Chest
Volume 113, Issue 4, April 1998, Pages 913-918
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Clinical Investigations: Exercise
Breathing Reserve at the Lactate Threshold to Differentiate a Pulmonary Mechanical From Cardiovascular Limit to Exercise

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Study objectives

Criteria used to define the respective roles of pulmonary mechanics and cardiovascular disease in limiting exercise performance are usually obtained at peak exercise, but are dependent on maximal patient effort. To differentiate heart from lung disease during a less effort-dependent domain of exercise, the predictive value of the breathing reserve index (BRI=minute ventilation [ V˙E]/maximal voluntary ventilation [MVV]) at the lactate threshold (LT) was evaluated.

Design

Thirty-two patients with COPD and a pulmonary mechanical limit (PML) to exercise defined by classic criteria at maximum oxygen uptake ( V˙o2max) were compared with 29 patients with a cardiovascular limit (CVL) and 12 normal control subjects. Expired gases and VE were measured breath by breath using a commercially available metabolic cart (Model 2001; MedGraphics Corp; St. Paul, Minn). Arterial blood gases, pH, and lactate were sampled each minute during exercise, and cardiac output (Q) was measured by first-pass radionuclide ventriculography (System 77; Baird Corp; Bedford, Mass) at rest and peak exercise.

Results

For all patients, the BRI at lactate threshold (BRILT) correlated with the BRI at V˙o2max (BRIMAX) (r=0.85, p<0.0001). The BRILT was higher for PML (0.73±0.03, mean±SEM) vs CVL (0.27±0.02, p<0.0001), and vs control subjects (0.24±0.03, p<0.0001). A BRILT ≥0.42 predicted a PML at maximum exercise, with a sensitivity of 96.9%, a specificity of 95.1%, a positive predictive value of 93.9%, and a negative predictive value of 97.5%.

Conclusions

The BRILT, a variable measured during the submaximal realm of exercise, can distinguish a PML from CVL.

Section snippets

Study Population

Consecutive maximum incremental cardiopulmonary exercise tests performed at the Massachusetts General Hospital (MGH) from 1991 to 1995, in which arterial blood was sampled during exercise, were evaluated. From this population, three groups were identified based on the type of exercise limit reached: normal (NL), PML, and cardiovascular limit (CVL). NL subjects consisted of individuals with suspected cardiopulmonary disease, but with a V˙o2max >80% predicted, maximum Q˙(Q˙max) >80%

RESULTS

The demographic characteristics of the three groups are shown in Table 1. There was no significant difference in age or Hb among groups. Weight was reduced in the PML group. There was an equal sex distribution in PML and CVL groups, but there was a male predominance in the NL group, β-Adrenergic blockers were being taken by nine (31%) CVL patients, one (12%) NL subject, and none in the PML group.

The PML group demonstrated a marked reduction in FEV1, with hyperinflation and severely depressed

DISCUSSION

Exertional intolerance is a common but exceedingly nonspecific feature of pulmonary and cardiovascular disease. This has provided incentive for the development of cardiopulmonary exercise testing diagnostic algorithms that can be used to identify the limiting organ system in patients with concurrent disease of the lungs and heart1,2,20 and to direct therapy. Most variables thought to be useful in assessing an organ's functional reserve are measured at peak exercise with the implicit assumption

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    Supported by a research fellowship of The Will Rogers Institute (Dr. Oelberg) and an AHA Grant-in-Aid (Dr. Systrom). Manuscript received July 9, 1997; revision accepted September 16, 1997.

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