Increased apparatus dead space and tidal volume increase blood concentrations of oxygen and sevoflurane in overweight patients: a randomised controlled clinical study.

BACKGROUND AND OBJECTIVE

General anaesthesia impairs respiratory function in overweight patients. We wanted to determine whether increased tidal volume (V(T)), with unchanged end-tidal carbon dioxide partial pressure (P(ET)CO₂), affects blood concentrations of oxygen and sevoflurane in overweight patients.

METHODS

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 V(T) (NV(T)) and one group with increased V(T) (IV(T)) achieved by increasing inspired plateau pressure 0.04 cmH₂O kg⁻¹ above initial plateau pressure. Extra apparatus dead space was added to maintain P(ET)CO₂ 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 (P(a)sevo, F(i)sevo, P(ET)sevo), paCO₂, P(ET)CO₂, V(t) and airway pressure were measured.

RESULTS

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 IV(T) 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).

CONCLUSION

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.