<qa>  What is the 4th leading cause of death among Canadians?Chronic Obstructive Pulmonary Disease chronic bronchitis and emphysema, is a pathologic condition characterized by lung parenchyma destruction and abnormal airway narrowing that leads to a limitation of air. In clinical practice, COPD is defined by its characteristically low airflow and abnormal pulmonary system resistance during lung function tests. In contrast to asthma, this limitation is poorly reversible and progresses over time. COPD is the 4th leading cause of death amongst Canadians. Severe exacerbations of COPD may require mechanical ventilation and intensive care unit admission. 