The effects of several gases (He, N2, N2O, and SF6) on gas trapping in excised lungs.

Rat lungs were ventilated in a saline filled plethysmograph with the trachea attached to a cannula extending through the chamber base. Because it was possible to connect the cannula to an anesthesia bag, the gases used to ventilate the lungs could easily be controlled. During the first part of the study, lungs were inflated-deflated at different rates for 10 cycles with one of four different gases (SF6, N2, He, or N2O). It was found that all gases were trapped at very slow ventilation rates. As the rates were increased, however, the amount of gas trapped in the lungs decreased at ventilation rates which were characteristic of each particular gas. Gas trapping decreased first in lungs ventilated with SF6, then N2, He, and finally N2O. The amount of gas trapped in the lungs at a given inflation-deflation rate was related to the solubility of the gas divided by the square root of its molecular weight. During the second part of the study the effect of different mixtures of SF6 and O2 on the amount of gas trapped was examined. All the results indicated that diffusion of gases through liquid walls of menisci or bubbles that occlude the airways is responsible for trapped gas in excised lungs.