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General anaesthesia impairs respiratory function in overweight patients. We wanted to determine whether increased tidal volume (VT), with unchanged end-tidal carbon dioxide partial pressure (PETCO2), affects blood concentrations of oxygen and sevoflurane in overweight patients.The present study is a prospective, randomised, clinical study. American Society of Anesthesiologists physical status I and II patients with BMI over 25 scheduled for elective surgery of the lower abdomen were randomly assigned to one of two groups with 10 patients in each. One group was ventilated with normal VT (NVT) and one group with increased VT (IVT) achieved by increasing inspired plateau pressure 0.04 cmH2O kg−1 above initial plateau pressure. Extra apparatus dead space was added to maintain PETCO2 at 4.5 kPa. Respiratory rate was set at 15 min−1, and sevoflurane was delivered to the fresh gas by a vaporiser set at 3%. Arterial oxygenation, sevoflurane tensions (Pasevo, Fisevo, PETsevo), paCO2, PETCO2, Vt and airway pressure were measured.The two groups of patients were similar with regard to sex, age, weight, height and BMI. Arterial oxygen tension (mean ± SD) was significantly higher in the IVT group (15 ± 4.3 vs. 10 ± 2.7 kPa after 60 min of anaesthesia, P < 0.05). Mean PETsevo did not differ between the groups, whereas arterial sevoflurane tension (mean ± SD) was significantly higher in the IVT group (1.74 ± 0.18 vs. 1.43 ± 0.19 kPa after 60 min of anaesthesia, P < 0.05).Ventilation with larger tidal volumes with isocapnia maintained with added apparatus dead space increases the tension of oxygen and sevoflurane in arterial blood in overweight patients.