VOLUME-PRESSURE CURVE OF THE RESPIRATORY SYSTEM PREDICTS EFFECTS OF PEEP IN ARDS - OCCLUSION VERSUS CONSTANT FLOW TECHNIQUE

The effects of positive end-expiratory pressure (PEEP) on static (''ra pid airway occlusion'' technique) and dynamic (''constant flow'' techn ique) volume-pressure (V-P) curves were studied in 19 patients with ad ult respiratory distress syndrome (ARDS). To describe the shape of bot h curves, the nonlinear coefficient of a second-order polynomial equat ion fitted to the static (static nonlinear coefficient) and dynamic (d ynamic nonlinear coefficient) V-P curves on zero end-expiratory pressu re (ZEEP) was used. Two distinct patterns were observed: (1) In ten pa tients, the static and dynamic V-P curves on ZEEP exhibited a convex s hape with a progressive decrease in slope with increasing inflation vo lume (nonlinear coefficients: negative). In these patients PEEP induce d a Volume displacement along the static and dynamic V-P curves on ZEE P (hyperinflation). (2) In nine patients, the static and dynamic V-P c urves on ZEEP showed a concave shape with a progressive increase in sl ope with increasing volume (nonlinear coefficients: positive) and PEEP shifted both curves upward along the volume axis (alveolar recruitmen t). A correlation (p < 0.0001) between static and dynamic nonlinear co efficients was found at all levels of PEEP. Both static and dynamic no nlinear coefficients on ZEEP were correlated (p < 0.0001) with the amo unt of lung volume recruited with PEEP, and the variations of cardiac index (Cl), O-2 delivery (Do(2)), right-to-left venous admixture (Qs/Q t), and Pao(2) with PEEP. Besides, the effects of PEEP on Cl, Do(2), Q s/Qt, and Pao(2) were less pronounced (p < 0.001) in patients with con vex V-P curves than in patients with concave V-P curves. We conclude t hat analysis of the dynamic V-P curve (''constant flow'' method) may r eplace the use of the static V-P curve (''occlusion technique'') to as sess the elastic properties of the respiratory system in ARDS patients . The dynamic V-P curve represents a simple and noninvasive clinical t ool which detects hyperinflation and predicts the effects of PEEP on a lveolar recruitment, hemodynamics, and gas exchange in ARDS patients.