Diffusion of nitrous oxide into the pleural cavity

We postulated that nitrous oxide transfer into the pleural cavity can occur by diffusion from the alveoli, independent of vascular transport. Under ge neral anaesthesia, six sheep were studied in two phases, a control and an e xperimental phase. The sheep were anaesthetized, intubated, and received po sitive pressure mechanical ventilation. A catheter was placed in the right pleural cavity and 150 ml air injected. The animals were ventilated with 10 0% oxygen. The inspired gas was changed to a mixture of 50% nitrous oxide a nd 50% oxygen, and the rate of increase of nitrous oxide concentration in t he pleural space was measured. The animals were then ventilated with 100% o xygen and then killed by exsanguination while ventilation was continued. Th e inspired mixture was changed to 50% nitrous oxide and 50% oxygen and the rate of increase in nitrous oxide concentration was measured in the pleural space again. During ventilation with nitrous oxide in the living animals, the concentration of nitrous oxide in the pleural cavity increased rapidly and decreased to zero during ventilation with 100% oxygen. During ventilati on without circulation, the rate of increase in the concentration of nitrou s oxide in the pleural cavity was the same as in the control phase. This su ggests that nitrous oxide enters the pleural space by diffusion, rather tha n by vascular delivery. This mechanism may explain the rapid increase in th e volume of pneumothorax if nitrous oxide is given in the inspired gas.