Near-infrared spectroscopy: A potential method for continuous, transcutaneous monitoring for compartmental syndrome in critically injured patients

Background: Near-infrared spectroscopy (NIRS) noninvasively measures tissue O-2 saturation (Sto(2)), and has been proposed as a means of monitoring for compartmental syndrome (CS). However, its specificity in hypoxemic, hypotensive patients with severely reduced systemic oxygen delivery has not been tested. We hypothesized that NIRS can differentiate muscle ischemia caused by shock from ischemia caused by CS, Methods: Nine swine were anesthetized and an NIRS probe placed over the anterolateral compartment of the hind leg, Compartment pressure was also measured. A nerve stimulator was placed over the peroneal nerve, and CS was defined as loss of dorsiflexion twitch. At 30-minute sequential intervals, mean arterial blood pressure was reduced to 60% of baseline (phlebotomy), fraction of inspired oxygen was reduced to 0.15, and compartment pressure was increased in one limb by interstitial albumin infusion until CS occurred. Results: Hypotension combined with hypoxemia reduced Sto(2) from 82 +/- 4% to 66 +/- 10%, CS further reduced Sto(2) to 16 +/- 12% (p < 0.0001). During hypotension + hypoxemia + CS, control limb Sto(2) was 70 +/- 15% (p = 0.0002 vs. experimental limb). Conclusion: NIRS detects muscle ischemia caused by CS despite severe hypotension and hypoxemia, making it potentially useful in critically injured, unstable patients.