Distributions of ventilation-perfusion ratios in acute respiratory failure.

shown a doubling of the extravascular lung water but we know that this is a small percentage of the total lung weight; therefore, doubling of the extravascular lung water cannot explain doubling of the dry lung weight. Dr. West: It should be noted that we are not sure that these lungs were indeed stiff and that pulmonary function studies should be made at this point. All that was shown was that the extravascular lung water was increased. I mpairment of pulmonary gas exchange is a hallmark of acute respiratory failure, but its physiologic basis cannot be accurately defined with traditional analytic methods. Techniques such as the Riley three-compartment analysis do not permit differentiation between blood which is shunted through completely unventilated alveoli, and blood perfusing areas in which the ventilation -perfusion ratio (VA/()) is iow but not zero. Although in theory the breathing of 100 percent 02 should allow such a separation, it is likely that 02 per se results in atelectasis of just those areas in which the VA/((ratio is Iow.1.2 Thus, the shunt estimated by this method may be considerably different from that present breathing room air, so that the breakdown of the total venous admixture into components due to shunt and ventilation-perfusion inequality may become subject to a large error. A possible additional factor contributing to the impaired gas exchange is increased resistance to diffusion of 02 and CO2 across the blood gas barrier. This further complicates the analysis, and most workers have been content to characterize the gas exchange abnormalities simply by the alveolar-arterial 02 difference, venous admixture, and physiologic dead space, numbers which-give little insight into the nature of the gas exchange lesion. We3 have recently described a method in which the distribution of VA/C> ratios can be measured as a virtually continuous function under steady-state conditions. In particular, it distinguishes between areas containing shunt and those in which the '' A/Q ratio is low on the one hand, and between areas containing dead space and those in which the VA/C> ratio is high on the other. The method can be applied at any desired level of concentration of inspired oxygen (Fio2). METHODS A mixture of the six gases, sulphur hexafluoride, ethane, cyclopropane, halothane, diethyl ether, and acetone is dissolved in a suitable medium such as 5 percent dextrose in water, then infused into a peripheral vein for 20 minutes at the rate of 5 …