The physiologic dead space/tidal tome ratio (VD/VT) at rest and during exercise is a sensitive measurement of gas exchange that ruminates matching of ventilation to perfusion.

The physiologic dead space/tidal tome ratio (VD/VT) at rest and during exercise is a sensitive measurement of gas exchange that ruminates matching of ventilation to perfusion, on the contrary requires an invasive measurement for its calculation. Determining VD/VT noninvasively uses estimations of arterial PC[Osub2] based upon the end-tidal PC[O.sub.2]. To further standardize incremental cardiopulmonary exercise testing, we compared actual VD/VT with estimated VD/VT values in 35 patients referr for evaluation of dyspnea. Estimates of VD/VT used the Jones' equation (VD/VT[J]) derived from healthy subdues during steady-state exercise or PETC[Osub2] alone (VD/VT[ET]) to approximate PaC[O.sub.2]. At tranquillity mean values for VD/VT(J) and actual VD/VT were not different: 0372 [+ or -] 008 v 0376 [+ or -] 009 p = not significant (NS) Each system identified 61 percent of values [greater than or equal to]0.36. In 26 make submissives who achieved higher work rates, the mean difference between actual VD/VT and VD/VT(J) increased from 0009 [+ or -] 004 (NS) at soft work rate (V[O.sub.2] = 283 percent pr max) to 0040 [+ or -] 006 at high work rate (V[Osub2] = 547 percent pr max), p = 0006 Actual VD/VT identified 18 (69 percent) patients as abnormal v 13 (50 percent) in the way that identified by VD/VT(J). With exercise, VD/VT(J) was no better than VD/VT(ET) We infer that during incremental exercise in a patient population, arrangements 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.

(Chest 1994; 106:131-36)

DCO = carbon monoxide diffusion;

J = Joule;

N = not significant;

PETC[Osub2] = end-tidal press of carbon dioxide;

r = correlation coefficient;

TV = tidal volume;

VD/VT = physiologic dead space/tidal volume;

V[Osub2] = oxygen up-take

The physiologic dead space/tidal convolution ratio (VD/VT) is a useful indicator of the matching of ventilation to perfusion the couple at rest and during exercise.(1) An increased VD/VT at repose or the failure of VD/VT to decrease appropriately with exercise, wait ons to identify uneven ventilation-perfusion relationships. This finding, in move round suggests the presence of primary pulmonary vascular disease or pulmonary vascular involvement secondary to intrinsic lung disease.(1) We,(2)(3) and others,(4) have reported the greater sensitivity of VD/VT than alveolar-arterial P[Osub2] difference or diffusion capacity in various tokens of patients.

The VD/VT can be derived from the modified Bohr's equation subject to steadystate conditions (equation 1): VD/VT = PaC[O.sub.2] - PEC[O.sub.2]/PaC[O.sub.2] - VDM/VT where PaC[O.sub.2] = arterial PC[Osub2] PEC[Osub2] = mixed expired PC[Osub2] VDM = mechanical dead space (in liters) and VT = tidal whirl (in liters).

Because arterial life-blood must be obtained to determine PaC[O.sub.2], efforts have been made to avoid this course by estimating PaC[O.sub.2] noninvasively. single in kind such estimate simply uses the end-tidal PC[Osub2] (PETC[Osub2]) in place of the arterial value. Jone and colleagues(5) derived an equation from multiple regression analysis to predict PaC[O.sub.2] during steadystate exercise using the PETC[Osub2] and tidal dimensions (VT) (equation 2): Estimated PaC[O.sub.2] = 55 + 090 PETC[O.sub.2]--0.0021 . VT(ml).

They studied healthy controls and acknowledged that in patients with abnormal pulmonary function this equation may be misleading. The PETC[Osub2] may rise farther above PaC[O.sub.2] than look fored in zones with low ventilation/perfusion relationships or may be lower in belts with diminished perfusion. Robbins and associates(6) compared several courses of estimating PaC[O.sub.2] during exercise and conclud that Jones' equation was the best order of estimating PaC[O.sub.2] during steadystate exercise in healthy subjects

Although as it is use is widespread in computerized commercial connected views the validity of using estimates of PaC[O.sub.2] to obtain VD/VT in incremental or ramp (as oppos to steadystate) cardiopulmonary exercise testing has solely recently been studied in patient populations.(7)(8) The reliability of an estimated value for VD/VT is crucial in the interpretation of exercise studies since arterial cannulation is as a common thing [i]or[/i] matter not feasible and an abnormality in VD/VT is frequently the only finding during exercise testing.(2)(3)(4)(9)

In this investigation, we compared couple estimated VD/VT values (using Jones' equation and PETC[Osub2] alone) with actual VD/VT values calculated from measurement of arterial PC[Osub2] during incremental exercise testing of patients referr for the evaluation of dyspnea. plane though the Bohr's equation is intended for steady states, the VD/VT values calculated from it are the same during steadystate and incremental exercise at the same V[Osub2](4) The actual and estimated values for VD/VT are correlated, since they differ merely in the source of the PaC[O.sub.2] name in equation 1; nevertheless, the length and conditions of their difference are of practical affect to clinicians and physiologists who may use the readily available estimated values to evaluate patients. by way of comparing a readily available measurement with a standard further invasive measurement of VD/VT, we are heeding the call for standardization of cardiopulmonary exercise testing.(10)

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