By analysis of the retractive forces of the lungs it was found that the pressure-volume characteristics of the lungs may be expressed by an exponential function. The curve described by such expression could be fitted to the experimental data obtained in 20 normal subjects. A half-inflation pressure (h) was defined which makes possible the evaluation of the retractive forces of the lungs by a measurement independent of lung size and accounting for known curvilinearity. H is a useful index of the stiffness of the organ and it is defined as the increase in transpulmonary pressure necessary to inflate the lungs halfway to the maximal pulmonary volume from any resting level. The mean value of h for the group was 7.58 ± 2.53 cm H2O. The half-inflation pressure is independent of the level of measurement within the inspiratory capacity and it does not vary with or depend on the size of the lungs. It may therefore be a more useful expression of the retractive forces of the lungs than compliance. pulmonary retractive forces; lung stiffness; compliance half-inflation pressure and lung size; VC and half-inflation pressure; FRC and half-inflation pressure; new expression for compliance; pressure-volume curve Submitted on March 4, 1963

The static pressure-volume curves of the lung, thoracic cage and total respiratory system were studied in the sitting, supine, prone and knee-elbow position in four subjects. The lung and thoracic-cage pressures were measured with the aid of an esophageal balloon and recorded on a water manometer. The analysis of these various components suggests that in the supine position the esophageal balloon is compressed by the mediastinal content, giving rise to an artifact in the recording of the lung pressure. If this artifact is taken into consideration, it would appear that the effect of posture on the compliance of the lung is negligible. Lung pressures calculated for various lung volumes during active breathing are compared with static values recorded on a water manometer. Submitted on December 19, 1958

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