The Effect of Inhaled Nitric Oxide on Dyspnea and Exercise Tolerance in COPD

Chronic Obstructive Pulmonary Disease (COPD) is a respiratory disorder typically caused by smoking and is characterized by airway obstruction. Exertional dyspnea (perceived breathlessness) is a hallmark of COPD regardless of severity and is the primary reason for exercise intolerance even in patients with mild COPD (defined using spirometric criteria as a forced expiratory volume in 1 s (FEV1)/forced vital capacity (FVC) <0.70 and a FEV1 ≥ 80%). Dyspnea in COPD has been shown to profoundly reduce patient quality of life, physical activity, and impair patients' ability to complete day-to-day tasks. Previous work in mild COPD has demonstrated that exertional dyspnea is the result of increased work of breathing during exercise, and that this increased work of breathing comes from: 1) an exaggerated ventilatory response to exercise (i.e. increased minute ventilation relative to carbon dioxide production, V̇E/V̇CO2), and 2) airflow limitation (i.e. expiratory flow limitation and resulting dynamic hyperinflation). A great deal of work has focused on improving airflow limitation in COPD; however, very little has been done to understand and treat the exaggerated ventilatory response to exercise in COPD.

Several previous studies in COPD have consistently shown an elevated ventilatory response (i.e. greater V̇E/V̇CO2) during exercise. The elevated V̇E/V̇CO2 response to exercise appears to be clinically important, as it independently predicts mortality in COPD and indicates that physiological abnormalities beyond airflow obstruction are important in determining disease severity, dyspnea, and risk of death. This increased V̇E/V̇CO2 in COPD appears to be secondary to increased deadspace ventilation(i.e. sections of the lung with ventilation, but no perfusion), and this increased deadspace ventilation results in a compensatory increase in total minute ventilation (i.e. increased V̇E/V̇CO2) to maintain effective alveolar ventilation and arterial blood gas homeostasis.

The underlying mechanism(s) for the increased deadspace ventilation and V̇E/V̇CO2 during exercise in mild to moderate COPD is currently unclear; however, pulmonary microvascular abnormalities and hypoperfusion of pulmonary capillaries are potential pathophysiologic mechanisms. Mild to moderate COPD patients have reduced pulmonary microvascular blood flow in nonemphysematous lung regions, which has led researchers to conclude that the low pulmonary perfusion in an intact pulmonary vascular bed is likely the result of pulmonary vascular dysfunction. Ventilation-perfusion (V̇A/Q̇) data in mild and moderate COPD shows substantial V̇A/Q̇ inequality at rest, with the V̇A/Q̇ distribution skewed towards regions of high V̇A/Q̇, which is indicative of increased deadspace. Consistent with this capillary hypoperfusion hypothesis, our recent work has shown a blunted pulmonary capillary blood volume response to exercise in mild COPD, when compared to age- and height-matched non-smoking controls. Importantly, the low pulmonary capillary blood volume was associated with increased V̇E/V̇CO2 during exercise, suggesting that low pulmonary perfusion (i.e. reduced pulmonary capillary blood volume) leads to increased deadspace.

Inhaled nitric oxide (NO) is commonly used to test for pulmonary vasodilatory responses in patients with pulmonary arterial hypertension (PAH), as it increases NO bioavailability and improves pulmonary vascular function. Previous work in PAH and heart failure (HF) patients has shown that standard doses (20-40 parts per million (ppm)) of inhaled NO can reduce pulmonary vascular resistance and increase peak oxygen consumption (V̇O2peak). If inhaled NO can reduce vascular dysfunction and increase perfusion in mild and moderate COPD, this would result in a reduction in V̇E/V̇CO2 and improved exercise tolerance.

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