Chest
Volume 108, Issue 5, November 1995, Pages 1246-1251
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Clinical Investigations: Airways Obstruction
Contribution of the Respiratory Muscles to the Lactic Acidosis of Heavy Exercise in COPD

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Patients with COPD usually are limited in their exercise tolerance by a limited ventilatory capacity. Lactic acidosis induced by exercise increases the stress on the ventilatory system due to CO2 generated by bicarbonate buffering and hydrogen ion stimulation. Patients with COPD are often observed to increase blood lactate levels at low levels of exercise. We wished to determine whether patients with COPD who experience lactic acidosis do so because of respiratory muscle production of lactate. Eight patients with moderate to severe COPD (FEV1=43.5±11.6% predicted) and 5 healthy subjects performed 10 min of moderate constant work rate exercise either breathing spontaneously or volitionally increasing their ventilation for 5 min to approximate the peak minute ventilation seen during incremental exercise. During volitional increased ventilation, 3% CO2 was added to the inspirate to prevent alkalosis and hypocapnia. In neither the healthy subjects nor the COPD group was the end-exercise lactate level significantly higher during volitional ventilation increase than during spontaneous ventilation. Further, in the COPD patients, the blood lactate levels during volitional ventilation increase were much lower than during maximal exercise (averaging 2.4 vs 5.3 mmol/L) despite similar ventilation levels (averaging 50 and 53 L/min). We conclude that it is unlikely that the respiratory muscles have an important influence on the blood lactate level elevation seen during maximal exercise in COPD patients.

Section snippets

Patients

A group of eight patients with moderate to severe COPD (FEV1 <65% of predicted) took part in this study after giving written informed consent. Before entering the study, full pulmonary function testing and incremental cardiopulmonary exercise testing were performed. Patients were included when they fulfilled the following criteria: (1) irreversible obstructive airway disease (<10% improvement in FEV1 after inhalation of ß2-agonist); (2) ability to exercise beyond the lactic acidosis threshold

Healthy Subjects

Average physical characteristics of the healthy subjects were as follows: age, 28.3±4.7 years; height, 174.4±4.7 cm; and weight, 78.6±13.9 kg. There were two women and three men in this group. In the incremental exercise test, average peak V˙CO2 was 2.32±0.56 L/min and peak minute ventilation ( V˙E) was 89.4±16.7 L/min. The lactic acidosis threshold averaged 1.27± 0.37 L/min.

The work rate chosen for the CWR averaged 94±22 W. The upper panel in Figure 1 shows the ventilatory time

Discussion

Impaired exercise tolerance is a prominent complaint of patients with obstructive lung disease. Unlike healthy subjects, patients with obstructive lung disease are often limited in their exercise tolerance by the level of ventilation they are able to sustain. Strategies to improve exercise tolerance have focused on either reducing the high ventilatory requirement for a given level of exercise or increasing the amount of ventilation that can be sustained. Improving the function of the

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Supported by an Established Investigator Award from the American Lung Association of California.

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