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 +/- 15.3 mL x cm H2O-1 vs 106.6 +/- 31.7 mL x cm H2O-1; p < 0.01) and Cst,w (112.4 +/- 47.4 mL x cm H2O-1 vs 190.7 +/- 45.1 mL x cm H2O-1; p < 0.01); (2) increased Rmin,L (4.7 +/- 3.1 mL x cm H2O x L-1 x s; vs 1.0 +/- 0.8 mL x cm H2O x L-1 x s; p < 0.01) and DR,L (4.9 +/- 2.6 mL x cm H2O x L-1 x s; vs 1.5 +/- 0.8 mL x cm H2O x L-1 x s; 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.

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 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.

Effects of carbon dioxide insufflation for laparoscopic cholecystectomy on the respiratory system.

The changes occurring in total respiratory system, lung and chest wall mechanics, lung volume and gas-exchange during abdominal insufflation with carbon dioxide for laparoscopic cholecystectomy were studied. Using the technique of rapid airway occlusion during constant flow inflation together with an oesophageal balloon, we computed compliance and maximum resistance of the respiratory system, subsequently apportioning it into its lung and chest wall components. Maximum resistance of the respiratory system was further divided into airway resistance and the viscoelastic properties of the lung and the chest wall. In 10 patients (group 1), we measured respiratory system, lung and chest wall mechanics (compliance and resistance), functional residual capacity, end-tidal carbon dioxide tension and oxygen saturation. In addition, arterial blood gas analysis and end-tidal carbon dioxide tension were measured in a second group of 10 patients (group 2). Measurements, in both groups, were obtained in the reverse Trendelenburg position, at 15 min after the induction of anaesthesia, 5 min and 45 min after abdominal insufflation and at 15 min after abdominal deflation. Tidal volume, respiratory rate, inspiratory flow and the fraction of inspired oxygen were similar in both groups and maintained constant during the procedure. We found that abdominal carbon dioxide insufflation caused: a reduction in compliance of the respiratory system (both lung and chest wall components) and of functional residual capacity; a marked increase in the maximum resistance of the respiratory system (mainly due to increases in the viscoelastic properties of the lung and chest wall); no change in oxygenation, but an increase in the end-tidal carbon dioxide tension (which was correlated closely with the arterial carbon dioxide tension). These changes were not affected by the duration of anaesthesia.

The prone positioning during general anesthesia minimally affects respiratory mechanics while improving functional residual capacity and increasing oxygen tension.

We investigated the effects of the prone position on the mechanical properties (compliance and resistance) of the total respiratory system, the lung, and the chest wall, and the functional residual capacity (FRC) and gas exchange in 17 normal, anesthetized, and paralyzed patients undergoing elective surgery. We used the esophageal balloon technique together with rapid airway occlusions during constant inspiratory flow to partition the mechanics of the respiratory system into its pulmonary and chest wall components. FRC was measured by the helium dilution technique. Measurements were taken in the supine position and after 20 min in the prone position maintaining the same respiratory pattern (tidal volume 10 mL/kg, respiratory rate 14 breaths/min, FIO2 0.4). We found that the prone position did not significantly affect the respiratory system compliance (80.9 +/- 16.6 vs 75.9 +/- 13.2 mL/cm H2O) or the lung and chest wall compliance. Respiratory resistance slightly increased in the prone position (4.8 +/- 2.5 vs 5.4 +/- 2.7 cm H2O.L-1.s,P < 0.05), mainly due to the chest wall resistance (1.3 +/- 0.6 vs 1.9 +/- 0.8 cm H2O.L-1.s, P < 0.05). Both FRC and PaO2 markedly (P < 0.01) increased from the supine to the prone position (1.9 +/- 0.6 vs 2.9 +/- 0.7 L, P < 0.01, and 160 +/- 37 vs 199 +/- 16 mm Hg, P < 0.01, respectively), whereas PaCO2 was unchanged. In conclusion, the prone position during general anesthesia does not negatively affect respiratory mechanics and improves lung volumes and oxygenation.

Effects of the prone position on respiratory mechanics and gas exchange during acute lung injury.

We studied 16 patients with acute lung injury receiving volume-controlled ventilation to assess the relationships between gas exchange and respiratory mechanics before, during, and after 2 h in the prone position. We measured the end-expiratory lung volume (EELV, helium dilution), the total respiratory system (Cst,rs), the lung (Cst,L) and the thoracoabdominal cage (Cst,w) compliances (end-inspiratory occlusion technique and esophageal balloon), the hemodynamics, and gas exchange. In the prone position, PaO2 increased from 103.2 +/- 23.8 to 129.3 +/- 32.9 mm Hg (p < 0.05) without significant changes of Cst,rs and EELV. However, Cst,w decreased from 204.8 +/- 97.4 to 135.9 +/- 52.5 ml/cm H2O (p < 0.01) and the decrease was correlated with the oxygenation increase (r = 0.62, p < 0.05). Furthermore, the greater the baseline supine Cst,w, the greater its decrease in the prone position (r = 0.82, p < 0.01). Consequently, the oxygenation changes in the prone position were predictable from baseline supine Cst,w (r = 0.80, p < 0.01). Returning to the supine position, Cst,rs increased compared with baseline (42.3 +/- 14.4 versus 38.4 +/- 13.7 ml/cm H2O; p < 0.01), mainly because of the lung component (57.5 +/- 25.1 versus 52.4 +/- 23.3 ml/cm H2O; p < 0.01). Thus, (1) baseline Cst,w and its changes may play a role in determining the oxygenation response in the prone position; (2) the prone position improves Cst,rs and Cst,L when the supine position is resumed.

Prone positioning improves pulmonary function in obese patients during general anesthesia.

We investigated the effects of prone position on functional residual capacity (FRC), the mechanical properties (compliance and resistance) of the total respiratory system, lung and chest wall, and the gas exchange in 10 anesthetized and paralyzed obese (body mass index more than 30 kg/m2) patients, undergoing elective surgery. We used the esophageal balloon technique together with rapid airway occlusions during constant inspiratory flow to partition the mechanics of the respiratory system into its pulmonary and chest wall components. FRC was measured by the helium dilution technique. Measurements were taken in the supine position and after 15-30 min of prone position maintaining the same respiratory pattern (tidal volume 12 mL/kg ideal body weight, respiratory rate 14 breaths/ min, fraction of inspired oxygen [FIO2]0.4). We found that FRC and lung compliance significantly (P < 0.01) increased from the supine to prone position (0.894 +/- 0.327 L vs 1.980 +/- 0.856 L and 91.4 +/- 55.2 mL/cm H2O vs 109.6 +/- 52.4 mL/cm H2O, respectively). On the contrary, the prone position reduced chest wall compliance (199.5 +/- 58.7 mL/cm H2O vs 160.5 +/- 45.4 mL/cm H2O, P < 0.01), thus total respiratory system compliance did not change. Resistance of the total respiratory system, lung, and chest wall were not modified on turning the patients prone. The increase in FRC and lung compliance was paralleled by a significant (P < 0.01) improvement of PaO2 from supine to prone position (130 +/- 31 vs 181 +/- 28 mm Hg, P < 0.01), while PaCO2 was unchanged. We conclude that, in anesthetized and paralyzed obese subjects, the prone position improves pulmonary function, increasing FRC, lung compliance, and oxygenation.