Combination of intravenously infused methylene blue and inhaled nitric oxide ameliorates endotoxin-induced lung injury in awake sheep

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Abstract
To evaluate the effects of a combination of methylene blue, an inhibitor of the nitric oxide pathway, and inhaled nitric oxide on endotoxin-induced acute lung injury in awake sheep. Prospective, randomized, controlled experimental study. University animal laboratory. Twenty-four yearling, awake sheep. The sheep were anesthetized and instrumented with vascular catheters. After 1 wk of recovery, the animals underwent tracheotomy and were subjected to intravenous infusions of Escherichia coli endotoxin 10 ng·kg−1·min−1 and isotonic saline 3 mL·kg−1·hr−1 for 8 hrs. The sheep were randomly assigned to three groups of eight animals each: a) the control group received endotoxin and saline; b) the INO group received endotoxin, saline, and inhaled nitric oxide 40 ppm for 5 hrs; and c) the MB/INO group received endotoxin, saline, and methylene blue 3 mg/kg as an intravenous bolus injection followed by a continuous infusion of 3 mg·kg−1·min−1 for 6 hrs in combination with inhaled nitric oxide 40 ppm for 5 hrs. Hemodynamic variables and blood gases were determined hourly. In the early phase of endotoxemia (0–2 hrs), methylene blue/inhaled nitric oxide reduced the increments in pulmonary arterial pressure, pulmonary microvascular pressure, and pulmonary vascular resistance index by 60% compared with the controls and to a greater extent than did inhaled nitric oxide alone. During the late phase, all the preceding variables returned closely to baseline following inhaled nitric oxide or methylene blue/inhaled nitric oxide but remained remarkably elevated in the control group. Inhaled nitric oxide and methylene blue/inhaled nitric oxide reduced the increase in extravascular lung water by 40% and 80%, respectively. Inhaled nitric oxide transiently attenuated the increase in venous admixture and did not prevent a decrease in arterial oxygenation. In the methylene blue/inhaled nitric oxide group, blood gases remained unchanged from baseline. In sheep, methylene blue/inhaled nitric oxide protects more efficiently against acute lung injury than inhaled nitric oxide alone, as indicated by a milder pulmonary hypertension, less extravascular lung water accumulation, and maintained gas exchange.