HANDBOOK of RESPIRATION 



1. ABSOLUTE LUNG VOLUMES, DEFINITIONS AND CONVERSIONS: ATPS. BTPS, AND STPD CONDITIONS 



Gas volume in the lung exists at ^ody Temperature and atmospheric Pressure and is completely Saturated 

 with water vapor at body temperature- -hence the designation BTPS. 



However, once the gas has been blown into a measuring device such as a spirometer, the temperature will 

 have dropped to the spirometer or Ambient Temperature; although the gas volume is still Saturated with water vapoi 

 at the lower ambient temperature the water vapor volume is reduced. The Pressure of the atmosphere is the same. 

 This condition is designated ATPS. 



Under average laboratory conditions (ATPS), the "true" lung volume (BTPS) will shrink, in response to the 

 ambient temperature and barometric pressure, to perhaps 93%, as shown in the figure below. If this lung volume is 

 then converted to conditions of Standard T[emperature and Pressure with all water vapor removed (or Dry), this 

 STPD value will be approximately 83% of the BTPS lung volume- -sometimes even less in accordance with the baro- 

 metric pressure (also as shown in the figure below). 



It must also be borne in mind that lung volume measurements are often made on closed breathing circuits 

 which contain a CO^ absorber. Any volume expired into such a system will, of course, be automatically reduced by 

 the percentage of CO^ in the expired air; for a Vital Capacity obtained after full inspiration and before maximal 

 expiration, this reduction may well be of the order of 2-3%. This discrepancy must be considered in making refer- 

 ence to "absolute volumes." 



All lung volumes are normally recorded at ATPS conditions. Conversion to BTPS conditions which repre- 

 sent true or anatomical lung volume requires knowledge of room or spirometer temperature and approximate baro- 

 metric pressure. 3,0 p„-pH20 



True lung volume (BTPS) = lung volume at ATPS x x " .p" , where t = spirometer temperature in 



degrees C; Pg = barometric pressure in mm Hg; and pH^O = vapor pressure of water at spirometer temperature t. 

 310 is absolute body temperature of 370C, and 47 mm Hg is the vapor pressure of water at 370C. 



BTPS 



% Lung Volume 



ATPS 

 At Various 

 Temps, °C 



/ 



760 



700 b50 



Barometric Pressure, mm Hg 



Contributor: Rahn, H. 



