THE PRESSURE-VOLUME DIAGRAM OF THE THORAX AND LUNG

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Abstract
This diagram relates the maximum voluntary in-spiratory or expiratory pulmonary pressure, P, as well as the relaxation pressure of the chest, Pr, (as abscissae) to various lung volumes (as ordinates). The difference between P and Pr represents the maximal inspiratory or expiratory muscle forces, Pm, at any specific lung volume. Thus Pm = P [long dash]Pr. Changes in the level of the tidal and residual air with pressure breathing can be plotted on this diagram as well as changes in the maximum positive and negative pressures that can be voluntarily exerted at altitude. The relaxation pressure is the resultant of 2 major compo-nents which oppose each other at most lung volumes, the positive lung elasticity, Pl, and the negative chest elasticity (including the diaphragm, etc), Pc. Thus Pr = Pl + Pc. Pr can be altered by posture. The elastic work of breathing at ambient pressure can be detd. from the slope of the relaxation pressure curve (70 ml./cm H2O). If the tidal volume, T, is expressed in cm.3, the work (g. cm.) is equal to T2/140.

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