Ventilation/Perfusion Relationships in Health and Disease

Mild to severe degrees of hypoxemia are commonly encountered in patients with pulmonary disease. Classically, four etiologies can result in hypoxemia: diffusion defect, shunting, alveolar hypoventilation, and ventilation/perfusion mismatching. Of these four possible causes, ventilation/perfusion abnormalities account for the major share of the severe hypoxemia observed in chronic obstructive pulmonary disease (COPD), with alveolar hypoventilation representing an additional contributing factor. The relatively high incidence of COPD in the patient population treated by respiratory therapists renders an understanding of ventilation/perfusion concepts important to the student and practitioner alike.