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 duri ng exercise is a sensitive measurement of gas exchange that reflects m atching of ventilation to perfusion, but requires an invasive measurem ent for its calculation. Determining VD/VT noninvasively uses estimati ons of arterial PCO2 based on the end-tidal PCO2. To further standardi ze 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]) de rived from healthy subjects during steady-tate exercise or PETCO(2) al one (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 Si percent of value s greater than or equal to 0.36. In 26 subjects who achieved higher wo rk rates, the mean difference between actual VD/VT and VD/VT(J) increa sed from 0.009 +/- 0.04 (NS) at impairment. low work rate (Vo(2) = 28. 3 percent pred max) to 0.040 +/- 0.06 at high work rate (VO2 = 54.7 pe rcent pred max), p = 0.006. Actual VD/VT identified 18 (69 percent) pa tients 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 duri ng incremental exercise in a patient population, methods for estimatin g VD/VT progressively underestimate this measurement; and therefore, ' 'normal'' estimated VD/VT values may fail to identify underlying pulmo nary and/or pulmonary vascular impairment.