Positive end-expiratory pressure improves end-expiratory lung volume but not oxygenation after induction of anaesthesia

Background and objective Induction of anaesthesia promotes collapse of dependent lung regions in both obese and nonobese patients. We hypothesized that end-expiratory lung volume (EELV) may be more sensitive than oxygenation to evaluate the effects of positive end-expiratory pressure (PEEP) after anaesthesia induction. Methods Forty patients (20 nonobese patients and 20 obese patients) were prospectively studied. After anaesthesia induction, PEEP was adjusted in a stepwise fashion [zero end-expiratory pressure (ZEEP), PEEP 5 cmH(2)O and PEEP 10 cmH(2)O]. At each step, we measured EELV, static elastance, gas exchange and dead space. Other than changing PEEP, respiratory settings were kept constant throughout. Results Anaesthesia induction and ZEEP both lowered EELV by 39% in nonobese patients and 59% in obese patients (both P<0.05), as well as oxygenation (P<0.05). Compared with ZEEP, in nonobese patients, PEEP 5 cmH(2)O and PEEP 10 cmH(2)O improved EELV (+15 and +40%, respectively, P<0.01) and elastance but not oxygenation. In obese patients, PEEP 10 cmH(2)O also improved EELV (49% vs. ZEEP and 30% vs. PEEP 5 cmH(2)O, P<0.01), elastance and dead-space fraction, with no effect on oxygenation. PEEP-induced changes of EELV correlated with changes of elastance (r(2) = 0.46, P=0.003), but not with oxygenation. Conclusion After induction of anaesthesia, mechanical ventilation with ZEEP is associated with a profound reduction in EELV. PEEP improves efficiently EELV and respiratory mechanics, with no major effect on oxygenation. EELV may be a useful indicator to guide PEEP setting in the operating room. Eur J Anaesthesiol 2010;27:508-513