WHEN ACL PATIENTS RECEIVE a well-placed continuous femoral nerve block, more than three-quarters of them will remain virtually pain-free for the first post-op week. They will experience very little "rebound" pain - the type that occurs after a single-shot block wears off2. Moreover, according to a 233-patient study we performed, they will achieve at least a one-point reduction in pain scores for every 33 hours of continuous nerve block they receive, depending on other pain relievers the patient is taking.
The operative phrase here, though, is well-placed. With some blocks - including interscalene, axillary and sciatic nerve blocks - placing a catheter in exactly the right spot can be challenging. It is all too easy to thread the catheter too far through the needle, causing the tip of the catheter to veer away from its target, thus directing the local anesthetic away from the target nerve.
For this reason, we use stimulating catheters when we want to achieve a continuous nerve block for a patient who is going home. Here's how. First, we use a needle with a stimulator to locate the target plexus. Then we slide the stimulating catheter through the needle, and disconnect the lead from the needle and attach it to the catheter. Just as a stimulating needle indicates when the needle is close to the target nerve by eliciting a twitch response, the stimulating catheter tells us when the catheter tip is close enough to the target nerve by eliciting the same response.
In our practice, we can place a stimulating catheter as quickly as a nonstimulating one by using a lower threshold for the twitch response. Our catheters are consistently successful when we use a threshold of 1.0 mA to even 2.0 mA, as opposed to the typical 0.5 mA that we use for singleshot blocks. Some authors have used ultrasound with or without nerve stimulation to verify both needle placement (visible with ultrasound) and catheter placement (halo of local anesthetic around visible nerve when catheter is properly located and injected).
Once the catheter is properly placed, we secure it with tape or glue, and use it for both post-op analgesia and to reduce anesthesia requirements during surgery.
It also pays to remember that pain control starts with blocking the appropriate nerves. Total knee replacement (TKR) pain, for example, is mediated (mostly) by both the sciatic and femoral nerves, and research suggests that coadministration of sciatic and femoral nerve block catheters helps reduce post-op pain scores, opioid requirements, and PONV.
With the tools we now have at hand, the time has come to set our standards high for post-op pain control. No more than 25 percent of patients should ever reach moderate-to-severe pain during the first week after significant elective knee surgery such as ACL, and less than 10 percent should experience PONV. With a well-placed continuous block, we can achieve this benchmark - and confidently send patients home knowing their pain scores will stay low.
1. Williams et al., Anesthesiology 104: 315-327, 2006.
2. Williams et al., Regional Anesthesia and Pain Medicine 32: 186-192, 2007.
3. Swenson et al., Anesthesia & Analgesia 103: 1436-43, 2006.
Dr. Williams is associate professor with the University of Pittsburgh Department of Anesthesiology. He practices at the University's South Side Hospital.