Effects of high PEEP and fluid administration on systemic circulation, pulmonary microcirculation, and alveoli in a canine model

This study aimed to determine the response of systemic circulation, pulmonary microcirculation, and alveoli to high positive end-expiratory pressure (PEEP) in a canine model. This study was conducted in nine mixed-breed dogs on mechanical ventilation under anesthesia. The PEEP was initially set at 5 cmH2O (PEEP5), the PEEP was then increased to 25 cmH2O (PEEP25), and then saline was used for fluid loading. Data were obtained at the following time points: PEEP5; PEEP25 prefluid loading; and PEEP25 postfluid loading. The images of subpleural lung microcirculation were assessed by sidestream dark-field microscopy, and the hemodynamic data were collected from pulse contour waveform-derived measurements. Compared with PEEP5, the lung microvascular flow index (MFI, 2.3 0.8 versus 0.9 0.8, P 0.001), lung perfused vessel density (PVD, 4.2 2 versus 1.5 1.8, P 0.004), lung proportion of perfused vessel (PPV, 93 14 versus 40 4, P 0.003), cardiac output (2.5 0.6 versus 1.4 0.5, P 0.001), and mean blood pressure (116 24 versus 91 31, P 0.012) were significantly lower at PEEP25 prefluid loading. After fluid loading, there were no significant differences in cardiac output or mean arterial pressure between the PEEP5 and PEEP25 postfluid loading levels. However, the lung microcirculatory MFI, PVD, and PPV at PEEP25 postfluid loading remain lower than at PEEP5. A significant increase in septal thickness was found at PEEP25 postfluid loading relative to septal thickness at PEEP25 prefluid loading (25.98 5.31 versus 40.76 7.9, P 0.001). Under high PEEP, systemic circulation was restored after fluid loading, but lung microcirculation was not. Moreover, the septal thickness of alveoli significantly increased after fluid loading.