Ultrasonography is increasingly being viewed as an everyday tool in obstetric anesthesia. For the administration of spinal or epidural anesthesia, it reduces needle redirection attempts in patients with difficult anatomy. In the less frequent scenario of a collapsed patient, it helps with assessment in the form of transthoracic echocardiography. Abdominal blocks require ultrasound guidance for safe and anatomically correct placement. Accurate assessment of gastric volume status with ultrasound would be a useful everyday skill if it is adopted into mainstream practice. We provide a summary of current opinions on the role of ultrasound in practice and highlight the potential for its future use in obstetric anesthesia.
Introduction
The indications for the use of ultrasound in the care of the obstetric patient have rapidly expanded in recent decades. Machines that are in use today are compact, portable, and safe. Ultrasonography facilitates the accurate evaluation of anatomical structures to provide procedural guidance for neuraxial blockade, abdominal blocks, vascular access, and difficult airway management. It is a cost-effective and noninvasive assessment and a diagnostic bedside tool used for transthoracic echocardiography (TTE), lung evaluation, and gastric volume assessment. The applications are continuing to expand, with the concept of whole-body point-of-care ultrasound beginning to be introduced into the anesthesiologist’s skill set.
Neuraxial anesthesia/analgesia
Improvements in ultrasound technology and image resolution since the 1980’s have transformed the role of ultrasonography for the performance of neuraxial anesthesia and analgesia. The utility of lumbar spine ultrasound to guide neuraxial procedures was first demonstrated by Cork et al., . who described techniques for visualizing neural structures and verified good correlation between ultrasound-predicted needle depth and the epidural space. In the 2000’s, Grau et al. significantly expanded research in the field and introduced visualization in the paramedian sagittal plane.
Performance of neuraxial anesthesia and analgesia may be uniquely challenging in the obstetric patient. The anatomic and physiologic changes of pregnancy contribute to greater difficulty in discerning anatomic landmarks because of edema, obesity, and an exaggerated lordosis. Softer and inhomogeneous interspinous ligaments, narrowing of the epidural space, and a smaller intrathecal space increase the likelihood of accidental dural puncture . The gravid uterus may limit the ability to achieve adequate flexion of the lumbar spine, and labor pain challenges the ability of the parturient to maintain positioning for the procedures. Evidence from a meta-analysis of the literature involving obstetric and nonobstetric patients supports improved precision and efficacy with the utilization of ultrasound for neuraxial techniques .
Techniques
It is beyond the scope of this article to discuss the details of the approaches to lumbar spine ultrasonography. Comprehensive descriptions may be found in reviews by Chin et al. and Carvalho and from online resources such as the New York School of Regional Anesthesia .
The timing of ultrasound assessment of the lumbar spine is typically immediately prior to the neuraxial procedure to identify landmarks. Real-time ultrasound guidance is still considered to be experimental . A curved-array low-frequency (2–5 MHz) probe is recommended for scanning in both the paramedian longitudinal and transverse planes ( Fig. 1 ). Lumbar vertebral levels and interspaces may be identified in addition to sacrum, articular process, ligamentum flavum, posterior dura mater, anterior dura mater, posterior longitudinal ligament, and vertebral body.
Accuracy and ease of placement
Preprocedural ultrasound scanning of the lumbar spine improves both the accuracy and ease of neuraxial techniques. For correctly identifying the vertebral level of a lumbar interspace, ultrasound favorably compares with gold standard radiologic modalities and is clearly superior to palpation. Furness et al. reported that ultrasound identified the correct intervertebral level in 71% of cases compared to lateral lumbar spine X-ray, whereas palpation only identified the correct interspace 30% of the time (p < 0.001). Watson et al. reported that compared with spine magnetic resonance imaging scan, ultrasonography correctly identified the L3-4 interspace 13/17 times; in the remaining four patients, the L2-3 interspace was misidentified as L3-4. Schlotterbeck et al. , and Whitty et al. reported that the agreement of palpation estimates with ultrasound occurred in only 36.4% and 55% of cases, with the palpation estimate usually being more cephalad. Halpern et al. reported an ultrasound accuracy rate of 68% for identifying vertebral levels of spinous processes compared with computed tomography scan.
Numerous studies have demonstrated that ultrasound-predicted depth to the epidural space is highly correlated with needle insertion depth , including studies of obese parturients . A recent meta-analysis determined the pooled Pearson product moment correlation coefficient between ultrasound-predicted and actual needle depth was 0.91 (95% CI, 0.87–0.94) using a random-effects model to account for heterogeneity . Ultrasound-measured depth was found to be accurate within 1–13 mm of actual needle insertion depth in 7 of 8 studies, with a mean difference of ≤3 mm. Ultrasound-predicted depth tends to underestimate actual needle insertion depth, which may be due to tissue compression by the probe or variations in needle angulation.
Ease of performance of neuraxial procedures is improved by preprocedural ultrasound scan, which facilitates the determination of the optimal puncture site by accurately locating the vertebral level and the midline and predicting the appropriate angulation and depth to the epidural space. Fewer attempts for epidural placement are required compared to the (blind) loss of resistance technique alone. The mean number of puncture attempts was reported to be 1.3 and 1.5 vs. 2.2 and 2.6 (p < 0.013 and p < 0.001), respectively, in ultrasound-guided and control groups from two randomized controlled trials (RCTs) .
For novices, preprocedural ultrasound among first-year anesthesia resident trainees resulted in fewer attempts compared to a no ultrasound group during the performance of labor epidural analgesia. The median number of attempts (range) were 1 (1–6) vs. 2 (1–6), respectively (p < 0.01) . A small study involving 30 parturients showed that only one attempt was required to complete combined spinal epidural anesthesia for 100% of a real-time ultrasound group, 70% of preprocedural ultrasound group, and 40% of a no ultrasound control group .
Despite evidence for the advantages of ultrasound assistance for neuraxial procedures, routine use has not been adopted because of the success with the blind technique. In patients with easily palpable lumbar spinous processes. Arzola et al. confirmed that among experienced fellows and residents, there was no difference in the epidural insertion time and the number of interspace levels attempted or needle passes between a palpation-only group and an ultrasound group for whom operators performed both the scan and the labor epidural catheter insertion. Authors from the same institution had earlier reported that for 61 patients, among whom 19% were determined to have poor anatomic landmarks by palpation, preprocedure ultrasound using the transverse approach resulted in no reinsertions of the epidural needle in 91.8% of patients, and there was no need to redirect the needle in 73.7% .
An expert panel from the American Society of Regional Anesthesia and Pain Medicine (ASRA) recently concluded that there is Level Ia evidence (from meta-analysis of RCTs) supporting grade A recommendations that neuraxial ultrasound increases the efficiency of lumbar neuraxial anesthesia (including technically difficult patients) and is accurate in the prediction of “depth-to-target” . They found Level IIa evidence supporting a grade B recommendation that neuraxial ultrasound is more accurate than palpation in identifying interspace level but is not as good as radiologic imaging.
Difficult landmarks
In patients with difficult anatomic landmarks, including impalpable spinous processes or iliac crests, obesity, scoliosis, or postspine surgery, ultrasound guidance is recognized to improve the accuracy and ease of performing neuraxial procedures. It may be used as a rescue technique when unanticipated technical difficulty is encountered. Creany et al. recently enrolled 20 parturients with impalpable spinous processes who were randomized to receive preprocedural lumbar ultrasonography or a palpation-only technique for spinal anesthesia for cesarean delivery. The authors reported significantly fewer needle passes in the ultrasound group [median 3 (IQR 1.8–3.2)] than in the palpation group [median 5.5 (IQR 3.2–7.2)], p = 0.03.
In obese parturients, there is greater difficulty in appreciating bony landmarks, including spinous processes and iliac crests, and in locating midline and lumbar interspaces. Obesity may also exacerbate the difficulty of achieving lumbar spine flexion by the parturient. During neuraxial procedures, multiple needle passes are often required, and the soft texture of interspinous ligaments from fatty infiltration increases the incidence of false-positive loss of resistance. Obesity is consequently associated with higher rates of inadvertent dural puncture, epidural venous puncture, and failure of epidural anesthesia .
A drawback of ultrasound imaging in obese patients is that visualization of structures may be more challenging as thick adipose tissue attenuates the ultrasound beam, leading to decreased resolution of images. Compression at the interface of the probe and skin improves the acoustic characteristics of adipose tissue but may lead to the underestimation of the calculated depth to the epidural space; pressure on the skin should be relieved during depth measurements to improve accuracy . Ultrasound estimates of the depth to the epidural space in obese pregnant women have been shown to strongly correlate with needle depth by either the sagittal or transverse approach and preprocedural scanning aids in selecting the appropriate needle gauge and length .
Preprocedural ultrasound was demonstrated to reduce both the number of needle passes and number of puncture levels for neuraxial procedures in obese parturients receiving spinal anesthesia for cesarean delivery . A first attempt success rate was 92% in the ultrasound-guided group (n = 25) vs. 44% when no ultrasound was used (n = 25) (p < 0.001). Balki et al. reported that for labor epidural analgesia, no redirection was required in 67.4% and no new insertion point in 76.1% of patients who underwent ultrasound-guided placement.
Scoliosis is commonly encountered among obstetric patients. Idiopathic scoliosis has a reported prevalence of up to 5.2% in the general population with approximately twice as many females affected than males . The deformity involves lateral curvature of the spine in the vertical axis and axial rotation of the vertebrae. Ultrasound imaging is useful in locating spinous processes, identifying the direction of asymmetry of the articular processes, selecting the optimal point for needle entry, and determining the depth and angle for needle placement .
Following spine surgery, neuraxial procedures are often technically difficult to perform because of scarring and distortion of the anatomic landmarks. Even uninvolved vertebrae below the level of surgery may undergo degenerative changes, causing retrolisthesis and spondylolisthesis, thus increasing the degree of difficulty. Ultrasonography may facilitate visualization of the vertebral levels affected by spinal instrumentation, identify uninvolved interspaces, and select acoustic windows that represent potential needle trajectories . Metallic rods create significant artifact on imaging and typically appear as a hyperechoic shadow.
Outcomes/safety
Ultrasound-guided neuraxial techniques are associated with improvement in several outcome measures. Lower maximal pain scores and higher subjective ratings for the quality of labor analgesia were reported among parturients randomized to preprocedural ultrasound scans and conventional epidural catheter placement (n = 300) . The investigators also found a lower incidence of severe headache [2.7% (4/150) vs. 10% (15/150), p < 0.011] in the ultrasound group. In another study, ultrasound guidance was also associated with fewer epidural catheter replacements (3/189) compared with a no ultrasound control group (10/181) (p < 0.02) .
Ultrasound-guided neuraxial techniques are associated with lower technical failure and complication rates. The combined risk ratio for technical failure was calculated to be 0.51 (95% CI, 0.32–0.80) with ultrasound guidance vs. palpation in one review . A prior meta-analysis by Shaikh et al. also found a reduced risk of failed procedures [risk ratio 0.21 (95% CI, 0.10 to 0.43), p < 0.001]; the absolute risk reduction of failed procedures was 0.063, and 16 ultrasound-guided procedures were needed to reduce one failure. They reported the risk of traumatic procedures to be lowered with ultrasound imaging, with a risk ratio of 0.27 (95% CI 0.11–0.67, p = 0.005).
Because serious complications such as epidural hematoma are so rare, it is not feasible to design prospective RCT’s to prove an increase in safety; however, surrogate measures such as greater accuracy of placement, fewer traumatic placements and fewer attempts may be reasonably expected to contribute to fewer complications. The expert panel from ASRA’s most recent assessment of ultrasound-guided regional anesthesia concluded that there was Level III evidence that neuraxial ultrasound reduces surrogate measures of potential neurologic injury; because of small numbers, the evidence is insufficient to assess safety outcomes .
Neuraxial anesthesia/analgesia
Improvements in ultrasound technology and image resolution since the 1980’s have transformed the role of ultrasonography for the performance of neuraxial anesthesia and analgesia. The utility of lumbar spine ultrasound to guide neuraxial procedures was first demonstrated by Cork et al., . who described techniques for visualizing neural structures and verified good correlation between ultrasound-predicted needle depth and the epidural space. In the 2000’s, Grau et al. significantly expanded research in the field and introduced visualization in the paramedian sagittal plane.
Performance of neuraxial anesthesia and analgesia may be uniquely challenging in the obstetric patient. The anatomic and physiologic changes of pregnancy contribute to greater difficulty in discerning anatomic landmarks because of edema, obesity, and an exaggerated lordosis. Softer and inhomogeneous interspinous ligaments, narrowing of the epidural space, and a smaller intrathecal space increase the likelihood of accidental dural puncture . The gravid uterus may limit the ability to achieve adequate flexion of the lumbar spine, and labor pain challenges the ability of the parturient to maintain positioning for the procedures. Evidence from a meta-analysis of the literature involving obstetric and nonobstetric patients supports improved precision and efficacy with the utilization of ultrasound for neuraxial techniques .
Techniques
It is beyond the scope of this article to discuss the details of the approaches to lumbar spine ultrasonography. Comprehensive descriptions may be found in reviews by Chin et al. and Carvalho and from online resources such as the New York School of Regional Anesthesia .
The timing of ultrasound assessment of the lumbar spine is typically immediately prior to the neuraxial procedure to identify landmarks. Real-time ultrasound guidance is still considered to be experimental . A curved-array low-frequency (2–5 MHz) probe is recommended for scanning in both the paramedian longitudinal and transverse planes ( Fig. 1 ). Lumbar vertebral levels and interspaces may be identified in addition to sacrum, articular process, ligamentum flavum, posterior dura mater, anterior dura mater, posterior longitudinal ligament, and vertebral body.
Accuracy and ease of placement
Preprocedural ultrasound scanning of the lumbar spine improves both the accuracy and ease of neuraxial techniques. For correctly identifying the vertebral level of a lumbar interspace, ultrasound favorably compares with gold standard radiologic modalities and is clearly superior to palpation. Furness et al. reported that ultrasound identified the correct intervertebral level in 71% of cases compared to lateral lumbar spine X-ray, whereas palpation only identified the correct interspace 30% of the time (p < 0.001). Watson et al. reported that compared with spine magnetic resonance imaging scan, ultrasonography correctly identified the L3-4 interspace 13/17 times; in the remaining four patients, the L2-3 interspace was misidentified as L3-4. Schlotterbeck et al. , and Whitty et al. reported that the agreement of palpation estimates with ultrasound occurred in only 36.4% and 55% of cases, with the palpation estimate usually being more cephalad. Halpern et al. reported an ultrasound accuracy rate of 68% for identifying vertebral levels of spinous processes compared with computed tomography scan.
Numerous studies have demonstrated that ultrasound-predicted depth to the epidural space is highly correlated with needle insertion depth , including studies of obese parturients . A recent meta-analysis determined the pooled Pearson product moment correlation coefficient between ultrasound-predicted and actual needle depth was 0.91 (95% CI, 0.87–0.94) using a random-effects model to account for heterogeneity . Ultrasound-measured depth was found to be accurate within 1–13 mm of actual needle insertion depth in 7 of 8 studies, with a mean difference of ≤3 mm. Ultrasound-predicted depth tends to underestimate actual needle insertion depth, which may be due to tissue compression by the probe or variations in needle angulation.
Ease of performance of neuraxial procedures is improved by preprocedural ultrasound scan, which facilitates the determination of the optimal puncture site by accurately locating the vertebral level and the midline and predicting the appropriate angulation and depth to the epidural space. Fewer attempts for epidural placement are required compared to the (blind) loss of resistance technique alone. The mean number of puncture attempts was reported to be 1.3 and 1.5 vs. 2.2 and 2.6 (p < 0.013 and p < 0.001), respectively, in ultrasound-guided and control groups from two randomized controlled trials (RCTs) .
For novices, preprocedural ultrasound among first-year anesthesia resident trainees resulted in fewer attempts compared to a no ultrasound group during the performance of labor epidural analgesia. The median number of attempts (range) were 1 (1–6) vs. 2 (1–6), respectively (p < 0.01) . A small study involving 30 parturients showed that only one attempt was required to complete combined spinal epidural anesthesia for 100% of a real-time ultrasound group, 70% of preprocedural ultrasound group, and 40% of a no ultrasound control group .
Despite evidence for the advantages of ultrasound assistance for neuraxial procedures, routine use has not been adopted because of the success with the blind technique. In patients with easily palpable lumbar spinous processes. Arzola et al. confirmed that among experienced fellows and residents, there was no difference in the epidural insertion time and the number of interspace levels attempted or needle passes between a palpation-only group and an ultrasound group for whom operators performed both the scan and the labor epidural catheter insertion. Authors from the same institution had earlier reported that for 61 patients, among whom 19% were determined to have poor anatomic landmarks by palpation, preprocedure ultrasound using the transverse approach resulted in no reinsertions of the epidural needle in 91.8% of patients, and there was no need to redirect the needle in 73.7% .
An expert panel from the American Society of Regional Anesthesia and Pain Medicine (ASRA) recently concluded that there is Level Ia evidence (from meta-analysis of RCTs) supporting grade A recommendations that neuraxial ultrasound increases the efficiency of lumbar neuraxial anesthesia (including technically difficult patients) and is accurate in the prediction of “depth-to-target” . They found Level IIa evidence supporting a grade B recommendation that neuraxial ultrasound is more accurate than palpation in identifying interspace level but is not as good as radiologic imaging.
Difficult landmarks
In patients with difficult anatomic landmarks, including impalpable spinous processes or iliac crests, obesity, scoliosis, or postspine surgery, ultrasound guidance is recognized to improve the accuracy and ease of performing neuraxial procedures. It may be used as a rescue technique when unanticipated technical difficulty is encountered. Creany et al. recently enrolled 20 parturients with impalpable spinous processes who were randomized to receive preprocedural lumbar ultrasonography or a palpation-only technique for spinal anesthesia for cesarean delivery. The authors reported significantly fewer needle passes in the ultrasound group [median 3 (IQR 1.8–3.2)] than in the palpation group [median 5.5 (IQR 3.2–7.2)], p = 0.03.
In obese parturients, there is greater difficulty in appreciating bony landmarks, including spinous processes and iliac crests, and in locating midline and lumbar interspaces. Obesity may also exacerbate the difficulty of achieving lumbar spine flexion by the parturient. During neuraxial procedures, multiple needle passes are often required, and the soft texture of interspinous ligaments from fatty infiltration increases the incidence of false-positive loss of resistance. Obesity is consequently associated with higher rates of inadvertent dural puncture, epidural venous puncture, and failure of epidural anesthesia .
A drawback of ultrasound imaging in obese patients is that visualization of structures may be more challenging as thick adipose tissue attenuates the ultrasound beam, leading to decreased resolution of images. Compression at the interface of the probe and skin improves the acoustic characteristics of adipose tissue but may lead to the underestimation of the calculated depth to the epidural space; pressure on the skin should be relieved during depth measurements to improve accuracy . Ultrasound estimates of the depth to the epidural space in obese pregnant women have been shown to strongly correlate with needle depth by either the sagittal or transverse approach and preprocedural scanning aids in selecting the appropriate needle gauge and length .
Preprocedural ultrasound was demonstrated to reduce both the number of needle passes and number of puncture levels for neuraxial procedures in obese parturients receiving spinal anesthesia for cesarean delivery . A first attempt success rate was 92% in the ultrasound-guided group (n = 25) vs. 44% when no ultrasound was used (n = 25) (p < 0.001). Balki et al. reported that for labor epidural analgesia, no redirection was required in 67.4% and no new insertion point in 76.1% of patients who underwent ultrasound-guided placement.
Scoliosis is commonly encountered among obstetric patients. Idiopathic scoliosis has a reported prevalence of up to 5.2% in the general population with approximately twice as many females affected than males . The deformity involves lateral curvature of the spine in the vertical axis and axial rotation of the vertebrae. Ultrasound imaging is useful in locating spinous processes, identifying the direction of asymmetry of the articular processes, selecting the optimal point for needle entry, and determining the depth and angle for needle placement .
Following spine surgery, neuraxial procedures are often technically difficult to perform because of scarring and distortion of the anatomic landmarks. Even uninvolved vertebrae below the level of surgery may undergo degenerative changes, causing retrolisthesis and spondylolisthesis, thus increasing the degree of difficulty. Ultrasonography may facilitate visualization of the vertebral levels affected by spinal instrumentation, identify uninvolved interspaces, and select acoustic windows that represent potential needle trajectories . Metallic rods create significant artifact on imaging and typically appear as a hyperechoic shadow.
Outcomes/safety
Ultrasound-guided neuraxial techniques are associated with improvement in several outcome measures. Lower maximal pain scores and higher subjective ratings for the quality of labor analgesia were reported among parturients randomized to preprocedural ultrasound scans and conventional epidural catheter placement (n = 300) . The investigators also found a lower incidence of severe headache [2.7% (4/150) vs. 10% (15/150), p < 0.011] in the ultrasound group. In another study, ultrasound guidance was also associated with fewer epidural catheter replacements (3/189) compared with a no ultrasound control group (10/181) (p < 0.02) .
Ultrasound-guided neuraxial techniques are associated with lower technical failure and complication rates. The combined risk ratio for technical failure was calculated to be 0.51 (95% CI, 0.32–0.80) with ultrasound guidance vs. palpation in one review . A prior meta-analysis by Shaikh et al. also found a reduced risk of failed procedures [risk ratio 0.21 (95% CI, 0.10 to 0.43), p < 0.001]; the absolute risk reduction of failed procedures was 0.063, and 16 ultrasound-guided procedures were needed to reduce one failure. They reported the risk of traumatic procedures to be lowered with ultrasound imaging, with a risk ratio of 0.27 (95% CI 0.11–0.67, p = 0.005).
Because serious complications such as epidural hematoma are so rare, it is not feasible to design prospective RCT’s to prove an increase in safety; however, surrogate measures such as greater accuracy of placement, fewer traumatic placements and fewer attempts may be reasonably expected to contribute to fewer complications. The expert panel from ASRA’s most recent assessment of ultrasound-guided regional anesthesia concluded that there was Level III evidence that neuraxial ultrasound reduces surrogate measures of potential neurologic injury; because of small numbers, the evidence is insufficient to assess safety outcomes .
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