A Nomogram for B.t.p.s.-volume Corrections in Pulmonary Ventilation Tests.

The conversion of a spirometer tracing into a volume of gas corrected to body temperature and pressure saturated with water vapour (B.T.P.S.volume) involves: (a) a factor to convert centimetres of tracing height to millilitres of gas at the existing (or 'ambient') temperature and pressure saturated with water vapour (A.T.P.S.-volume), this factor being 300 for the Godart apparatus in use in this Unit, since 1 cm. is equivalent to 300 ml. of gas; (b) conversion of A.T.P.S.-volume to B.T.P.S.-volume by the application of Boyle's and Charles' laws and the known values of water vapour partial pressure at different temperatures. Detailed investigation of a single patient requires that these steps should be repeated many times. Hence a welcome contribution was the provision of a set of factors for the second calculation in the range 20-37'C. (Comroe, Forster, Dubois, Briscoc, and Carlsen, 1962), barometric pressure changes between 750 and 770 mm. Hg having been shown to be insignificant. These factors are therefore primarily dependent upon laboratory temperature. This paper extends the range of the Comroe factors to 15-40'C., as these extremes have occasionally been required, and presents an easily constructed nomogram for the complete calculation.