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

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 PEEP_5, 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.NEW & NOTEWORTHY An excessively high positive end-expiratory pressure (PEEP) can impair the systemic circulation and alveolar microcirculation. In the high-PEEP condition, fluid loading restored the systemic circulation but did not affect the impaired lung microcirculation. The septal thickness of the alveoli significantly increased after fluid loading in the high-PEEP condition.