|Year : 2015 | Volume
| Issue : 2 | Page : 49-53
Ultrasound for obstetric neuraxial anesthetic procedures: Practical and useful?
Jason Lie1, Santosh Patel2
1 North Western Deanery, The Royal Oldham Hospital, Manchester, United Kingdom
2 Pennine Acute NHS Trust, The Royal Oldham Hospital, Manchester, United Kingdom
|Date of Web Publication||11-Sep-2015|
Dr. Santosh Patel
Consultant Anaesthetist, Pennine Acute NHS Trust, The Royal Oldham Hospital, Manchester
Source of Support: None, Conflict of Interest: None
|How to cite this article:|
Lie J, Patel S. Ultrasound for obstetric neuraxial anesthetic procedures: Practical and useful?. J Obstet Anaesth Crit Care 2015;5:49-53
|How to cite this URL:|
Lie J, Patel S. Ultrasound for obstetric neuraxial anesthetic procedures: Practical and useful?. J Obstet Anaesth Crit Care [serial online] 2015 [cited 2021 May 18];5:49-53. Available from: https://www.joacc.com/text.asp?2015/5/2/49/165130
Central neuraxial blocks (CNBs), namely spinals and epidurals, are routine procedures in obstetric anesthetic practice. According to the Third National Audit Project,  approximately 325,000 spinals, 293,000 epidurals and 42,000 combined spinal-epidural were performed annually in the United Kingdom (UK). Obstetric spinals and epidurals made up 45% of all neuraxial procedures in the census phase of the national audit and approximately 25% of laboring women in the UK received epidural analgesia. Our recent audit (unpublished data) demonstrated that procedural failure was responsible for 50% cases that needed general anesthesia for caesarean sections. Failure rate for epidural analgesia can be as high as 20%. 
| Challenges of Landmark Technique|| |
Weight gain, pelvic rotation, hyperlordosis, tissue edema, and suboptimal positioning predispose difficulty in the performance of neuraxial block during pregnancy. Traditionally, an imaginary horizontal line joining the two superior parts of the posterior iliac crests (Tuffier's line, Jacoby's line or the intercristal line) is used as an anatomical landmark, as it is believed to pass through the L 4 vertebral body  for the estimation of vertebral levels for CNBs. During pregnancy, this landmark can be potentially positioned higher than the L 4 or L 5 vertebral levels.  It is of utmost importance to identify the vertebral levels correctly to avoid needle trauma to the spinal cord, which usually ends at L 1-2 level in adults, but this can have significant variability from the lower half of T 12 to the upper half of L 3 vertebra.  Major permanent complications, although rare, continue to be reported when the vertebral level is misidentified. , This could have potential medico-legal implications to the anesthesiologist. The level at which the procedure is performed also affects the level of the neuraxial blockade. 
This high variability of the landmark we traditionally use for CNBs has unsurprisingly meant that we, as anesthesiologists, are extremely poor at identifying the correct and therefore, safe vertebral level. Our data  demonstrated an accuracy rate of only 45.4% (n = 91) and that was reduced further to 39.2% if we consider obstetric patients only (n = 51). Worryingly, 10.2% of CNBs were performed at or above the L 2 vertebra. These accuracy rates are like to be overestimation because of the small number and the limitations of retrospective ultrasound (US) scanning. Inaccuracy in identifying correct landmarks was highlighted by Broadbent et al.  who utilized magnetic resonance imaging (MRI) to confirm the position of their estimated vertebral level by landmark technique. The space was correctly identified in only 29% of cases, and the error made by 51% showed a tendency to think the interspinous space they identified was lower than it actually was. Srinivasan et al. also demonstrated inaccuracy of the landmark technique for patients undergoing elective lower segment caesarean section (LSCS). They compared two groups: Group A performed spinal at or above the level when the Tuffier's line intersected a space, and Group B performed below when this line intersected a spinous process. The actual space performed was then identified by a blinded investigator using US. They found that 45.5% of those in Group A would be performing the procedure at or above L 2-3 intervertebral space.
| Usefulness of Ultrasound|| |
Estimation of intervertebral space by US is more accurate than by landmark technique as it allows the operator to count the number of spinous processes or laminae upward from the sacrum.  Lee et al. found that in >40% of pregnant population the level identified clinically was at least one space above that identified by US. It is also interesting that different sonoanatomy of the spine had been reported in those with previous unintentional postdural puncture headache.  Evidence suggests that US imaging of the spine by an experienced operator can achieve an accuracy rate of >90% (compare to 32% for novices).  In a randomized control study in nonobstetric population with difficult surface anatomical landmarks undergoing spinal anesthesia first attempt success was twice in US group.  The study population included were patients with body mass index (BMI) more than 35 kg/m2 and poorly palpable spinous processes; moderate to severe lumbar scoliosis; or previous lumbar spine surgery. Similar anatomical landmarks are not uncommon in parturients.
The efficacy and safety records of using US for catheterization of epidural space were reviewed by Shaikh et al.  and more recently by Perlas et al.  Both meta-analysis showed beneficial outcomes with US-facilitated spinal or epidural techniques. The numbers for failure rate, incidence of traumatic procedures, number of attempts and needle redirections were all reduced with the use of US. Perlas et al.  suggested that US not only complements the landmark palpation but it has potential to improve safety of neuraxial anesthetic techniques. Use of US for epidural procedure has been recommended by National Institute for Health and Clinical Excellence since 2008. 
Several factors can cause procedural difficulty during spinal or epidural technique including obesity, spinal deformity, and previous spinal surgery. Ultrasonography can be useful to identify midline, to predict the depth of epidural space and to direct the insertion needle. It can demonstrate running course of spinal canal (e.g. in scoliosis) and the location of any surgical metals in the back. In our audit of failed procedural neuraxial technique, obesity was the most common cause. Our audit of high risk obstetric outpatient clinic showed 42% and 23% referred cases were morbidly obese, and with lumbar musculoskeletal problems respectively.  Some of our colleagues have started using US in high risk obstetric antenatal clinics for anticipated difficult cases for neuraxial anesthesia or analgesia. In anticipated difficult cases, ultrasonography can be performed; images can be stored or printed for records. It facilitates advanced discussion with women with regards to potential difficulties for neuraxial techniques.
In obese pregnant patients, ultrasonography can be useful to reduce number of epidural attempts and the rate of epidural catheter replacements  and may prevent accidental dural puncture. The first attempt success rates under US guidance were 92% in comparison to 44% using a conventional technique in obese parturients undergoing spinal anesthesia for elective LSCS.  In addition, number of attempts, and duration of procedure were significantly less with the use of US in obese women. In a small study  of obese (BMI >35 kg/m 2 ), US measured depth of epidural space correlated with the depth measure by MRI (70.1 [standard deviation (SD) 13.1] vs. 72.4 [SD 12.8]; P = 0.29). However, US imaging was difficult in women with BMI >45 kg/m 2 . Increased number of reflective interfaces, exaggerated attenuation, and phase aberration (uneven speed of sound waves through irregularly shaped adipose layers) are some of the causes for technical difficulty with the use of US in obese women.  Technological advancements are necessary for optimal use of US in women with morbid and super obesity.
| Technique and Training|| |
Below is a brief step-by-step guide on scanning the lumbar spines using US with the patient in sitting position. It is possible to perform the scan with the patient in the lateral decubitus position, but the spine may not be in a straight line making it more difficult.
- Step 1: Paramedian sagittal approach at the lower back to identify the sacrum and the desired level of the spine. Orient the probe toward midline to achieve "saw-tooth" appearance indicating lamina and interlaminar space [Figure 1].
|Figure 1: Paramedian sagittal (a and b) view (with median probe orientation) showing classic "saw-tooth" appearance (c and d) representing lamina and interlaminar space|
Click here to view
- Step 2: Count up the levels from sacrum (L 5 S 1 ) to your desired level.
- Step 3: Rotate probe by 90° at your desired vertebra (e.g., L 3 for L 3-4 space). Transverse view at spinous process shows "tower" sign indicating the spinous process [Figure 2].
|Figure 2: Transverse spinous process (a and b) approach showing "tower" like appearance of the spinous process (c and d)|
Click here to view
- Step 4: Slowly move probe caudally into desired intervertebral space (e.g., L 3-4 ). "tower" sign will disappear as the probe is moved into space either caudally or cephalad. A "flying bat" sign will then emerge [Figure 3].
|Figure 3: Transverse interspinous (a) view showing "flying bat appearance" (highlighted with yellow line). Posterior dark midline is represented by interspinous ligament. Lamina, articular process and transverse process are seen (b and c). Anterior complex (anterior dura, anterior epidural space, posterior longitudinal ligament, and the posterior aspect of the vertebral body) can be visualized deep to spinal canal (b and c)|
Click here to view
- Step 5: Mark midline and level on side of probe using surgical marker, followed by an indentation on the skin using a needle cap to keep insertion point visible after cleaning with antiseptic solution.
Currently, there is no US probe portable and sophisticated enough to allow real time performance of spinal or epidural. Therefore, the gap between time of the scan and the actual performance of the procedure should be kept as minimal as possible to minimize alterations of the identified intervertebral space. In terms of the number of scans required to be competent in achieving >90% accuracy in intervertebral level for CNBs, Halpern et al. showed that a novice would require 23-39 US scans to reach that target.
There is certainly a steep learning curve.  However, training by attending workshops may improve its wider use and clinical applications. One of the authors (JL) has established a training course which consists of a mixture of lectures, workshops and plentiful hands-on practice. The use of virtual spine website (http://www.usra.ca/vspine.php) has also shown to improve performance amongst novice anesthesiologists learning this technique.  The use of epidural simulator in future may also allow the measurement in real time the depth of needle tip during insertion and interspinous pressure changes through a pressure measurement device as the epidural needle is advanced through the tissue layers. 
| Limitations|| |
Currently, there is no evidence-based curriculum available for trainees in UK or novice anesthesiologists. In fact, less than 50% obstetric units have dedicated anesthetic US machine in the UK.  Our obstetric unit experience also suggests that despite availability, not all our colleagues use the US routinely. Financial constraint, lack of experience, unavailability of curvilinear probes, difficult real-time visualization of needle and catheter, the urgency of cesarean section and poor evidence of its benefit pertaining to US of spine are some of the reasons against its routine use. 
In addition to the learning curve, time delay is another concern for routine use of US for elective LSCS. Breach et al.  demonstrated that with adequate preparation and training, the use of US needed only additional 4.5 min for the total set up time of the procedure. However, it reduced around 1 min the time for the actual procedure itself with potential reduction in number of failed procedures, needle passes and traumatic complications. There are no data for use of US during emergency LSCS. Correct identification of landmarks with US does not guarantee successful completion of procedure. When the landmarks are easily palpable, US may not affect time or technical aspects of spinal or epidural procedure.  Technology and technique for real-time US-guided spinal and epidural procedures remains elusive at present.
| Conclusion|| |
We predict US would become increasingly common in identifying the midline (as a minimum) to reduce the number of failed attempts and needle redirections. It has the potential to improve patient safety and satisfaction in subgroup patients with anticipated difficulty in success of neuraxial techniques. In future, with technological advancements real-time US-guided CNBs may be feasible with higher success in anticipated difficult cases.  The use of US is a valuable skill for all obstetric anesthesiologists.
The images ([Figure 1], [Figure 2], [Figure 3]) are modified by Dr. Santosh Patel with the help of medical illustrator Mr. Harry Heyes of Central Manchester University Hospitals. The original images are courtesy of 'Ultrasound of the Spine Workshop'.
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[Figure 1], [Figure 2], [Figure 3]