We investigated the effects of body mass index (BMI) on functional residual capacity (FRC), respiratory me-chanics (compliance and resistance), gas exchange, and the inspiratory mechanical work done per liter of venti-lation during general anesthesia. We used the esopha-geal balloon technique, together with rapid airway oc-clusion during constant inspiratory flow, to partition the mechanics of the respiratory system into its pul-monary and chest wall components. FRC was meas-ured by using the helium dilution technique. We stud-ied 24 consecutive and unselected patients during general anesthesia, before surgical intervention, in the supine position (8 normal subjects with a BMI 125 kg/m', 8 moderately obese patients with a BMI >25 kg/m2 and ~40 kg/m', and 8 morbidly obese pa-tients with a BMI 240 kg/m'). We found that, with in-creasing BMI: 1. FRC decreased exponentially (Y = 0.86; P < 0.01) 2. the compliance of the total respiratory system and of the lung decreased exponentially (Y = 0.86; P < 0.01 and Y = 0.81; P < 0.01, respectively), whereas the compliance of the chest wall was only minimally af-fected (Y = 0.45; P < 0.05) 3. the resistance of the total respiratory system and of the lung increased (Y = 0.81; P < 0.01 and Y = 0.84; P < 0.01, respectively), whereas the chest wall resis-tance was unaffected (r = 0.06; P = not significant) 4. the oxygenationindex (Pao,/PAo,) decreased expo-nentially (Y = 0.81; P < 0.01) and was correlated with FRC (Y = 0.62; P < O.Ol), whereas Pace, was unaf-fected (r = 0.06; P = not significant) 5. the work of breathing of the total respiratory system increased, mainly due to the lung component (Y = 0.88; P < 0.01 and Y = 0.81; P < 0.01, respectively).
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