RELIABILITY OF ESTIMATION OF FUNCTIONAL RESIDUAL CAPACITY IN EMPHYSEMATOUS PATIENT

Abstract

A comparison was made between various methods of measuring functional residual capacity (FRC) by nitrogen washout in a group of 7 patients with advanced chronic pulmonary emphysema and in 6 normal control subjects. Prolonged N2 washout while breathing oxygen was carried out for an average period of 39 minutes in such a fashion that estimation of the FRC could be made at the end of the usual 7 minutes, at the end of 15 minutes, and at the termination of the washout. The FRC was calculated at these times by the conventional formula of Darling and associates and also computed by the integration extrapolation technique of Emmanuel and co-workers. Using FRCTC[degree][degree] (the FRC computed from extrapolation of the prolonged washout) as a standard of comparison, was been found that in normal subjects the FRC measured by conventional methods at 7 minutes (FRC7) does not differ significantly from the values estimated at the later periods. This is in marked contrast to the findings in the group of emphysematous patients in whom the FRC7 is underestimated by 16.1%, the FRC15 is underestimated by 7.0%, and the FRC15[degree][degree] is overestimated by 4.2%. As in these same patients the difference between the FRC calculated at the end of the prolonged washout by conventional methods (FRCTC) and FRCTC[degree][degree] is only 1.0%, it is concluded that a good approximation of actual lung volume can be made even in severe emphysema by a prolonged washout (30 minutes or longer). However, values approximating FRCtc[degree][degree] by less than [plus or minus]10% can be achieved by the shorter FRC15[degree][degree] method. These methods of measuring FRC by N2 washout in emphysema suffer from dependence on the following assumptions: (1) that the mean alveolar N2 concentration can be measured at any given end point; (2) that the final phase of washout from the most poorly ventilated group of alveoli has been reached by the time of the termination of the washout procedure; and (3) that the tissue N2 contribution rate and magnitude in emphysema are similar to those of normal lungs. These assumptions are discussed.