Left ventricular filling and cardiac output may decrease during mechanical ventilation inspiration; verification in a sheep model of ARDS

Introduction: Mechanical ventilation (MV) can decrease cardiac output (CO) and oxygen delivery to the periphery leading to organ failure and mortality. MV decreases CO by decreasing the RV venous return and increasing the pulmonary vascular resistance (PVR) and RV afterload. The pulmonary vessel diameters are determined by the transmural gradient between the intravascular pressure and the surrounding alveolar (P_A) or pleural (P_pl) pressures. Some argue that high P_A augments LV filling. We hypothesized that P_A can decrease LV filling when the pulmonary vessels collapse, as P_A approaches the intravascular pressure.

Methods: The effects of MV on the circulation were investigated in a sheep model of ARDS. ARDS was induced by repeated lung lavages. A volume controlled ventilator generated the tube pressure required to delivered the desired tidal volume. Two impedance catheters were inserted into the right and left ventricles to provide the pressure-volume loops of both ventricles. Swan-Ganz catheter was inserted to measure the pulmonary artery (PAP) and pulmonary capillary wedge (PCWP) pressures. Intrapleural catheter was inserted to measure the P_pl. The positions of all the transducers were verified by fluoroscopy.

Results: Inspiration decreased RV EDV and RV stroke-volume based on RV PV-Loops. The PAP pulse-pressure decreased during inspiration when a large inspiratory drop in RV stroke-volume occurred. The PCWP fluctuated with the changes in P_pl. Inspiration yielded: (1) Slighter drop in the PCWP from the v-wave to y-wave, (2) Smaller changes in LV pressure throughout the diastolic filling. (3) Reduced LV filling volume based on the impedance catheter. Although inspiration increases the transpulmonary pressure and it should augment LV filling, these three observations suggest that LV filling decreases during inspiration and they support the above hypothesis.

Conclusions: An increase in the inspiratory pressure and prolonged inspiration can decrease LV filling and CO due to collapse of postcapillary pulmonary vessels.