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March 2021

The Mechanics of Pulmonary Ventilation in Normal Subjects and in Patients with Emphysema*


Abstract: DONALD L. FRY M.D. Bethesda Maryland RICHARD V. EBERT M.D. Chicago Illinois; WILLIAM W. STEAD M.D. Denver CoIorado and CYRUS C. BROWN M.D. Minneapolis Minnesota * From the Department of Medicine Medical School University of Minnesota The Variety Club Heart Hospital and the Veterans Administration Hospital Minneapolis Minn. This study was supported in part by grant from the Minnesota Heart Association. Published with approval of Chief Medical Director. The statements and conclutions published by the authors are the result of their own study and do not necessarily reflect the opinion or policy of the Veterans Administration. SUMMARY 1. The intrathoracic pressure exclusive of the pressure required to overcome the retractive force of the lung was measured and shown to be functionally related to the rate of respirator flow in both normal and emphysematous subjects. This pressure increment at any given level of flow represents the pressure required to overcome both the resistance to air flow through the bronchopulmonary passages and the frictional resistance of the intrathoracic tissues to deformation. The resistance to gas flow is composed of resistance to laminar gas flow and resistance to turbulent gas flow. 2. Pressure-flow curves were obtained from normal subjects and from patients with pulmonary emphysema. Studies were made with subjects breathing air and breathing argonoxygen. Comparison of the pressure-flow relationships obtained with these two gases demonstrated that tissue friction was negligible both in normal and emphysematous subjects. 3. The resistance to gas flow was evaluated in normal subjects and patients with pulmonary emphysema. The resistance was markedly increased in the patients with emphysema. 4. The resistance to both laminar gas flow and the resistance caused by eddying turbulence are important in normal subjects and patients with emphysema. As the rate of flow is increased the resistance caused by eddying turbulence becomes progressively greater. 5. A change in the pressure-flow relationship with change in the amount of lung inflation was demonstrated. This was observed particularly during expiration. More pressure was required to produce a given flow at lesser degrees of lung inflation. This was marked in emphysematous subjects. The three variables pressure flow and lung inflation were expressed on a three-dimensional graph. 6. During expiration in emphysematous subjects the pressure-flow curve bends rapidly to approach an asymptote of flow. This accounts for the inability of patients with emphysema to exceed a certain flow regardless of the pressure exerted. In emphysema there is an increase in resistance to air flow in the bronchioles and a decrease in the elastic force of the lung for a given degree of inflation of the lung. As a result collapse of the unsupported bronchioles tends to occur when the pressure drop between the alveoli and the cartilaginously supported end of the bronchiole exceeds the elastic pressure of the lung. ___MH

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