Metabolic Acidosis during Exercise in Patients with Chronic Obstructive Pulmonary Disease

Metabolic Acidosis during Exercise in Patients with Chronic Obstructive Pulmonary DiseaseUse of the V-slope Method for Anaerobic Threshold Determination
Patients with chronic obstructive pulmonary disease (COPD) generally have limited exercise capacity because of reduced ventilatory capacity. During a progressive exercise test, for example, a patient with COPD will reach his maximum work rate and be unable to continue exercise when the minute ventilation (Ve) is very close to the maximum voluntary ventilation (MW) measured at rest. Thus, in most patients with COPD, maximum work rate is increased while breathing oxygen, because oxygen attenuates the ventilatory drive and lowers Ve. In contrast, the maximum work rate is decreased by increased physiologic dead space, a factor that increases the ventilatory requirement.

Most often, treatment of the dyspnea of patients with COPD is designed to improve pulmonary mechanics by the use of bronchodilators and the prevention and treatment of infection. However, reduction of the metabolic acidosis during exercise by exercise training is also a potential approach to reducing ventilatory drive and thereby exertional dyspnea. Modification of hydrogen ion (H+) stimulus to breathing has a major effect on ventilatory drive in normal subjects and would also presumably affect the ventilatory drive in COPD patients.
Because the hydrogen ion produced by the dissociation of lactic acid reacts with bicarbonate to produce carbon dioxide, lactic acidosis increases the patients C02 load. Further, the hydrogen ion itself stimulates ventilation, thereby lowering arterial Pco2 as compensation for the fall in pH. Both the increase in C02 generated by buffering and the respiratory stimulation by the hydrogen ion are perceived by the patient as breathing stimuli, whether or not the patient is able to respond with an increased Ve. In a patient with COPD, the occurrence of metabolic acidosis would be expected to have a marked influence on exercise capacity, because the patients maximum Ve will be reached at a lower work rate than if the metabolic acidosis did not occur.
The frequency of metabolic acidosis during exercise in patients with COPD is unclear. In fact, in those whose obstructive lung disease is very severe, exercise capacity may be so reduced as to preclude the development of metabolic acidosis during exercise. The presence or absence of metabolic acidosis during exercise in a patient with COPD may have important implications for the choice of exercise training work levels in individual rehabilitation programs.
Observation of the increase in ventilation and changes in gas exchange have been used to identify the development of metabolic acidosis during progressive exercise. The oxygen uptake (VoJ above which metabolic acidosis occurs has been called the anaerobic threshold (AT). In the past, it has sometimes been difficult to use noninvasive gas exchange techniques to detect the AT in patients with COPD and subjects with insensitive chemoreceptors to the H+ stimulus. However, Beaver and colleagues recently presented a method for identifying the AT that did not depend on finding an increase in ventilation at the Vo2 at which metabolic acidosis occurs. This method, called the V-slope method, may be especially useful in patients with ventilatory limitation, because the reduced ventilatory capacity in these patients may result in an attenuated or absent increase in Ve above the At.
The purpose of this study was to compare a simplified modification of the V-slope method with direct iiieasurements of metabolic acidosis in: (1) patients vHth COPD, whose ventilatory response to metabolic abidosis would be predictably poor, and (2) normal subjects. In addition, we investigated how frequently patients with moderately severe or severe airway obstruction develop metabolic acidosis during exercise.


Category: Pulmonary Disease

Tags: lung disease, metabolic acidosis, Pulmonary function