Near infrared spectroscopy offers accuracy, consistency in monitoring injuries
Because acute compartment syndrome (ACS) can be such a devastating condition, accurate and timely diagnosis is essential. Improving the process to assess injuries for ACS is imperative across trauma care, but it may have elevated priority in the military setting, where high-energy injuries that are especially associated with ACS may be frequently encountered.
Clinical examination—along with observation and measurement of intramuscular pressure (IMP)—have stood as the primary means of monitoring for ACS. However, these methods can be imprecise or unreliable in some patients, such as those who are obtunded or intoxicated.
A study presented at the annual meeting of the Society of Military Orthopaedic Surgeons (SOMOS) revealed successful use of a physiologic monitoring method—near infrared spectroscopy, or NIRS—that could fulfill the ideal criteria of painlessly and continuously monitoring the patient with severe extremity injury over the 48-hour “window of worry” for ACS. The study was the recipient of the 2016 SOMOS Founder's Award.
The study, said its presenter, COL Brett Freedman, MD, of the Mayo Clinic, is “one of more than 15 basic science and clinical studies that we have completed to date on the road to developing and validating an NIRS-based decision support tool for diagnosing ACS.”
In this investigation, he and his co-researchers sought to evaluate NIRS as a “continuous, noninvasive means” (Fig. 1) of monitoring patients with lower extremity trauma at risk for ACS. The study involved 23 severely injured patients without high-energy lower extremity trauma and 91 patients with lower extremity trauma, of whom 79 had unilateral injury. Among patients with lower extremity injury, ACS would develop in 7. In all, 77 percent of patients were male, with a mean age of 39 years (range 18 to 65).
Medical staff placed NIRS sensors on the patients over 12 sites—one over each of the four muscle compartments of both lower legs, on the volar forearm and deltoid of one arm, and on the plantar surfaces of both feet. All care was directed by the attending physician. Treating clinicians were blinded to the NIRS values. ACS was based solely on the provider's clinical diagnosis.
As the authors explain in their SOMOS paper, increased perfusion can be expected in injured limbs not complicated by ACS. NIRS, they note, “contains all of the potential benefits of the ideal monitor for ACS. It is continual, noninvasive, and responsive. It provides real-time data that are simple to interpret, and it directly measures perfusion in somatic tissue, which is the physiologic parameter that determines ACS.”
In NIRS monitoring (Fig. 2), Dr. Freedman explained, “as IMP increases and the compartments become tight, NIRS values drop from above a control to even with, and then below [a control].” A control limb is crucial to determine if changes within an injured extremity are due to local conditions (ACS or vascular injury) versus other systemic changes. Isolated drops in injured extremities without changes in the control site would indicate local ischemia worrisome for ACS. Simultaneous drops in both injured extremities and control sites would point toward a systemic etiology. “Without the use of a control site, the ability to determine local versus systemic concerns is impossible,” the authors write.
NIRS is best used as a trending device that compares perfusion in “like compartments” between injured and uninjured extremities, Dr. Freedman explained. “For instance, if you sustain a severe tibia-fibula fracture of the left leg, then the best means for using NIRS to detect ACS would be to continuously measure NIRS values in the left anterior leg compartment and compare those to the uninjured right anterior leg compartment, and so on for the four compartments of the leg. There should be no more than 1 to 2 percent difference in healthy individuals between “like compartments.” In injured legs not complicated by ACS, there should be hyperemia, which increases the perfusion values in the injured over the uninjured leg.”
Among the patients in the study, average NIRS values of four leg compartments ranged from 72 to 78 percent (SD 0.6–0.9 percent) and 69 to 72 percent (SD 0.8–1.3 percent), respectively, among injured and contralateral limbs of patients with unilateral injury (n = 79), and 71 to 73 percent (SD 6–0.9 percent) in both legs among patients without leg injury (n = 23). Adjusted differences between leg compartments and internal control values ranged from 3.2 to 8.5 percent among unilateral injured and –0.2 to 2.3 percent among non–leg injury patients. Effect of time was clinically insignificant in all models. ACS limbs showed NIRS values that were at least 3 percent lower than internal reference values in at least one compartment prior to fasciotomy. The transition from hyperemia to hypoperfusion was observed in all but one.
These results, the authors write, “confirm the feasibility of NIRS oximetry as a decision-support tool in the management of ACS. Sustained hyperemia of +3 percent above internal reference values are statistically consistent with the absence of ACS, and therefore do not warrant concern in an asymptomatic patient. Index-reference differentials of +2 percent or less warrant heightened surveillance. An index-reference difference of –3 percent or less is prevalent in the ACS state.”
This study also confirms, the authors write, “that the best site for control measurements is the contralateral anterior compartment of an uninjured leg. In bilateral lower extremity injuries, the volar forearm represents the best comparative site. These sites offer insight into normal perfusion and allow a measuring stick by which to determine levels of concern for ACS.”
In an interview with AAOS Now, Dr. Freedman said that his interest in studying ACS and exploring better ways to diagnose it arose in 2008 while he and Michael S. Shuler, MD, his co–primary investigator throughout the process, were completing their fellowships (Dr. Freedman in spine surgery, Dr. Shuler in hand surgery) at Emory University Medical Center, where Dr. Shuler had served his residency.
“The Emory Orthopaedic Residency program provides primary trauma coverage for Grady Memorial Hospital, one of the busiest metropolitan Level 1 trauma centers in the country,” Dr. Freedman explained. “One of the duties of being a resident on the orthopaedic trauma service at a busy Level 1 trauma center is diagnosing ACS.
“Michael saw NIRS monitors being used by anesthesiologists to noninvasively and continuously measure the perfusion of tissue 2 to 3 cm below the surface of the skin,” continued Dr. Freedman. “He came up with the bright idea of seeing if this same technology could be used to measure perfusion in injured legs at risk for ACS. His early work used existent and suboptimal technology. But it showed that spectroscopy, which is the most accurate means for measuring the composition of tissue, could be used in the clinical setting of severe leg injury to detect changes in perfusion that would indicate critical ischemia indicative of ACS.
“From there, we wrote our first Department of Defense grant in 2008, and for the last 8 years we have been continuously modifying existing NIRS technology for optimal performance in the task of trying to identify threshold values of perfusion change that, when present, indicate impending or existent ACS and warrant surgical decompression via fasciotomy.”
Asked about how this work will affect clinical practice, Dr. Freedman said, “Acknowledging our significant bias and hubris, we firmly believe that the validation of an NIRS-based decision support tool for diagnosing ACS is a ‘game changer.’ This will revolutionize the civilian and, even more importantly, the military management of ACS. Patients at risk of ACS will be placed on NIRS telemetry-like status for the 48 hours of at-risk time. If a critical threshold in NIRS values is crossed it will alarm the team to examine the patient. Gone are the days of every-2-hour rounds for the resident through the day and night for serial exam. In real time and with continuous monitoring, we will know the leg is fine or heading in the direction of critical hypoperfusion for which fasciotomy is indicated. Also, NIRS not only can be used to diagnose ACS, it can be used to ensure that the fasciotomy is complete. Failure to perform a complete fascial decompression was a common reason for delayed or missed diagnosis of ACS, which can lead to significant increase in amputation and mortality.”
When NIRS devices are fully operational, they will be applied on the patient like a 12-lead EKG, and, while the patient is at risk for ACS, he or she will be wirelessly monitored. “The training will be minimal,” Dr. Freedman said. “This will radically improve the diagnosis of one of the most commonly missed and litigated conditions in orthopaedics.” The absence of a current reimbursement model, however, “presents a hurdle that needs to be crossed prior to widespread use.”
Co-authors with Dr. Freedman are Michael S. Shuler, MD; Mellisa Roskosky, MSPH; Tracy Kinsey, MSPH; and Dale Glaser, PhD.
The authors' disclosure information can be accessed at www.aaos.org/disclosure
Terry Stanton is senior science writer for AAOS Now. He can be reached at firstname.lastname@example.org
- Acute compartment syndrome is a devastating complication of traumatic limb injuries and has traditionally been monitored by clinical observation, which can be unreliable.
- NIRS offers a noninvasive means of continuous physiologic monitoring throughout the 48-hour “window of worry.”
- This study of patients with high-energy limb injuries demonstrated the feasibility of NIRS oximetry as a decision-support tool in the management of ACS, including confirmation of complete fasciotomy.
- Prospective trials of the technology are planned to occur at four major medical centers.