Acute Compartment Syndrome (ACS)
“What are the odds?”
I have discussed in lecture and in conversation the reasonable consideration of continuous peripheral nerve blocks when ACS is a potential risk and strategies we can utilize to optimize patient care and minimize the possibility of delaying the diagnosis of ACS. I was pleased to see this topic discussed in an article, “Pro-Con: Regional Anesthesia in the Patient at Risk for Acute Compartment Syndrome” in the most recent issue (May 2013) of ASRA NEWS, and this has prompted me to discuss some of my opinions and experience on the topic. I would highly recommend that you read the same article and consider studying some of the references as well.
I think it is always a reasonable question to ask before any intervention, “What are the risks of DOING this?” Having observed thousands of outcomes after performing continuous nerve blocks for an expanded range of cases and circumstances, I am now more pressed to ask the question, “What are the risks of NOT doing this?” Certainly, everything we ever do as clinicians is based on a risk-benefit ratio to the patient, and this scenario is no different. It is my estimation that within the context of a nerve block program with at least a moderately developed infrastructure and a thoughtful treatment strategy, the likelihood of masking the initial symptoms of an ACS and delaying a diagnosis is nearly non-existent. I would opine further that WITH a CPNB in a mature block program, you are MORE LIKELY to detect the earliest symptoms of an ACS. Allow me to explain.
The first consideration we should address is the patient population that you are likely to be treating, and the scenario in which the particular patient you are planning to block received their injury. If the patient was involved in a high-speed collision and sustains a midshaft tibia-fibula fracture, then an ACS is a real possibility. They might have up to a 1 in 10 chance in developing an ACS. In fact, if the clinical scenario warrants, the surgeon may need to consider performing a fasciotomy or fasciotomies preemptively. If the patient falls a short distance and lands awkwardly against something which leads to their midshaft radius-ulna fracture (primarily due to their poor bone quality), though we will remain vigilant, unless there are other significant factors, this patient has essentially no real risk of an ACS. For the sake of argument, we will still identify both patients as ‘at risk’ for an ACS; however, the extent to which I will alter my treatment plan will differ between these patients.
The second consideration is the essential element in the diagnosis of an ACS that occurs no matter the initial presenting symptom: a change! This is critical no matter what post-operative analgesia plan is selected. If the patient receiving IV morphine complains of escalating pain, and the response is to continue to give morphine more often, then the diagnosis will be delayed. If ongoing evaluation of pain to passive stretch is not performed, or if we are not actively seeking changes in sensation, then the diagnosis can be delayed. In neither scenario would I prescribe maintaining an anesthetic (vs analgesic) nerve block which would prohibit all neurologic evaluation of change to the nerve distribution that is at risk for an ACS.
Let me dispel another myth regarding ischemic pain.
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Even with a general anesthetic and two anesthetic dose nerve blocks (femoral and sciatic) in a patient who has had zero hemodynamic response to surgical stimulus during a total knee arthroplasty, we still see a slow rise in blood pressure and heart rate after about an hour due to the tourniquet-induced ischemic pain. Actually, with the addition of a perioperative dose of 1000 mg of IV Acetaminophen, we have been able to blunt this response. Just the same, ischemic pain is intense, and it is very difficult to mask. Even in this scenario, patients have still indicated aching throughout the leg after the extended use of a tourniquet. They do not have pain where the end of their femur or tibia was cut off and drilled or the big incision on their knee. They have diffuse pain, or it is centered in their calf. This is ischemic pain, and continuing a low infusion of dilute local anesthetic pot-operatively is not going to block this massive barrage of nociceptive input. No doubt, ‘some’ nociceptive input will be blocked. You could argue that the very earliest and smallest change in pain might be blunted at the initial onset of ischemia if a patient were completely pain-free before the pain reached the threshold where it would overcome the nerve block. Possibly for a very short time it may be difficult to discern a difference at rest. Regularly using passive vs dynamic pain as a marker of change ends this theoretical discussion. That is, extending the wrist or the calf will elicit a change in response that will be easily recognized.
Now, let’s talk about thoughtful strategies for performing continuous nerve blocks in patients at risk for ACS. The most conservative approach would be to place a perineural catheter, and wait until the surgeon is confident that the risk of ACS has passed before dosing it. I think this is better than nothing, but I find it unnecessarily prudent. Another strategy would be to dose a preoperatively placed perineural catheter with either a shorter-acting anesthetic concentration of local anesthetic or with a longer-acting analgesic concentration of local anesthetic (possibly with a smaller volume). In either scenario, plan to utilize an infusion rate that will necessitate some degree of analgesic ‘rescue’ regimen and/or allow for monitoring of a change in pain intensity with passive stretch. Further, do not allow the patient to utilize unmonitored patient-controlled local anesthetic boluses (or at all). This regimen allows the patient to have ‘adequate’ analgesia, avoid significant opioid doses and side-effects while allowing reliable neurologic evaluation. This may allow you to cover their breakthrough pain adequately with opioid pills instead of with an IV PCA which is a very concrete end-point with real cost savings. Optimal local anesthetic coverage may begin when the surgeon is comfortable that the necessary period of time to monitor for ACS has passed. Also, some use bupivacaine and ropivacaine in their infusions. In this instance, I would recommend ropivacaine as it tends to minimize loss of motor strength.
A reasonable Acute Pain Service should be evaluating and documenting the extent and type of sensation and loss as well as any loss of motor strength associated with a continuous peripheral nerve block at least daily. The RN and patient must be aware of the need to report a change. As in every ACS situation, any change should prompt further evaluation and closer monitoring. The protocol about whether to turn off the infusion if there are suspicious signs or symptoms should be made with the understanding that sensations should increase. That is, affected distributions of numbness should diminish, not extend when the infusion is stopped unless there is, indeed, an ACS. Further, pain will increase whether there is an ACS or not.
Because there will be heightened concern about masking an ACS, there is likely to more vigilance and patient education. Pay attention to the sensory distributions you are blocking as well as the sensory distributions more likely to manifest with symptoms of an ACS. The RN needs to be familiar with the different nerve block distributions as well. This highlights the need to invest in education to your entire hospital staff. I’m not sure what to say about studies that blame a femoral nerve block for delaying the diagnosis of a sciatic nerve block distribution ACS*. It is not necessary to withhold adequate analgesia from our patients, and a thoughtful plan can allow you to prevent suffering for your patients in a safe manner. This being said, a high index of suspicion and vigilance with any plan and clinical scenario is your #1 ally to prevent a delay in the diagnosis of ACS. I would recommend reading the following recent article by Aguirre et al for additional discussion on this topic: Anesthesiology 2013; volume 118: pages 1198-1205.
*Hyder et al, Compartment Syndrome in Tibial Shaft Fracture Missed Because of a Local Block, Journal of Bone and Joint Surgery, p 499-500,vol 78-B, No. 3, May 1996
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