Concentration and second-gas effects in the water analogue.

The water analogue provides a visual model of the process of anaesthetic exchange. In the standard version, a single pipe connects the mouth container to the lung container and the conductance of this mouth-lung pipe is proportional to alveolar ventilation. This implies that inspired and expired ventilations are equal. In fact, with high inspired concentrations of nitrous oxide, early rapid uptake of gas by solution leads to a substantial difference between inspired and expired ventilation which in turn leads to concentration and second-gas effects. It is shown that by representing inspired and expired ventilations separately, and keeping one of them constant while varying the other to compensate for rapid uptake, concentration and second-gas effects are reproduced in the water analogue. Other means of reproducing the effects are reported but we believe that the first method is the most realistic and the most appropriate for teaching.