By Will Mauldin, PhD
More than 3.5 million neuraxial anesthesia procedures are performed in the U.S. every year in the labor and delivery department alone (Centers for Disease Control and Prevention, 2017). Studies show that the optimal site for delivery is identified in as few as 14% of patients (Lee et al., 2011). Depending on clinician skill, patient body mass index (BMI), and patient age, between 20% and 80% of needle placements fail, resulting in repeated passes and potential risks of long-term headaches, back pain, and rarely paralysis (Macario, Scibetta, Navarro, & Riley, 2000). Clearly, neuraxial and epidural anesthesia continue to pose major healthcare challenges, with a failure rate that adds significantly to healthcare costs.
In fact, the path to effective neuraxial anesthesia delivery has existed for decades—ultrasound visualization. But practical issues in implementation have intervened. As a result, even in today’s technological age, highly trained anesthesia providers continue to deliver neuraxial—and most prominently epidural—anesthesia wearing a virtual blindfold, using spinal palpation alone to determine the optimal site for injection.
While ultrasound has proven beneficial in directing physicians to the preferred delivery site, the modality involves a steep learning curve and skill set unfamiliar to most anesthesia providers. Additionally, ultrasound is better suited to soft-tissue rather than bony imaging. Ultrasound equipment also is often unwieldy and difficult to maneuver to the bedside, where most neuraxial pain blocks are administered. So the difficulties continue.
However, emerging technologies combining automated image guidance with enhanced bone image display are beginning to overcome these issues. Increasingly, they are making ultrasound a viable tool to enhance patient safety and clinical efficiency for these challenging procedures.
This article provides an overview of neuraxial imaging, the use of image guidance to enhance neuraxial anesthesia needle placement, and a novel technology that optimizes ultrasound guidance for these procedures. It also suggests a pathway to a new standard of care for neuraxial anesthesia placement.
Neuraxial anesthesia background
A brief refresher course: Neuraxial anesthesia includes epidural and spinal as well as combined spinal-epidural (CSE) procedures. In labor and delivery, epidurals are the most common, typically administered in patients presenting for vaginal births. They involve placement of a catheter into the lumbar spine epidural space. The catheter enables continuous delivery of an anesthetic agent throughout labor and delivery. By contrast, spinal anesthesia involves a single injection of the anesthetic agent into the spinal column subarachnoid space. The provided pain relief is sufficient for the course of the surgical intervention. In addition to cesarean sections in labor and delivery, spinal anesthesia has a variety of indications, including in orthopedics and vascular surgery.
In labor and delivery, neuraxial placement location is generally acceptable anywhere between intervertebral levels L2 through L5. These intervertebral levels are more easily accessible compared to the thoracic spine and provide pain relief to the lower body. Additionally, placement in the lumbar spine mitigates risk of severe neurologic injury because the conus medullaris of the spinal cord terminates at higher intervertebral levels.
The manual palpation standard of care for neuraxial anesthesia has been used for decades to estimate the spinal midline and appropriate intervertebral space for anesthesia administration. For epidurals, following palpation and needle insertion, the provider experiences a sudden loss of resistance to pressure, signaling access into the epidural space.
Current neuraxial anesthesia delivery challenges
Studies reveal that palpation identifies the appropriate intervertebral level in only 14%–64% of cases, with patient anatomy as well as provider skill and experience largely determining the outcome (Lee et al., 2011). Patients with above-normal BMI have a significantly higher risk of epidural failure. For example, in obese subjects whose spinal bony structures are less apparent through palpation, first-attempt success rates have been reported as low as 50% (Chin et al., 2011). Patients with atypical spinal conditions such as scoliosis or who have had surgeries that affect the spine are also significantly at risk for epidural anesthesia failure. In addition, needle placement failure can increase with patient age due to higher rates of scoliosis and stenosis.
During labor and delivery, anesthesiologists are compelled to warn women about possible long-term symptoms and other serious consequences—most often, about the potential for ongoing pain from the most effective pain-reducing technique medicine offers them today. Frequently, the consequence is increased anxiety during an already stressful time.
Anesthesia delivery through misplaced needles compromises patient safety. Needle trauma may cause lengthy backaches, while severe cerebrospinal leakage results in ongoing “spinal headaches.” Cephalad spread of anesthetic may cause other significant problems. In extremely rare cases, spinal hematomas can bring on paralysis.
Additionally, prolonged procedures due to multiple needle passes and redirects, patient dissatisfaction, and costs are associated with adverse outcomes.
The value of ultrasound image guidance
Recent years have seen the use of a variety of medical imaging modalities to guide medical procedures. Ultrasound, in particular, is well-suited to support numerous image-guided procedures because system functionality and portability have increased, while costs remain low. The modality also avoids the risks of ionizing radiation.
- Anesthesia in general
In anesthesia, ultrasound guidance is beginning to replace palpation as the standard of care for central venous access, peripheral nerve blockades, and related needle guidance procedures (Rupp et al., 2012), primarily due to technological innovations that dramatically boost guidance precision. Thus far, this trend has not similarly impacted neuraxial anesthesia, largely due to a range of practical challenges. But as clinical evidence increasingly highlights the benefits of ultrasound guidance in this setting, and as spine-specific technologies begin to eliminate barriers, the situation is changing.
- Neuraxial anesthesia
The value of neuraxial and epidural anesthesia image guidance has been proven by more than a decade of clinical trials and over 30 studies. The data effectively demonstrates that neuraxial ultrasound accurately identifies the appropriate intervertebral space for epidural administration far more successfully than palpation (Lee et al., 2011). Ultrasound also strongly predicts needle insertion depth to the epidural space (Seligman, Weiniger, & Carvalho, 2017; Tran et al., 2009) and reduces attempts for procedure completion, enhancing patient safety and clinical efficiency.
The recently revised AANA Analgesia and Anesthesia for the Obstetric Practice Guidelines cite neuraxial ultrasound as a useful adjunct in patients with difficult-to-palpate anatomic landmarks (AANA Board of Directors, 2017).
Neuraxial ultrasound challenges
While the benefits of ultrasound guidance for neuraxial anesthesia placement are increasingly clear, a range of limiting factors has prevented widespread adoption:
- Steep ultrasound learning curve
For both image acquisition and interpretation, ultrasound requires a significant skill set and training not consistent with the anesthesiology provider’s background. Studies show that learning curves for the modality are significant (Margarido, Arzola, Balki, & Carvalho, 2010).
Use of trained sonographers or radiologists for anesthesiology procedures is cost-prohibitive. Scheduling also is often problematic for labor and delivery.
- Bony anatomy
A thorough understanding of the nuances of bony spinal structures imaged under ultrasound is crucial for neuraxial anesthesia guidance because the procedure relies on identification of specific bony landmarks. Ultrasound is inherently better suited to soft-tissue imaging without extensive, experimental post-processing. This is because the physical characteristics of bone alter the reflectivity, absorption, and scattering of high-frequency ultrasound waves, creating image distortion.
In particular, high reflectivity results in a characteristically bright, less nuanced image when the ultrasound beam is positioned perpendicular to the spinal bone surfaces. However, the appearance is angle-dependent. Successful ultrasound interpretation must account for these and a range of additional complexities.
Recently, to utilize ultrasound’s benefits in broader clinical applications, academic research has examined segmentation and enhancement of ultrasound data for bone surfaces and spinal bone in particular (Tran et al., 2009). However, human data is extremely limited.
Bedside availability of ultrasound is limited in many hospitals, particularly in labor and delivery. Navigating standard consoles through cramped locations can be problematic and difficult to arrange for short anesthesia scans. While today’s portable scanners could provide an answer, their functionality is generally optimized for specific soft-tissue imaging applications.
A novel technology overcomes challenges
Recently, the first ultrasound system dedicated to neuraxial image guidance was introduced commercially, following clearance by the FDA. The system combines automated 3-D image spinal navigation with sophisticated image reconstruction optimized for bony anatomy. A flexible, handheld form factor enables easy and convenient operation by the anesthesia provider.
Relying on a pre-procedural scouting approach, the device is moved along the patient’s spine to identify the midline and interlaminar space at the desired vertebral level. These landmarks, along with a calculated depth to the spinous process and epidural space, appear as overlays on the device display in real time. Users then mark this precise location and proceed with the neuraxial anesthesia placement.
Ongoing clinical trials and research prove the device’s efficacy (Seligman et al., 2017; Tiouririne, Dixon, Mauldin, Scalzo, & Krishnaraj, 2017; Powlovich, Tiouririne, & Singla, 2017; Van Hecke, Pandin, Naik, Estruch, & Van Obbergh, 2017), as well as its benefits in promoting procedure safety, efficiency, cost-cutting, and patient satisfaction. The introduction may launch a path toward a new image-driven neuraxial anesthesia standard of care by encouraging an image-guided approach to needle insertion and the growth of similar technologies aimed at improving placement.
Specific benefits include:
- Reduction of the ultrasound learning curve through image interpretation automation
Using this device, providers with even minimal ultrasound experience can take advantage of effective image guidance. The system relies on automated 3-D navigation of the lumbar spine to identify relevant landmarks. Based on these, it computes, visualizes, and pinpoints the optimal needle insertion point and other relevant data. Its algorithms use proven image processing techniques, including pattern recognition and image segmentation. Studies validate that the device’s success rate in identifying the epidural location exceed 94% (Tiouririne et al., 2017).
- Enhanced spinal anatomy image reconstruction
Helping to enable this automated navigation is an innovative image reconstruction technology that compensates for ultrasound’s bony structure distortion with a fivefold increase in bone-to-tissue contrast compared to conventional ultrasound technology. In addition to driving spinal navigation accuracy, this data is presented to the user as an enhanced, easy-to-understand image to further guide clinical decision-making. Also contributing to image quality is a specific ultrasound transducer type known to limit certain bone imaging artifacts present in a conventional ultrasound system (Mauldin, Owen, Tiouririne, & Hossack, 2012).
- Form factor supporting bedside care
Designed for flexible and convenient use, the handheld technology is battery-operated with a touch-screen interface and rotatable display. The device can be easily enclosed in a sterile cover.
In addition to supporting neuraxial anesthesia placement, the technology offers bone and tissue modes to support further applications, including central line placement and paraspinal injections.
A growing number of studies prove its effectiveness
Clinical trials and additional research underscore the benefits of the ultrasound device and its image-guided approach to needle placement.
A recent randomized trial conducted at the University of Virginia Medical Center and funded by the National Institutes of Health National Institute of Biomedical Imaging and Bioengineering compared anesthesiology residents’ success placing spinal anesthesia in C-section patients using the device and using conventional methods. It found that for residents with prior spinal anesthesia experience, first-attempt needle placement using the device more than doubled in patients with above-normal BMI. In addition, 80% fewer patients reported dissatisfaction in pain control (University of Virginia, 2016).
A second clinical trial conducted at Stanford University Medical Center found that the device successfully identified the location and depth for optimal epidural anesthesia administration with essentially equivalent accuracy to traditional ultrasound images read by an experienced interpreter (Seligman et al., 2017). A study recently published in Investigative Radiology found that the device accurately identified key neuraxial landmarks in ultrasound images of the lumbar spine acquired in the transverse plane (Tiouririne et al., 2017).
Additional research continues to emerge, and the device is now in clinical use in a number of academic institutions and other hospitals worldwide.
Today, neuraxial anesthesia delivery remains fraught with significant problems in patient safety and satisfaction, provider efficiency, and procedure cost. While ultrasound image guidance has reduced similar concerns in other anesthesia procedures, the same has not been true for spinal procedures due to barriers that include a steep ultrasound learning curve, suitability for bony imaging, and modality accessibility.
Growing research has demonstrated the value of ultrasound image guidance for neuraxial procedures. The recent introduction of an ultrasound guidance system optimized for spinal applications is proving that an image-guided approach can reduce placement failures and complications, while adding efficiencies and cutting costs. Potentially, it will open a path to a new standard of care for neuraxial anesthesia placement from manual spine palpation to image guidance.
About the Author
Will Mauldin holds a PhD in biomedical engineering from the University of Virginia, where he also served as an assistant research professor in the Department of Biomedical Engineering. He holds a bachelor’s degree in applied science from the University of North Carolina, Chapel Hill. He is co-founder and CEO of RIVANNA, where he engineered and commercialized a novel ultrasound-based epidural guidance device.
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