Metabolic Acidosis during Exercise in Patients with Chronic Obstructive Pulmonary Disease: Discussion

Further, in patients with COPD with limited ability to increase Ve, the additional C02 produced may not be easily eliminated; this can lead to an elevation in arterial Pco2. We found that those patients who developed metabolic acidosis during exercise tended to have a higher mean exercise Ve and, possibly more important, tended to use a higher proportion of their ventilatory capacity (higher maximum Ve/MW ratio) at the time they stopped exercising, although the mean values of these variables were not significantly different. Similarly, in five of 12 patients with severe COPD reported by Kanarek et al who developed metabolic acidosis during exercise, mean maximum Ve/M W% was 103 percent compared with 74 percent for those who stopped exercising without developing metabolic acidosis.

Previously, noninvasive evidence of the presence of metabolic acidosis during exercise depended on recognizing increased Ve as a result of the increased CO£ load presented to the respiratory system and any ventilatory compensation for the metabolic acidosis. However, it is apparent that this finding depends on an appropriate ventilatory response to the added C02 load, and it might be expected that patients with reduced ventilatory response to C02, either because of insensitive or abnormal ventilatory control or because of abnormal respiratory system mechanics, might be less able to develop a detectable Ve increase at the time of metabolic acidosis. In contrast, the V-slope method or the modification of the V-slope method presented here does not depend on the ventilatory response because the buffering of metabolic acidosis by bicarbonate will occur regardless of the degree of ventilatory response. The additional C02 produced from this reaction results in the Vco2 increasing relative to the Vo2. Thus, at the work rate at which lactic acid production begins, Vco2 will begin to increase faster than Vo2, and there will be a discernible change in the slope of the Vco2 vs Vo2 plot (Fig IB). The V-slope was successful in identifying ten of the 14 COPD patients in whom metabolic acidosis was found.

Category: Pulmonary Disease

Tags: lung disease, metabolic acidosis, Pulmonary function