Cyclic changes in right ventricular output impedance during mechanical ventilation

In a context such as acute respiratory distress syndrome, where optimum tid al volume and airway pressure levels are debated, the present study was des igned to differentiate the right ventricular (RV) consequences of increasin g lung volume from those secondary to increasing airway pressure during tid al ventilation. The study was conducted by combined two-dimensional echocar diographic and Doppler studies in 10 patients requiring mechanical ventilat ion in the controlled mode because of acute respiratory failure. Continuous monitoring of airway pressure on echocardiographic and Doppler recordings provided accurate timing of each cardiac event during the respiratory cycle , with particular attention being paid to end-expiratory and end-inspirator y atrial diameters, RV dimensions, and pulmonary artery and tricuspid flow estimated by the velocity-time integral (PA(VTI) and T-VTI, respectively). At baseline, lung inflation during the inspiratory phase of mechanical vent ilation produced a drop in PA(VTI) from 14.3 +/- 2.6 cm at end expiration t o 11.3 +/- 2.1 cm at end inspiration. This drop occurred without reduction in right atrial diameter or in RV diastolic dimensions. It was not preceded but was followed by a decrease in T-VTI, thus confirming an increase in RV outflow impedance. Manipulation of tidal volume without changing airway pr essure and manipulation of airway pressure without changing tidal volume de monstrated that tidal volume, but not airway pressure, was the main determi nant factor of RV afterloading during mechanical ventilation.