.3 mLxcm H2O
1vs 106.6±31.7 mLxcm H2O1; p<0.01) and Cst,w (112.4±47.4 mLxcm H2O1vs 190.7±45.1 mLxcm H2O1; p<0.01); (2) increased Rmin,L (4.7±3.1 mLxcm H2OxL1xs; vs 1.0±0.8 mLxcm H2OxL1xs; p<0.01) and DR,L (4.9±2.6 mLxcm H2OxL1xs; vs 1.5±0.8 mLxcm H2OxL1xs; p<0.01); (3) reduced FRC (0.665±0.191 L vs 1.691±0.325 L; p<0.01); (4) increased work performed to inflate both the lung (0.91±0.25 J/L vs 0.34±0.08 J/L; p<0.01) and the chest wall (0.39±0.13 J/L vs 0.18±0.04 J/L; p<0.01); and (5) a reduced pulmonary oxygenation index (PaO2/PAO2 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.
Key Words: anesthesia • gas-exchange • mechanical ventilation • morbid obesity • respiratory mechanics
Submitted on November 30, 1994
Accepted on June 8, 1995
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