Prediction of Impact of End-expiratory Pressure Changes on Pulmonary Shunt Fraction in Patients with ARDS – the Significance of EstimatedPulmonary Recruitment

Overview

The use of PEEP is the integral part of ventilatory strategy in patients with ARDS. Optimization technique of PEEP setting is controversial. Thegoal 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 evalutionof the mechanical properties of respiratory system. The prospective observational study enrolled 7 patients with pulmonary artery catheter who metAmerican-European consensual criteria for ARDS. During data recording the patients were ventilated in CMV ventilation mode with constantinspiration 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 intrinsicPEEP was not presented. With tested values of PEEP we assessed; quasistatic compliance of respiratory system Crsq, shunt fraction Qs/Qt, differencebetween end-expiratory lung volume ∆EELV with tested values ofPEEP and baseline value ofPEEP (the technique ofPEEP changes), andwe calculatedso called estimated lung recruitment (ELR = ∆EELV – Crsqbaseline x /PEEPtested – PEEPbaseline/). The capability of the prediction of optimal PEEPvalue by the value of Qs/Qt was tested for the smallest PEEP with maximal compliance Crsqmax,, maximal PEEP with maximal Crsqmax and for thecombined method measuring Crsq and ELR.We assessed sensitivity, specificity and so called probability ratio (LR) of tested methods for the predictionof the smallest shunt fraction Qs/Qt. The combination of compliance and ELR permits better prediction of PEEP value with the smallest Qs/Qt. The useof the combined method evaluating Crs and ELR can be valuable for the determination of PEEP.

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