ATS Pulmonary Function Laboratory Manual

ATS Pulmonary Function Laboratory Management & Procedure Manual | 3rd Edition

BSA ( m 2

) = 0 . 007184 × weight ( Kg ) 0 . 425 × height ( cm ) 0 . 725

Appendix 8. 2 Calculations for FRC He The initial Helium content of the apparatus is redistributed to include the subject’s lung volume at the time of con- nection to the apparatus (FRC He ) as follows: V app × F He 1 = ( V app + FRC He ) ( F He 2 ) Rearranging to solve for FRCHe yields: FRC He = V app ( F He 1 − F He 2 ) F He2 Where: Vapp is the known initial volume of the spirometer apparatus, F He 1 is the initial He fraction, and F He 2 is the He fraction after equilibration. 1. FRC He should be corrected for BTPS. 2. No corrections for He absorption should be made (1). 3. Vl includes the dead space of the valve and mouthpiece, which must be subtracted. 4. Correction factors for N 2 excretion during the He equilibration and corrections for He concentration when the respiratory quotient differs from 1.0 can be ignored. 5. Switching Error: In practice, patients are not always at FRC when they are switched into the spirometer cir- cuit. Corrections for this should be made from the spirometer trace when reporting FRC He . Some comput-

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