Author: Michael Vlessides
Using the supine position for ultrasound-guided sciatic nerve blocks in the popliteal fossa may be better than the more common prone and lateral positions, particularly for trauma patients. This superiority involves better imaging time, image clarity and nerve distance to the skin, a study has concluded.
The superior visualization may allow practitioners to more accurately block the sciatic nerve, particularly in trauma patients. “In the trauma population, it is sometimes very challenging to turn your patients prone, especially if they’re already asleep and intubated or have multiple lines and tubes,” said Ron E. Samet, MD, an assistant professor of anesthesiology at the University of Maryland School of Medicine, in Baltimore.
“Therefore, we often have to use a supine approach. And in doing so, we noticed that with the patient supine, we were often able to visualize the sciatic nerve significantly better than with the prone or lateral positions.
“So we began to think that maybe the ultrasound image is better with the patient supine, because gravity pulls the leg downward toward the probe, whose vector is pushing upward against the leg. With opposing vectors, there is better compression of the tissues, possibly making it easier to visualize the sciatic nerve,” he said.
“On the other hand, if the patient is prone, the sciatic nerve is being pulled by gravity away from the probe. Pushing the probe on the posterior thigh can’t compress the tissues as well, and may generate worse image capture.”
To study their hypothesis, Dr. Samet and his colleagues enrolled 38 informed and consenting volunteers into the study; participants were imaged using a FUJIFILM SonoSite X-Porte ultrasound machine. Each volunteer was scanned four times, once on each leg in the prone and supine positions, by one of five regional anesthesiologists with experience performing popliteal blocks. Images were obtained of the popliteal artery and tibial nerve at the popliteal crease, nerve bifurcation and sciatic nerve.
Scanning time began when the ultrasound probe was placed on a volunteer’s skin, and ended when the practitioner found the best image of the sciatic nerve.
“We took those images and used computer analysis to identify the gray scale of the sciatic nerve relative to its background to define whether or not it was more recognizable when the patient was supine or prone,” Dr. Samet said.
Image Preference Favors the Supine
The study revealed that the ratio of the gray scale of the sciatic nerve versus the background showed a mean of 1.830 for the supine position and 1.750 for the prone position (P=0.034). Similarly, the ratio of the gray scale of the sciatic nerve versus that of the area immediately surrounding the nerve showed a mean ratio of 1.651 for the supine position and 1.551 for the prone position (P=0.0046).
“So we found, via computer analysis, the P values were quite significant for the ability to visualize the sciatic nerve against its background,” he said.
Additionally, the mean skin-to-nerve distance was found to be 1.61 cm in the supine position and 1.74 cm in the prone position (P=0.008). The mean time to acquire the best sciatic nerve image also favored the supine approach (36.3 vs. 47.1 seconds; P=0.0023). In a second part of the study, the prone and supine sciatic nerve images of each leg were placed side by side for comparison, randomizing which image was on the right or left without revealing whether the image was taken with the patient in the supine or prone position. Sixty-one anesthesiologists identified the sciatic nerve and chose their preferred image, if they had one.
Observers preferred an image in 61 of the 76 images (76.3%), with 53.8% preferring supine positioning and 22.5% preferring prone.
“If you want to perform a sciatic nerve block of the popliteal fossa under ultrasound, the common teaching was to turn these patients prone,” Dr. Samet noted in his presentation at the 2018 Joint World Congress on Regional Anesthesia and Pain Medicine and annual meeting of the American Society of Regional Anesthesia and Pain Medicine (abstract 4788).
“In fact, keeping the patient supine, placing the probe underneath an elevated leg, and having the leg rest on the probe may actually define the sciatic nerve quicker and easier.
“This might ultimately make it easier to perform the block in that position rather than turning them prone.”
As promising as these results may be, not all of Dr. Samet’s audience members agreed that supine positioning was optimal for this approach. One attendee noted that the problem with the supine position for him was not the ultrasound image but rather developing cramps in his hand, and he had to lift the leg and hold it at an angle, especially when doing a catheter technique.
“What we usually do at the trauma center is use a Mayo stand or a leg raise device to rest the leg on,” Dr. Samet replied. “It is also helpful to rest one’s elbow on the bed to avoid tiring and hand cramps.”
In the end, Dr. Samet believes the study has the potential to change regional practice. “This is arguably the first study to evaluate the influence of a patient’s position on the ease of ultrasound identification of nerves and surrounding structures,” he concluded, “and may have an impact on nerve block success rates, especially in the difficult-to-image patient.”