Estimated vs actual values for dead space/tidal volume ratios during incremental exercise in patients evaluated for dyspnea.
The physiologic dead space/tidal volume ratio (VD/VT) at rest and during exercise is a sensitive measurement of gas exchange that reflects matching of ventilation to perfusion, but requires an invasive measurement for its calculation. Determining VD/VT noninvasively uses estimations of arterial PCO2 based on the end-tidal PCO2. To further standardize incremental cardiopulmonary exercise testing, we compared actual VD/VT with estimated VD/VT values in 35 patients referred for evaluation of dyspnea. Estimates of VD/VT used the Jones' equation (VD/VT[J]) derived from healthy subjects during steady-state exercise or PETCO2 alone (VD/VT[ET]) to approximate PaCO2. At rest, mean values for VD/VT(J) and actual VD/VT were not different: 0.372 +/- 0.08 vs 0.376 +/- 0.09, p = not significant (NS). Each method identified 61 percent of values > or = to 0.36. In 26 subjects who achieved higher work rates, the mean difference between actual VD/VT and VD/VT(J) increased from 0.009 +/- 0.04 (NS) at low work rate (VO2 = 28.3 percent pred max) to 0.040 +/- 0.06 at high work rate (VO2 = 54.7 percent pred max), p = 0.006. Actual VD/VT identified 18 (69 percent) patients as abnormal vs 13 (50 percent) so identified by VD/VT(J). With exercise, VD/VT(J) was no better than VD/VT(ET). We conclude that during incremental exercise in a patient population, methods for estimating VD/VT progressively underestimate this measurement; and therefore, "normal" estimated VD/VT values may fail to identify underlying pulmonary and/or pulmonary vascular impairment.
Zimmerman
M I
MI
Pulmonary Physiology Laboratory, Mount Sinai School of Medicine, New York.
Miller
A
A
Brown
L K
LK
Bhuptani
A
A
Sloane
M F
MF
Teirstein
A S
AS
D000328:Adult; D000368:Aged; D004417:Dyspnea / Q000175:diagnosis / Q000209:etiology / Q000503:physiopathology*; D005080:Exercise Test; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D011653:Pulmonary Diffusing Capacity; D012126:Respiratory Dead Space; D013990:Tidal Volume