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(Chest. 1994;106:131-136.)
© 1994 American College of Chest Physicians

Estimated vs Actual Values for Dead Space/Tidal Volume Ratios During Incremental Exercise in Patients Evaluated for Dyspnea

Mark I. Zimmerman M.D., F.C.C.P.1; Albert Miller M.D., F.C.C.P.1; Lee K. Brown M.D., F.C.C.P.1; Anand Bhuptani M.D.1; Mark F. Sloane M.D., F.C.C.P.1; and Alvin S. Teirstein M.D., F.C.C.P.1

1 From the Pulmonary Physiology Laboratory, Division of Pulmonary and Critical Care Medicine, The Mount Sinai School of Medicine, New York

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 ge0.36. 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.

Submitted on February 12, 1993
Accepted on November 19, 2007




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