TIDAL VOLUME, LUNG HYPERINFLATION AND ARTERIAL OXYGENATION DURING GENERAL-ANESTHESIA

Impaired pulmonary oxygen (O2) exchange is common during general anaes thesia but there is no clinical unanimity as to methods of prevention or treatment. We studied 14 patients at risk for pulmonary dysfunction because of increased age, obesity, cigarette smoking, or chronic lung disease. Pulmonary O2 exchange was measured during four conditions of ventilation: awake spontaneous, conventional tidal volume (CV(T), 7 m l.kg-1) or high tidal volume (HV(T), 12 ml.kg-1) controlled ventilatio n, and five min after manual hyperinflation (HI) of the lungs. The F(I )O2 was controlled at 0.5, and F(ET)CO2 was kept constant by adding de ad space during HV(T). Eight patients were ventilated with N2O/O2 and six with air/O2. Arterial blood gases were used to calculate the (A-a) DO2. In seven patients (A-a)DO2 worsened after induction of anaesthesi a, while in seven there was no change or an improvement. Manual HI sig nificantly reduced (A-a)DO2, but changing tidal volume (VT) had no eff ect. Using a multivariate model to predict O2 exchange, obesity and ty pe of surgery were significantly associated with worsening, while leve l of V(T) and inspiratory gas (N2O or N2) were not significant predict ors. Thus patient and surgical factors were more important determinant s of pulmonary gas exchange during anaesthesia than were tidal volume or inspiratory gas. Manual HI is a simple and effective manoeuvre to i mprove gas exchange.