San Francisco—Where 30 mL once stood as a standard dose, 10 mL now does the job.
So concluded a recent study by Illinois researchers of sciatic nerve block in patients undergoing total knee arthroplasty, which found that although 2.5 and 5 mL doses of local anesthetic provided suboptimal analgesia, amounts between 10 and 30 mL were comparably effective when paired with ultrasound guidance.
“We do a lot of regional techniques for total knee replacements, and we performed this study because we’re always looking for the minimum dose that will still offer the benefit we’re aiming for,” said Jessica J. Buren, MD, a fellow in regional anesthesia and acute pain medicine at Northwestern University Feinberg School of Medicine, in Chicago.
Dr. Buren and her colleagues enrolled 139 adults undergoing total knee arthroplasty into the trial. Each underwent a sciatic nerve block using the infragluteal-parabiceps approach under ultrasound guidance and nerve stimulator assistance.
“We use ultrasound guidance to visualize the nerve sheath, which we call the common investing extraneural layer [CIEL],” said Dr. Buren, who presented the findings at the 2013 annual meeting of the American Society of Anesthesiologists (abstract 1315). “With the nerve stimulator, the minimum current needed to get a response is very different if the needle is outside the CIEL or inside the CIEL.”
Participants were randomized into 14 groups receiving 0.5% bupivacaine or 0.5% ropivacaine with epinephrine in amounts ranging from 2.5 to 30 mL. After local anesthetic administration, a blinded clinician assessed sensory and motor function every two minutes for 10 minutes, and every five minutes thereafter until one hour before surgery, researchers said.
After surgery, the clinician assessed the patient’s motor and sensory block every four hours until the anesthetic was completely resolved. A block was considered successful if it produced complete motor block and insensitivity to pinprick at the one-hour mark in the sural, tibial, deep and superficial peroneal nerves.
“We found that in subjects where we used 5 mL or less of local anesthetic, we had 30 of 40 subjects with a successful block,” Dr. Buren told Anesthesiology News. “By comparison, in the patients who received 10 mL or greater of local anesthetic, 97 of 99 had a complete block.
“What’s more,” she continued, “the patients who received 10 mL and greater had less pain behind the knee in the first 24 hours after surgery. They also had a longer duration of motor block [up to 56 hours], of course, which is not what we were aiming for, but is what we expected.”
These findings are consistent with much of current literature on the subject, said Chad M. Brummett, MD, assistant professor of anesthesiology at the University of Michigan Health System, in Ann Arbor, who moderated the poster session. “This is probably going to be the biggest change in regional anesthesia practice in the coming years: volume and concentration. It will be interesting to see how important a role concentration begins to play as we drop volume. Certainly we can expect to see local anesthetic toxicity drop off.”
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