Influence of tidal volume on pulmonary gas exchange during general anaesthesia

Background and objective: General anaesthesia impairs respiratory function. The present studies were performed to compare arterial concentration of sevoflurane, oxygen and carbon dioxide in normal and overweight patients ventilated with increased tidal volume (VT), or normal tidal volume with and without PEEP. Methods: Prospective, randomised, clinical studies. ASA І and II abdominal surgery patients were randomly assigned to be ventilated with normal VT (NVT) with and without PEEP to 10 cmH2O or with increased VT (IVT) achieved by increasing inspired plateau pressure 0.04 cm H2O kg-1. Extra apparatus dead space was added to maintain PETCO2 at 4.5 kPa. Arterial oxygenation, sevoflurane tension (Pasevo, Fisevo, PETsevo), PaCO2, PETCO2, stroke volume, cardiac output, VT and airway pressure were measured. Results: The groups of patients compared were similar regarding gender, age, and BMI. Arterial oxygen and sevoflurane tension was generally higher in the IVT group (P < 0.05) whereas mean FiO2 and PETsevo did not differ between the groups. Arterial carbon dioxide was significantly lower with IVT than NVT ventilated without PEEP but in the presence of PEEP in the NVT group, the groups were similar. Cardiac output decreased significantly less in the IVT group compared to the NVT group with PEEP (5 and 33 % respectively). Conclusion: Isocapnic ventilation with larger tidal volumes maintained with added apparatus dead-space increases the arterial oxygen and sevoflurane tension as well as carbon dioxide elimination in normal and overweight patients, and in overweight patient also preserves cardiac output better than in the presence of PEEP