Chronic obstructive bronchopulmonary disease: II. Oxygen transport in two clinical types

Abstract

Two contrasting types of patients with chronic airway obstruction were distinguished by purely clinical criteria: the emphysematous type (herein referred to as PP patients) and the bronchial type (BB patients). In the autopsy cases there was somewhat more extensive emphysematous destruction of the lungs and much less evidence of active bronchial mucous gland hyperplasia in the former type than in the latter. These two types of patients were studied physiologically and were found to have equivalent degrees of ventilatory impairment. However, PP patients had larger lungs, more uniform distribution of inert gas (helium) and hyperventilated both at rest and during exercise. BB patients had higher carbon dioxide tensions and much lower arterial oxygen saturations than did PP patients. The severe hypoxemia in BB patients was not caused by alveolar hypoventilation per se nor could it be satisfactorily explained by calculations of gas and blood flow distribution in the lungs. The most consistent measured difference relevant to pulmonary oxygen transport in the two types was in pulmonary blood flow: PP patients had subnormal and BB patients normal cardiac outputs both at rest and during exercise. The higher ratio of total ventilation to total blood flow in PP patients probably accounted for the nearly normal arterial oxygenation. The hypoxemia in BB patients is considered to be a form of diffusion impairment resulting from too rapid flow of red blood cells through hypoventilated alveoli. The most consistent measured difference relevant to tissue oxygenation in the two types was in the systemic oxygen transport (product of total blood flow and arterial blood oxygen content). Because of their low cardiac outputs, PP patients, although their arterial blood had a higher oxygen pressure, delivered less oxygen to their tissues than BB patients, who maintained a normal cardiac output despite a greater degree of pulmonary hypertension. These findings may account for the cachexia of the former and the cor pulmonale of the latter patients.