Case Study
A multiparous woman (gravida 3, para 2) was admitted to the delivery suite at 39 weeks’ gestation following the onset of labor. She described a pain score of 5/10 and requested labor analgesia prior to the initiation of oxytocin for labor augmentation.
Her medical history was positive for severe obesity. On further questioning, the previous obstetric history revealed two spontaneous vaginal deliveries with several problems related to her epidural analgesia. On both occasions, multiple attempts were required to site the epidural. Furthermore, the first epidural provided a unilateral block that could not be optimized, and the second epidural “did not work.”
On physical examination, the patient weighed 145 kg and measured 160 cm in height to give a body mass index (BMI) of 57 kg/m2. It was impossible to palpate bony landmarks on examination of her spine.
The patient adopted the sitting position, and a preprocedure spinal ultrasound assessment was performed. Successful identification of the L3–L4 interspace was achieved. The ideal insertion point for the epidural puncture was marked based on the identification of the midline and interspace, and the depth to the epidural space was estimated at 9.5 cm. Following this, the back was cleaned and draped, and the procedure was undertaken with a 17-gauge, 11.5-cm Tuohy needle. Loss of resistance to saline was achieved at approximately 9.75 cm, and an epidural catheter was sited uneventfully at the first attempt.
A test dose of 3 ml bupivacaine 0.125 percent was negative, and the epidural was loaded with a further 12 ml bupivacaine 0.125 percent plus fentanyl 50 µg. Satisfactory analgesia was achieved. The patient remained comfortable during labor and delivery with a patient-controlled epidural analgesia pump administering bupivacaine 0.0625 percent with fentanyl 2 µg/ml (bolus 5 ml, lockout 10 minutes, continuous infusion 10 ml/h, maximum 20 ml/h).
Key Points
It is not uncommon to encounter patients who have had a previous difficult placement of an epidural, sometimes due to obesity or problems identifying anatomic landmarks.
Spinal ultrasound imaging performed before the procedure to facilitate epidural insertion may be considered for such patients.
Discussion
Conventional placement of spinal or epidural anesthesia is a blind technique that relies on palpation of anatomic landmarks to identify the optimal puncture point. Typical landmarks that are examined include the iliac crests, tips of the spinous processes, and the intervertebral spaces. It is not unusual to fail to identify these landmarks in the setting of obesity, anatomic spinal variations, or edema.
Furthermore, studies have demonstrated that even when the bony landmarks are easily identified, experienced clinicians often fail to accurately identify the desired interspace. One investigation showed that when compared with MRI, anesthetists were correct in their assessment in only 29 percent of cases, and the error in their assessment could be as much as four interspaces. In 94 percent of the errors, the actual level was higher than assumed.1 The safety implications of these inaccuracies for central neuraxial blockade are concerning, especially in the context of spinal anesthesia.
Advantages of Spinal Ultrasound Assessment
The use of ultrasound as an adjunct in anesthesia has risen significantly, and ultrasound-assisted nerve blockade, peripheral or central, is no exception. In the United Kingdom, the National Institute for Health and Care Excellence (NICE) has issued guidance regarding the facilitation of epidural space catheterization with the use of spinal ultrasound.2 A significant source of knowledge can be provided by preprocedural spinal ultrasound to facilitate the placement of neuraxial anesthesia. It assists in the identification of the desired intervertebral space, the optimal insertion point, the angle of needle insertion, and the depth to the epidural space.
A technique that minimizes the number of puncture points and needle redirections can reduce trauma and subsequent infection, bleeding, bruising, back pain, post–dural puncture headache, and nerve damage. In addition, patient satisfaction can be improved and the efficacy of epidural anesthesia can be enhanced.3 A recent systematic review and meta-analysis concluded that there was a reduction in the number of failed attempts, traumatic attempts, insertion attempts, and needle redirections with the use of ultrasound when performing epidural placements or lumbar punctures.4 Novice residents have also shown a significantly higher success rate of epidural insertion with the use of ultrasound.5
The accuracy of ultrasound in identifying the desired interspace is superior to the conventional palpatory technique. One study showed a success rate of 71 percent in the ultrasound group versus 30 percent in the palpatory group, with errors greater than one space occurring in the palpation group but not in the ultrasound group.6
Preprocedural spinal ultrasound prior to spinal or epidural anesthesia can provide the clinician with knowledge that can help select the most appropriate needle length. For example, if the depth of the epidural space is measured on ultrasound to be greater than 8 cm, an extralong needle can be selected from the outset. This can reduce unnecessary multiple passes and trauma, as well as the risk of infection.
One may predict difficult neuraxial anesthesia in certain patient groups. Assessment of surface landmarks in obesity can be challenging, and identification of the intervertebral space and midline can prove problematic. In scoliosis, spinal anatomy is distorted and associated with lateral curvatures of the spine along the longitudinal plane but also with varying degrees of rotation around the longitudinal axis. Therefore, determining the optimal puncture site in either of these clinical scenarios may be impossible by palpation, and preprocedural spinal ultrasound certainly can facilitate success in these settings. Chin et al.7 demonstrated a twice-as-high first-attempt success rate and a twofold difference in the number of needle insertion attempts and number of needle passes for spinal anesthesia with ultrasound compared with the conventional landmark technique in patients with difficult predicted spines (body mass index > 35 kg/m2, moderate to severe lumbar scoliosis, or previous lumbar spine surgery).
Technique
Spinal ultrasound assessment can prove to be challenging for two reasons. First, the spinal structures are protected by a complex encasement of bones that provides only a narrow acoustic window for the ultrasound beam. Second, the desired structures to be imaged are deeper than those imaged for peripheral nerve blocks or placement of central vascular catheters. Therefore, a low-frequency (5–2 MHz) curved ultrasound probe is used, which allows deeper penetration, albeit at the expense of a lower resolution.
Spinal sonoanatomy can be assessed via two useful acoustic windows: the longitudinal paramedian oblique approach and the transverse midline approach, both providing invaluable information.8
Longitudinal Paramedian Oblique Approach
The probe is held vertically over the sacral area, parallel to the long axis of the spine, and initially placed 2–3 cm to the left or to the right of the midline and angled slightly to target the center of the spinal canal (Figure 6.1a). The sacrum is visualized as a continuous hyperechoic (bright) line. Slowly, the probe is moved cephalad until a hyperechoic sawlike image is visualized (Figure 6.1b). The teeth of the saw represent the lamina of each vertebrae, and the spaces between the teeth indicate the intervertebral spaces. One can then mark on the skin the exact level of each interspace from L5–S1 to L1–L2.
Figure 6.1 Longitudinal paramedian oblique approach. (a) Orientation of the ultrasound probe over the sacrum and lower lumbar spine. (b) Corresponding sonogram to a. Hyperechoic image of the sacrum and of the saw sign, the teeth of which represent the laminae of the lumbar vertebrae and the gaps of which indicate the interspaces. (c) Orientation of the ultrasound probe more cephalad on the lumbar spine. (d) Corresponding sonogram to c. Laminae of two adjacent vertebrae and elements of an interspace; from superficial to deep, there is visualization of the posterior unit, the intrathecal space, and the anterior unit. (Posterior unit: ligamentum flavum and posterior dura mater; anterior unit: anterior dura mater, posterior longitudinal ligament, and vertebral body.)
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