Specific Aim 1: Prospectively enroll and follow for 6 months a sample of 200 patients. Patients will receive continuous tissue perfusion monitoring using NIRS in all 4 leg compartments and intramuscular pressure (IMP) via indwelling catheters placed in the anterior and deep posterior compartments. These measures will be blinded and not provided in real time to treating physicians. All clinical care, including diagnosis of ACS, will be according to current standard-of-care practiced at each institution.