Influence of systemic hypotension on skeletal muscle ischemia-reperfusion injury after 4-hour tourniquet application.

OBJECTIVE Tourniquet use for extremity hemorrhage control is common in military trauma. Tourniquet use may be accompanied by systemic hypotension, but this phenomenon has not been studied. We aimed to define the muscle effects of the combined insult of tourniquet-induced skeletal muscle ischemia-reperfusion injury (I-R) and hemorrhagic hypotension. DESIGN After a 33% carotid arterial hemorrhage, Sprague-Dawley rats underwent 240-min hindlimb ischemia induced by pneumatic tourniquet. Control animals were not hemorrhaged. No resuscitation was given. After tourniquet release, muscles were reperfused for 120 min and then dissected. Weights and mitochondrial viability assays (nitroblue tetrazolium method) were performed on the plantaris (PL), and soleus (SO). Histologic analysis was performed on the PL and SO. Muscle edema is expressed as the ratio of tourniquet limb to contralateral limb muscle weight. SETTING Animal laboratories of the United States Army Institute of Surgical Research. STUDY ANIMALS: Twelve Sprague-Dawley rats. RESULTS The mean arterial pressure of hemorrhaged animals was 38 +/- 3 mm Hg before tourniquet placement and 86 +/- 4 mm Hg before release, both significantly (p < 0.05) lower than controls at the same time points. Pre- tourniquet mortality was 38% with hemorrhage and 0% without. All muscles experienced edema, with weight ratios greater than 1. The PL experienced significantly (p < 0.05) less edema with hemorrhage. Viability was unaffected by hemorrhage in all muscles, as was tissue inflammation. No differences in inflammation were observed with hemorrhage. CONCLUSIONS Systemic hypotension modulates the impact of 4 hours of tourniquet ischemia by decreasing muscle edema but minimally altering measures of muscle viability. Compartment anatomy and muscle fiber type both influence muscle response to the combined insult of hypotension and I-R. In this model, hypotension did not worsen the skeletal muscle I-R observed after the use of a tourniquet for 4 hours.