Spinal Sonography for Ultrasound-Guided Lumbar Neuraxial Anesthesia

Summary

The protocol described here provides the basics of spine sonoanatomy and a quick, straightforward method for performing ultrasound-guided neuraxial anesthesia. Additionally, two handheld devices that enhance portability are presented, one of which uses pattern recognition software to aid in epidural space localization.

Abstract

Neuraxial anesthesia is one of the few remaining forms of regional anesthesia that relies on palpation and tactile feedback techniques to facilitate catheterization into the epidural space. Over two decades ago, spine ultrasonography was demonstrated to provide reliable guidance for locating the epidural space. Compared to the palpation technique, preprocedural ultrasonography has been shown to result in fewer needle punctures and fewer traumatic procedures, particularly in patients with abnormal or distorted spine anatomy (e.g., scoliosis, obesity). Despite its utility, the ultrasound-guided neuraxial technique is still marginally used, even for patients with abnormal anatomy. Some experts attribute this to cost, a relatively high success rate without ultrasound, and a lack of technical expertise, which is often tied to formal education and regular practice. Several proponents of the ultrasound technique emphasize that proficiency requires practice on patients with normal spine anatomy, though this training may not be as challenging as once thought. This protocol was designed to help all providers learn the basics of lumbar spine anatomy and how to apply this knowledge clinically. Through a series of videos, we will provide step-by-step instructions for performing neuraxial ultrasonography and offer practical tips for troubleshooting in cases of difficult anatomy.

Introduction

Lumbar epidural analgesia provides the dual benefit of providing effective labor analgesia and the best way of avoiding the use of general anesthesia¹. The latter has been associated with anesthetic and surgical complications as well as an increased risk of postpartum depression^2,3. Hence, it is not surprising that anesthesiologists have evaluated many techniques over the years to decrease the incidence of epidural catheter failures. Several techniques (e.g., combined spinal and dural puncture epidural) evaluated over the years have been shown to reduce the incidence of epidural catheter failures^1,4,5. Yet, to the best of the authors understanding, the ultrasound-guided neuraxial technique is the only technique that has demonstrated a decrease in the rate of failed epidural catheters and the number of epidural attempts, particularly when performed by relatively inexperienced providers⁶.

There is mounting high-quality evidence to demonstrate that ultrasound-guided neuraxial anesthesia decreases the number of needle manipulations, provides an excellent correlation between the estimated and actual depth from skin to epidural space, and reduces traumatic procedures^{7,8,9,10,11,12}. Besides, the traditional anatomical landmark approach has proven inferior to the ultrasound technique or imaging for identifying the desired interspace for instrumentation^13,14. The abovementioned benefits are noticed in patients with normal and abnormal anatomy. Yet, the evidence suggests that patients with abnormal anatomy benefit the most from using ultrasound guidance^9,11,15,16. Perhaps these advantages prompted the National Institute for Health and Excellence (NICE) to determine that there was enough evidence to recommend the routine use of ultrasound guidance for establishing neuraxial anesthesia^6,17. Close to two decades after that recommendation, this technique is scarcely, rather than routinely utilized.

Some cited reasons for this slow embrace include a high success rate without ultrasound, lack of access to the technology, additional time to obtain imaging, and lack of formal training^18,19,20,21. While it is conceivable that access to ultrasound and the image quality were less than optimal when this technique was first described by Cork et al. in 1980, imaging quality and accessibility to ultrasound have improved^22,23. Besides availability, portability has also increased without compromising image quality^24,25,26. Hence, we have overcome most of the obstacles that have slowed the acceptance of this technique. The hurdles to overcome are the relatively high success rate without ultrasound, additional time to obtain imaging, and lack of formal training.

While the overall success rate of epidurals is high, the number of needle attempts is not often reported. Given that ultrasound-guided neuraxial anesthesia has been shown to decrease the number of needle manipulations (attempts and redirections) and failed catheters, it is conceivable that this technique may also improve patient satisfaction¹⁶. Besides the high success rate, the last two hurdles are time and formal training^{15,16,27,28,29}. Regarding formal training, this is perhaps the rate-limiting factor. The skepticism surrounding the use of this technique perpetuates the lack of formal training. With the protocol below and enough practice (in patients with normal anatomy), most providers will achieve proficiency and seize the benefits of this procedure, even in the most challenging cases^9,17,21.

Protocol

All procedures involving human participants in the study were conducted in accordance with the ethical standards of the institutional guidelines for research and the 1964 Declaration of Helsinki, including its later amendments or comparable ethical standards. The protocol was developed based on input from previously published articles in the academic literature^30,31,32. Imaging studies were conducted on the authors themselves for normal images and as part of routine educational spine sonoanatomy. The following sections discuss the use of preprocedural ultrasound-guided neuraxial anesthesia, but real-time ultrasound guidance is not addressed. Details of the equipment used in this study are listed in the Table of Materials.

1. Probe selection

1. Select a low-frequency (2-5 MHz) curvilinear probe if utilizing a traditional ultrasound device. See Figure 1A.
2. For a handheld device (see Table of Materials), select a curvilinear frequency probe. Click on presets and select abdomen. See Figure 1B and Figure 2A,B.
3. For an automated device (see Table of Materials), choose the spine option from the scanning menu. See Figure 1C.

2. Machine preset

1. Set the scanning depth at 8 cm.
   NOTE: A study by Sutton et al.³³ demonstrated that most patients would have a skin-to-epidural space distance of 4-6 cm (76%), whereas depths between 2-4 cm or >6 cm range from 16% to 2%, respectively. Given the current obesity epidemic, depths >6 cm are probably more common than the cited 2%. Another great reason to use ultrasound is to help discern when a needle longer than the typical 3.5-inch Tuohy needle is needed. If using the AU, the device will automatically adjust according to the estimated depth.

3. Scanning technique

1. Ask the patient to sit in the traditional sitting position with shoulders relaxed, chin to chest, arms resting on their thighs, and a slouched posture (pushing the belly button towards the provider)³⁴.
2. Apply ultrasound gel to the transducer.

4. Longitudinal paramedian view

1. Place the ultrasound probe angled towards the midline over the sacral area, about 3 cm from the midline (Figure 3).
2. Move the probe cephalad until a solid hyperechoic line is visualized.
   NOTE: A solid, continuous hyperechoic line will be noticed. This is the sacrum.
3. Continue to scan the cephalad until the "saw sign" is visible.
   NOTE: The teeth of the "saw" are formed by the lamina. The space between represents the interspace. With a good ultrasound window, the interspace looks like an equal sign (Figure 2). The equal sign - the blue line in Figure 3 and Figure 4 represents the posterior complex (PC), composed of the ligamentum flavum and the posterior dura matter. The white line in Figure 3 and Figure 4 represents the anterior complex (AC) - formed by the anterior dura, posterior longitudinal ligament, and vertebral body. The sacrum is only visualized when utilizing this view. Hence, this is the ideal view for estimating at which level the block is being performed. After locating the sacrum, the levels are counted as they appear (e.g., L5-S1, L5-L4, L4-L3, L3-L2).

5. Transverse view

1. Place the ultrasound probe at the predicted midline.
2. Scan the cephalad or caudal until a long hypoechoic (dark) line is noted (Figure 5).
   NOTE: This hypoechoic line represents the spinous process, and you can mark this as the midline. The spinous process may be noted to be at an angle for patients with scoliosis. Angle the probe to the left or to the right to correct for the curvature. This angle should be considered to guide your epidural needle's trajectory.
3. Continue to scan the cephalad or caudal until the pattern shown in Figure 4 is localized.
   NOTE: The interspace can be recognized and marked when the articular processes, transverse processes, and posterior (PC) and anterior complex (AC) come into view. Once these structures have been identified, the interspace can be marked on the patient's back by drawing a line on the lateral side of the probe.
4. Place the device over the suspected midline (users utilizing the AU device).
   NOTE: This device has pattern recognition software programmed to identify the previously described bony landmarks. A 2D and 3D spine representation image will appear on the upper and lower portion of the screen, respectively, of your screen, and a crosshair icon will show when the space is correctly identified.

6. Measurements

1. Once the posterior complex (PC) has been located, select the caliper (traditional US) and adjust the measure from the skin to the posterior complex (Figure 6A,B).
2. For BU users, click on freeze image (Figure 7A), then click on Actions, select line (Figure 7B), and measure as shown in the previous step.
3. Draw a straight line from the skin to the PC.
   NOTE: This measurement will be the estimated depth from skin to epidural space. It is important to ensure no pressure is applied between the probe and the skin; otherwise, the measurement will underestimate the actual depth.
4. For Automated device users (see Table of Materials), place the device on the patient's back, and the device will automatically provide the estimated depth (Figure 8).
   NOTE: As described above, applying pressure on the skin should be avoided to improve measurement accuracy.

7. Epidural placement

1. After obtaining the information from the steps above, clean and drape the patient following the institutional protocol.
2. Ensure that the insertion point is the midline and interspace marking intercept.
   NOTE: The midline refers to the vertical centerline along the patient's back. The needle must be placed here for optimal results. The vertebrae in the spine have small gaps (interspaces) between them. The needle should be inserted in one of these gaps. The interspace marking intercept refers to the specific point where these guidelines (midline and interspace) meet.
3. Proceed with the epidural placement utilizing the loss of resistance technique.
   NOTE: The loss of resistance technique is a common method for identifying when the needle has reached the epidural space. The medical practitioner will gently push the needle and feel for a "loss of resistance" as the needle passes through the ligament and enters the epidural space, indicating the correct position for administering the anesthesia. The information from the steps above should be used as guidance, not as a substitute for your clinical judgment or practice. For instance, the estimated depth measure in step 18 could underestimate or overestimate the actual needle depth.

8. Follow-up procedures

1. Ensure the catheter is securely taped to the patient's back to prevent it from moving or dislodging.
2. Document the insertion site and note the catheter's depth for future reference.
3. After completing epidural catheterization, evaluate patient's pain management. This is usually done 15-30 min after the procedure.
4. Ensure the catheter is functioning properly and delivering medication consistently.

Results

The main results from this research have focused on image quality and proficiency in performing ultrasound-guided neuraxial anesthesia. When comparing the quality of images from the BU to those of a mid-range ultrasound machine, it was determined that the former is a good alternative for obtaining spine anatomy images²⁶. In terms of proficiency, in a prospective cohort analysis, first attempt success rate (defined as the number of skin needle punctures), number of needle passes (defined as the num...

Discussion

The main findings of this research are that the use of US-guided neuraxial anesthesia results in an overall increased first-attempt success. That is, fewer needle attempts and passes are needed to identify the epidural space²⁹. The abovementioned findings are in agreement with those of several meta-analysis studies when comparing preprocedural US-guided neuraxial anesthesia to the landmark technique^7,8,9<...

Materials

Name	Company	Catalog Number	Comments
ACCURO	Rivanna Medical	NA	Described throughout the manuscript as the automated device
Butterfly iQ+	Butterfly Network	iQ+	Described throughout the manuscript as the handheld device
Traditional ultrasound	SonoSite	SonositePX	Select a low-frequency (2-5 MHZ) curvilinear probe  if utilizing a traditional ultrasound device.