ATS Pulmonary Function Laboratory Manual

chapter  19

2. Inspired volume ( ˙ V i) is needed for other calculations, and if room air is inspired, can be obtained from: ˙ V = ˙ V × 1 − F O 2 − F CO 2 0 . 7904 where: Fe O 2 = mixed expired O 2 concentration. Fe CO 2 = mixed expired CO 2 concentration. 3. ˙ V co 2 and ˙ V o 2 are calculated from timed gas collection and concentration of mixed expired gases (either direct measurement of a mixed expired gas sample, or calculated from analysis of expired gas flow and con- centration), corrected to ml/min at STPD conditions (40): ˙ V O 2 = ˙ V , BTPS × F O 2 − ˙ V , BTPS × F O 2 × PB − 47 0 . 863

PB − 47 0 . 863

˙ V CO 2 =

˙ V , BTPS × F CO 2

− ˙ V , BTPS × F CO 2

×

where:

˙ V e and ˙ V i are calculated above 0.863 = conversion from L/min to ml/min and the ratio of (760 × 310/273) × 0.001 is the factor for the conversion from BTPS to STPD conditions. Fi O 2 = fractional concentration of inspired O 2 Fi CO 2 = fractional concentration of inspired CO 2 Fē O 2 = fractional concentration of mixed expired O 2 Fe CO 2 = fractional concentration of mixed expired CO 2

4. There are variables of interest that are derived from ˙ V e, ˙ V co 2 , ˙ V o 2

and heart rate (HR): the ventilatory /HR), and the gas exchange ratio or respiratory

equivalents ( ˙ V e/ ˙ V o 2 exchange ratio (RER = ˙ V co 2

, and ˙ V e/ ˙ V co 2

), the oxygen pulse ( ˙ V o 2

/ ˙ V o 2

) (3, 8, 50).

5. Measures of pulmonary dead space to tidal volume ratio can be done either with analysis of arterial blood gases using the classic Bohr equation (which gives the physiological dead space exactly) (51): V D /V T % = [( Pa CO 2 Pa CO 2 − P CO 2 ) / ] − ( V D , valve/V T ) ] × 100 , = mixed expired partial pressure of CO 2 Vd,valve = respiratory valve and mouthpiece dead space, or by a calculation erroneously called “non-inva- sive Vd/Vt” is made by assuming that Pet CO 2 reflects the partial pressure of CO 2 in arterial blood. Non − invasive V D /V T % = [( P ET CO 2 − P E CO 2 ) /P ET CO 2 ] − ( V D , valve/V T ) × 100 The latter equation is usually misleading and can lead to radically wrong Vd/Vt values because Pet CO 2 can be considerably different than Pa CO 2 in the presence of significant gas exchange abnormalities which yield alveolar dead space (52). Utilization of “non-invasive Vd/Vt” is inappropriate in CPET. where Pe CO 2

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