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Unformatted text preview: A RAPID PLETHYSMOGRAPHIC METHOD FOR MEASURING THORACIC GAS VOLUME: A COMPARISON WITH A NI- TROGEN WASHOUT METHOD FOR MEASURING FUNC- TIONAL RESIDUAL CAPACITY IN NORMAL SUBJECTS1 By ARTHUR B. DuBOIS, STELLA Y. BOTELHO, GEORGE N. BEDELL, ROBERT MARSHALL, AND JULIUS H. COMROE, JR. (From the Department of Physiology and Pharmacology, Graduate School of Medicine, University ofPennsylvania,Philadelphia,Pa.) (SubmittedforpublicationOctober3,1955;acceptedOctober31,1955) In general, two principles have been employed inthemeasurement of the volume of gas in the lungs. The dilution techniques, using either a closed (rebreathing) or open (non-rebreathing) systemare themost commonly used; thephysical methods, based on Boyle's law relating pressure and volume, requirespecial equipment and have been used relatively infrequently. This report presents a rapid, practical physical method for measuring lung volumes plethysmographically; it has been found to give reproducible values which, inhealthy subjects, do not differ signifi- cantlyfromvaluesobtainedbythedilutionmethod ofDarling,Cournand,andRichards (1). Two types ofphysical (pneumatometric) tech- niqueshavebeendescribedpreviously,bothbased on Boyle'slaw. One of these isthe decompres- sionmethod used by Hitchcock,Edelmann, Shel- den, and Whitehorn (2), Willmon and Behnke (3),and Dejoursand Rahn (4);ithasbeenused in experimentalanimals (5,6) aswellasinnor- malman. In thedecompressionmethod,thepres- sure aboutthebodyand inthetracheaand lungs israisedequallyaboveatmospheric soastocom- press thegasinsidethelungs andoutsidethebody. Thetotalnumberofmoleculesistherebyincreased inallpartsofthelunginfreecommunication with an open airway. The pressures in the chamber and in the trachea are then released simultane- ously. Duringreturntonormalatmosphericpres- sure, theamount of gas which flows out of the trachea is measured; this is the amount of gas whichhadbeenaddedtothelunggasbytheprevi- 'This investigation was supported in part by a re- search grant, H-406from the National Heart Institute ofthe NationalInstitutesof Health, U. S.Public Health Service. ous increment of pressure. The pulmonary gas volume atatmospheric pressure can then be cal- culated by applicationofBoyle'slaw. Since the method measures the gas volume which flows in andoutof thetracheaduringcompressionand de- compression, itmeasures thevolume of lunggas which isin communicationwiththetrachea. The othertype,based on voluntarycompression or decompression of lung gas, was described in principle by Pfluger in 1882 (7). It requires measurements of changesinalveolargaspressure and volume while the subject makes voluntary respiratory efforts against a closed airway. It givesvaluesforthevolumeofgasinthelungs and thorax, whether in free communication with the airway or not....
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