Low tidal volume in association with low positive end expiratory pressure in acute respiratory distress syndrome: a suboptimal strategy? A computed tomography-based analysis

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Abstract

To characterize an experimental model of pulmonary embolism by studying hemodynamics, lung mechanics and histopathologic derangements caused by pulmonary microembolism in pigs. To identify lung alterations after embolism that may be similar to those evidenced in pulmonary inflammatory conditions. Materials and methods Ten Large White pigs (weight 35-42 kg) were instrumented with arterial and pulmonary catheters, and pulmonary embolism was induced in five pigs by injection of polystyrene microspheres (diameter ~300 M), in order to obtain a pulmonary mean arterial pressure of twice the baseline value. Five other animals injected with saline served as controls. Hemodynamic and respiratory data were collected and pressure x volume curves of the respiratory system were performed by a quasi-static low flow method. Animals were followed for 12 hours, and after death lung fragments were dissected and sent to pathology. Results Pulmonary embolism induced a significant reduction in stroke volume (71 18 ml/min/bpm pre vs 36 9 ml/min/bpm post, P < 0.05), an increase in pulmonary mean arterial pressure (27 4 mmHg pre vs 39 6 mmHg post, P < 0.05) and pulmonary vascular resistance (193 122 mmHg/l/min pre vs 451 149 mmHg/l/min post, P < 0.05). Respiratory dysfunction was evidenced by significant reductions in the PaO 2 /FiO 2 ratio (480 50 pre vs 159 55 post, P < 0.05), the dynamic lung compliance (27 6 ml/cmH 2 O pre vs 19 5 ml/cmH 2 O post, P < 0.05), the increase in dead space ventilation (20 4 pre vs 47 20 post, P < 0.05) and, the shift of pressure x volume curves to the right, with reduction in pulmonary hysteresis. Pathology depicted inflammatory neutrophil infiltrates, alveolar edema, collapse and hemorrhagic infarctions. Conclusion This model of embolism is associated with cardiovascular dysfunction, as well as respiratory injury characterized by a decrease in oxygenation, lung compliance and hysteresis. Pathology findings were similar to those verified in inflammatory pulmonary injury conditions. This model may be useful to study pathophysiology, as well as pharmacologic and ventilatory interventions useful to treat pulmonary embolism.

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