Pressure-volume curves of the respiratory system.

The quasi-static pressure-volume (P-V) curve of the respiratory system describes the mechanical behavior of the lungs and chest wall during inflation and deflation. To eliminate resistive and convective acceleration effects, the measurement of volume and pressure must be performed during short periods of apnea or during very slow flow. There are 3 main techniques for acquiring quasi-static P-V curves: the supersyringe method, the constant flow method, and the multiple-occlusion (or ventilator) method. For the information to be interpreted correctly, one must understand the interaction between the lungs and the chest wall, the effects of the supine position, and the meaning of hysteresis. The P-V curve has been studied in many disease states, but it has been applied most extensively to patients with acute respiratory distress syndrome, in hopes that it might allow clinicians to customize ventilator settings according to a patient's individual respiratory mechanics and thus protect the patient from ventilator-induced lung injury. However, lack of standardization of the procedure used to acquire P-V curves, difficulties in measuring absolute lung volume, lack of knowledge regarding how to use the information, and a paucity of data showing a benefit in morbidity and mortality with the use of P-V curves have tempered early enthusiasm regarding the clinical usefulness of the quasi-static P-V curve.