Chest
Volume 94, Issue 5, November 1988, Pages 931-938
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Clinical Investigations
Metabolic Acidosis during Exercise in Patients with Chronic Obstructive Pulmonary Disease: Use of the V-slope Method for Anaerobic Threshold Determination

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Patients with chronic obstructive pulmonary disease (COPD) usually have limited exercise tolerance owing to low ventilatory capacity. Because metabolic acidosis induced by exercise increases ventilatory drive, decreasing the hydrogen ion stimulus may improve exercise capacity. However, in those with mechanical limitation to ventilation or chemoreceptor insensitivity, identifying metabolic acidosis may be difficult using gas exchange methods that depend on the ventilatory response to the acidosis. We compared a modification of a gas exchange method (V-slope) for determining the lactate (anaerobic) threshold (ATX which is independent of ventilatory response with a method using the change in blood standard bicarbonate (HCO3) level in COPD and normal subjects during cycle incremental exercise. In 43 normal subjects, the V˙o2 at which metabolic acidosis was identified using the two method correlated (r = 0.75), although mean values differed. In 22 patients with moderately severe to severe COPD, eight who had a change in standard HCO3 <2.0 mEq/L between rest and 2 min of recovery from exercise (group 1) were contrasted with 14 whose blood standard HCO3 fell by >2.5 mEq/L (group 2). Mean VC was higher and FEV1/VC was lower in group 2, but mean FEV1, maximal voluntary ventilation, and diffusing capacity for carbon monoxide were not different. The degree of obstruction did not correlate strongly with the degree of exercise metabolic acidosis. The AT determined by the V-slope method was compared with that from standard HCO3; good correlation between these methods was found (r = 0.98), although mean values were different. The V-slope method predicted metabolic acidosis in 10/14 who had a fall in HCO3 more than 2.5 mEq/L. A significant proportion of patients with COPD seem to develop metabolic acidosis during exercise. The V-slope gas exchange method may be useful in selecting those patients with COPD who develop exercise metabolic acidosis and might therefore benefit from exercise training.

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MATERIAL AND METHODS

A group of 22 patients with moderately severe to severe COPD was studied. For comparison, a group of 56 subjects was identified from a previous study of the cardiopulmonary responses of normal men during cycle incremental exercise.6 These subjects had no evidence of heart, lung, peripheral vascular, or musculoskeletal disease and were not anemic. As previously reported, this group included men with hypertension in whom an elevated blood pressure did not by itself limit exercise capacity,

Normal Subjects

The mean values for age, size, and pulmonary function of the 56 normal subjects are shown in Table 1. As described above, all had normal pulmonary function. Exercise capacity expressed as maximum oxygen uptake as percent predicted value6 and AT as determined by both the modified V-slope method and from changes in standard bicarbonate are shown for individual subjects in Table 2. In 43 normal subjects, the AT could be determined by both methods; in the other 13, an AT could be clearly selected

DISCUSSION

During an incremental exercise test, about two thirds of our patients with chronic airflow obstruction developed metabolic acidosis. This finding may be surprising in view of the understanding that many of these patients with COPD are limited in their exercise capacity by exertional dyspnea, presumably due to reduced ventilatory capacity. In fact, it may have been thought that these patients would have such low exercise capacity that they would be unable to develop metabolic acidosis during

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Supported by Public Health Service grant HL-11907.

Manuscript received January 12; revision accepted April 26.

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