Effect of Positive Expiratory Pressure on Right Ventricular Function in Patients With Respiratory Distress Syndrome

Background: The ventilatory strategy in patients with Adult Respiratory Distress Syndrome (ARDS) is still controversial concerning the finest level of positive expiratory pressure (PEP). In fact, PEP allows optimisation of lung recruitment and oxygenation. However, high PEP may be detrimental on hemodynamics, notably impairing filling and ejection of the right ventricle (RV). At present, it is not known whether these adverse effects of PEP are independent or not of plateau pressure. Objectives: The aim of the present study is to explore the effect of PEP variations (with constant plateau pressure) on RV function (assessed using trans-oesophageal echocardiography, TOE) in patients with ARDS. The assumption tested is that a high level of PEP increases the impedance to RV ejection independently of the level of plateau pressure. The effect of PEP on the right ventricular preload will also be checked via the analysis of the respiratory variability of the diameter of superior vena cava. Respiratory system properties will be assessed as follows: alveolar dead space determination using expired CO2, alveolar recruitment calculation using pressure-volume curves. Methods: The patients are ventilated according to three consecutive strategies (A, B and C), using the same plateau pressure (\<30 cm H2O), but different PEP levels: low PEP in strategy A and high PEP in strategies B and C. In order to maintain a constant plateau pressure, the increase in PEP level in strategies B and C is accompanied by a decrease in tidal volume. This decrease in tidal volume is compensated by an increase in respiratory frequency (strategy B) or a decrease of instrumental dead space by removal of heat and moisture exchanger filter (strategy C). The first ventilatory strategy tested is "A". After that, the patient is randomised for strategies "B then C", or "C then B". At the end of each strategies the following explorations are performed: TOE, respiratory system pressure-volume curves, expired CO2 analysis, and arterial blood gas analysis.