The use of PEEP is the integral part of ventilatory strategy in patients with ARDS. Optimization technique of PEEP setting is controversial. The goal of the study was to find the algorithm which can predict the value of PEEP with the smallest pulmonary shunt fraction Qs/Qt based on the evalution of the mechanical properties of respiratory system. The prospective observational study enrolled 7 patients with pulmonary artery catheter who met American-European consensual criteria for ARDS. During data recording the patients were ventilated in CMV ventilation mode with constant inspiration flow, tidal volume of 5–7 ml.kg-1, inspiration pause of 0.5–1 sec; I:E ratio and frequency of ventilation were determined so that intrinsic PEEP was not presented. With tested values of PEEP we assessed; quasistatic compliance of respiratory system Crsq, shunt fraction Qs/Qt, difference between end-expiratory lung volume ∆EELV with tested values ofPEEP and baseline value ofPEEP (the technique ofPEEP changes), andwe calculated so called estimated lung recruitment (ELR = ∆EELV – Crsqbaseline x /PEEPtested – PEEPbaseline/). The capability of the prediction of optimal PEEP value by the value of Qs/Qt was tested for the smallest PEEP with maximal compliance Crsqmax,, maximal PEEP with maximal Crsqmax and for the combined method measuring Crsq and ELR.We assessed sensitivity, specificity and so called probability ratio (LR) of tested methods for the prediction of the smallest shunt fraction Qs/Qt. The combination of compliance and ELR permits better prediction of PEEP value with the smallest Qs/Qt. The use of the combined method evaluating Crs and ELR can be valuable for the determination of PEEP.

        Key words: PEEP – aerating compliance – ARDS – pulmonary shunt