Physiological Regulation of Pulmonary Microcirculation under Mechanical Ventilation at Different Cardiac Outputs and Positive End-Expiratory Pressures in a Porcine Model.

This study was performed to visualize the hemodynamic effects of pulmonary microcirculation and ventilation/perfusion (V/Q) matching after mechanical ventilation under different cardiac outputs and positive end-expiratory pressures (PEEPs). Ten experimental pigs were randomly divided into high and low tidal volume groups, and ventilation/perfusion were measured by electrical impedance tomography (EIT) at different PEEPs. Then, all the pigs were redivided into high cardiac output (CO) and low CO groups and measured by EIT at different PEEP levels with a low tidal volume. Additionally, sidestream dark field (SDF) was used to measure pulmonary microcirculation. Hemodynamic parameters and respiratory mechanics parameters were recorded. As PEEP increased at high tidal volume, blood flow was impaired at a higher PEEP (20 cmH2O) compared with low tidal volume (shunt: 30.01 ± 0.69% vs. 17.95 ± 0.72%; V/Q ratio: 65.12 ± 1.97% vs. 76.57 ± 1.25%, p < 0.01). Low tidal volume combined with an appropriate PEEP is the best option from the match between ventilation and pulmonary blood flow. Increasing PEEP can solve the problem of excessive shunt at high CO, and the V/Q ratio tends to match. At low CO, the increased dead space can reach as high as 64.64 ± 7.13% when PEEP = 20 cmH2O. With increasing PEEP, the microcirculation index deteriorates, including total vessel density (TVD), proportion of perfused vessel (PPV), perfused vessel density (PVD), and microcirculatory flow index (MFI). The periodic collapse of pulmonary capillaries or interruption of blood flow obviously occurred with high PEEP. The hemodynamic parameters indicated that the transpulmonary capillary wall pressure (Pcap) of the low CO group was negative at PEEP = 5 cmH2O, which determines the opening and closing of the pulmonary microcirculation and controls lung perfusion and the production of extravascular lung water. Therefore, it is essential to couple macrocirculation and pulmonary microcirculation during mechanical ventilation by improving shunting and optimizing Pcap.