Table. 5—

Corrections for barometric pressure(PB), ambient water vapour pressure (PH2O), partial pressure of CO2 and temperature

H2O removed from sampled gas; CO2 does not interfere with analysers
VA,BTPS = (VI,ATPDVD,INSTVD,ANAT)×(FI,Tr/FS,Tr)×(PB/(PB–47))×(310/(273+T))
VA,STPD = (VI,ATPDVD,INSTVD,ANAT)×(FI,Tr/FS,Tr)×(PB/760)×(273/(273+T))
H2O and CO2 removed from sampled gas
VA,BTPS = (VI,ATPDVD,INSTVD,ANAT)×(FI,Tr(1+FA,CO2)/FS,Tr)×(PB/(PB–47))×(310/(273+T))
VA,STPD = (VI,ATPDVD,INSTVD,ANAT)×(FI,Tr(1+FA,CO2)/FS,Tr)×(PB/760)×(273/(273+T))
  If no measurement of FA,CO2 is available, then it may be assumed to be 0.05
H2O in sampled gas equilibrated to room air; CO2 does not interfere with analysers. If FI,Tr is read by the analysers, the equations are the same as for when H2O is removed from sampled gas. If tank values (i.e. dry gas concentrations) are used for FI,Tr, then the following equations are used
VA,BTPS = (VI,ATPDVD,INSTVD,ANAT)×(FI,Tr/FS,Tr)×((PBPH2O)/(PB–47))×(310/(273+T))
VA,STPD = (VI,ATPDVD,INSTVD,ANAT)×(FI,Tr/FS,Tr)×((PBPH2O)/760)×(273/(273+T))
Neither H2O nor CO2 removed from sampled gas, no interference with analysers, heated sample tubing to prevent condensation
VA,STPD = (VI,ATPDVD,INSTVD,ANAT)×(FI,Tr/FS,Tr)×((PB–47)/760)×(273/(273+T))
  • In these calculations, room temperature (T) is measured in Celsius and gas pressures are measured in mmHg. In all four cases, the inspired volume (VI) is the measured volume of inhaled dry gas and, thus, is considered under ambient temperature, ambient pressure, and dry (ATPD) conditions. The conversion to body temperature, ambient pressure, saturated with water vapour (BTPS) and standard temperature, pressure and dry (STPD) may require factors to compensate for the diluting or concentrating effects of adding or deleting H2O or CO2 at the gas sampling site. Therefore, standard gas condition conversion formulae must be adjusted as described previously. VA: alveolar volume; VD,INST: instrument dead space; VD,ANAT: anatomic dead space; FI,Tr: fraction of tracer (Tr) gas in the inspired test gas; FS,Tr: fraction of the Tr gas in the alveolar sample, which may differ from the fraction of alveolar Tr gas, depending on the effects of CO2 and H2O as noted; FA,CO2: fraction of CO2 in the alveolar sample.