Sidestream-dark field videomicroscopy for the in vivo evaluation of the pulmonary alveoli and microcirculation in mechanically ventilated pigs

Introduction: Under mechanical ventilation, the pulmonary microcirculation can be affected by the expansion or collapse of alveoli, resulting in a change in pulmonary vascular resistance. Sidestream-dark field (SDF) videomicroscopy has been used in animal models to assess pulmonary microcirculation in vivo. This study aimed to evaluate whether different mechanical ventilation settings could affect the alveolar size and pulmonary vessels in a porcine model. Methods: This experimental study was conducted in four healthy pigs on mechanical ventilation under general anesthesia. The ventilation was initially set at a tidal volume (VT) of 8 mL/kg, PEEP 5 cmH2O, and FiO2 50%. Access to the thoracic cavity was obtained through surgical thoracotomy. The subpleural pulmonary microcirculation was assessed using SDF videomicroscopy at different ventilator settings: VT 8 mL/kg, PEEP 5 cmH2O, FiO2 50%; VT 12 mL/kg, PEEP 5 cmH2O, FiO2 50%; VT 8 mL/kg, PEEP 12 cmH2O, FiO2 50%; VT 8 mL/kg, PEEP 5 cmH2O, FiO2 100%. We calculated the diameter of the alveoli and extra-alveolar microvessels. Results: Comparing VT 8 mL/kg and VT 12 mL/kg, we observed a significant increase in alveolar diameter (89 [70.6–114.7] μm vs. 94.6 [78.3–115] μm, p = 0.04) and a significant decrease in vessels diameter (10.4 [8.6–12.7] μm vs. 9.2 [7.6–11.2] μm, p < 0.01). We did not observe a significant difference in alveolar and vessels diam- eters after changing the PEEP from 5 to 12 cmH2O. Increasing the FiO2 from 50 to 100%, the alveolar diameter significantly raised (86.7 [69.6–112.6] μm vs. 94 [72.7–122.5] μm, p = 0.03) as well as the vessels diameter (10.4 [8.5–12.5] μm vs. 12.2 [10.3–14.7] μm, p < 0.01). Subpleural pulmonary microcirculation is shown in the Figure. Conclusions: Mechanical ventilation affects alveolar and pulmonary vessel size. SDF microscopy represents a valid tool to assess the sub- pleural pulmonary microcirculation in vivo in porcine models.