LARGE TIDAL VOLUME VENTILATION DOES NOT IMPROVE OXYGENATION IN MORBIDLY OBESE PATIENTS DURING ANESTHESIA

Eight morbidly obese patients (body mass index [BMI] = 46) were studie d during general anesthesia and controlled mechanical ventilation. To evaluate the effect of large tidal volume ventilation on oxygenation a nd ventilation, the baseline 13 mL/kg tidal volume (VT) (calculated by the ideal body weight) was increased in 3 mL/kg volume increments to 22 mL/kg, while ventilatory rate (RR) and inspiratory time (TI) were k ept constant. Each volume increment was maintained for 15 min. Gas exc hange was assessed by measuring the arterial blood oxygen tensions, an d calculating the indices of alveolar-arterial oxygen tension differen ce [P(A-a)O-2] and arterial/alveolar oxygen tension ratio (a/A). Peak inspiratory airway pressure (Ppeak), end-inspiratory airway pressure ( Pplateau), and compliance of the respiratory system (CRS) were recorde d using the Capnomac Ultima(TM) (Datex, Helsinki, Finland) on-line res piratory monitor. Increasing tidal volumes to 22 mL/kg increased the r ecorded Ppeak (26.3 +/- 4.1 vs 37.9 +/- 3.2 cm H2O, P < 0.008), Pplate au (21.5 +/- 3.6 vs 27.7 +/- 4.3 cm H2O, P < 0.01), and CRS (39.8 +/- 7.7 vs 48.5 +/- 8.3 mL/cm H2O) significantly without improving arteria l oxygen tension and resulted in severe hypocapnia. Since changes in a rterial oxygenation were small and not statistically significant, mech anical ventilation of morbidly obese patients with large VTs seems to offer no advantage to smaller (13 mL/kg ideal body weight) VTs.