e face many challenges in the practice of obstetric anesthesiology. Preeclampsia, maternal hemorrhage and difficult airways all contribute to ongoing management issues. More recently the ever-rising cesarean delivery rate, advanced maternal age and morbid obesity serve to remind us that we need new tools to ensure the safety of our patients. One of the technologies that has been developed to help guide us involves the use of high-frequency sound waves. Ultrasound imaging has now been used in a wide variety of applications for pregnant patients, especially vascular access and regional blocks.

Vascular Access With Ultrasound
Analysis of a large number of clinical trials supports the use of ultrasound for central vascular access. Major adverse outcomes such as carotid artery puncture and failure to cannulate the vessel have been reduced. While these studies did not involve pregnant patients, the findings are particularly applicable for several reasons. First, parturients have a limited tolerance of Trendelenburg position, and performance time therefore is important. Second, the coagulopathy and edematous state that potentially result from preeclampsia can make surface landmark-based approaches to central line placement especially problematic. The consequences of carotid artery puncture in these patients can be catastrophic, sometimes leading to complete airway obstruction.^1,2 Third, pregnancy can result in a clinically relevant hypercoagulable state. Predisposition toward central venous thrombosis can occur, especially when related to ovarian hyperstimulation syndrome. The exquisite sensitivity of ultrasound imaging allows detection of jugular venous thrombosis.

Peripheral Nerve Blocks
Direct nerve imaging with ultrasound has made a major impact on the practice of regional anesthesia.³ Because ultrasound offers real-time visualization of peripheral nerves, block needle and local anesthetic injection, it would seem to be the ideal imaging modality for regional blocks. A prior ASA NEWSLETTER article asked the question of whether ultrasound imaging would become a standard practice for peripheral nerve blocks.⁴ A key component in this issue is the simplicity of the approach, which must be comparable to alternative anesthetic techniques.

Sonogram of the Brachial Plexus in the Neck The contributions to the brachial plexus are seen in short axis view between the scalene muscles. In this region, the nerves appear monofascicular in their echotexture. Large tickmarks are 10 mm apart.

Sonogram of the Ilioinguinal Nerves in the Abdominal Wall The nerves are seen in short axis view between the internal oblique and transversus muscles. Large tickmarks are 10 mm apart.

Very small nerves (1-2 mm diameter) can now be imaged with current ultrasound technology. For more information, visit http://nerveatlas.ucsf.edu. Imaging may even progress to the point where incidental nerves within the realm of anatomic variation can be identified, i.e., nerves without names. The use of ultrasound imaging for peripheral nerve blocks in pregnant patients is a relatively rare event (only a few in my personal experience). Ultrasound guidance, however, can produce complete blocks with minimal volumes of local anesthetic and help to avoid general anesthesia. Examples of peripheral nerve sonograms in clinical practice are shown above.

Neuraxial Sonography
Many of the first descriptions of ultrasound imaging of the epidural space resulted from its use in parturients.^5,6 This imaging was originally advocated as a means to navigate needle placement in patients with spinal deformity or instrumentation.⁷ Although a quarter of a century has gone by since the original reports, today there is still little use of ultrasound imaging for neuraxial procedures. There are several reasons why it has not yet gained popularity.

Ultrasound imaging of the neuraxis is made difficult by surrounding bony structures. Mature bone absorbs sound waves, thereby causing extensive acoustic shadowing. The absorption of sound waves by bone is markedly larger than for soft tissue (attenuation coefficients of 15 and 0.75 dB/[cm-MHz], respectively). Neuraxial imaging with ultrasound therefore involves identification of acoustic windows that allow penetration of the sound beam. Paramedian longitudinal imaging planes are generally preferred for visualization of neuraxial structures. Shadowing can prevent imaging of the epidural space, local anesthetic distribution and catheter location, all of which are important to regional blocks. The larger acoustic window provided by incompletely ossified bone is one of the reasons why ultrasound-guided neuraxial interventions are used in neonates and infants.⁸

The attenuation of soft tissue mandates the use of lower-frequency sound waves for imaging deeper neuraxial structures. These low frequencies have less axial resolution, and the image quality is therefore less appealing than for more superficial regional blocks. By comparison, with follow-up postpartum examinations, ultrasound visualization of the neuraxis is known to be more difficult in the parturient.⁹

One reason why online use of ultrasound for epidural block (imaging during the intervention) has not gained popularity is that the angle of needle approach is close to parallel to the sound beam. This insonation will only result in weak backscatter echoes from the needle and potentially ambiguous localization of the needle tip. Most practitioners who do utilize ultrasound for epidural placement use an offline technique with skin markings (imaging prior to the intervention).¹⁰ The principal information obtained is the depth of the epidural space. Some of the accuracy in the estimated depths is lost because of probe compression, local anesthetic infiltration and discrepancies in needle angle. This technique, however, also can be used to ascertain the optimal interspace, the angle and position (midline/paramedian) of approach. Ultrasound depiction of the ligamentum flavum and dura can be especially valuable.

Current problems with obstetric anesthesia include clinical issues amenable to anatomic imaging guidance. Examples include the need for fast subarachnoid block for urgent cesarean section and difficult epidural placement.

The use of ultrasound is now firmly embedded in the practice of obstetrics. In this patient population, it is particularly important to reduce exposure to ionizing radiation. As ultrasound imaging continues to improve and becomes more affordable, and as practitioners develop expertise with this technique, it will have an expanded role in guiding obstetric anesthesia procedures.

References:
1. Ball DR. Ultrasound-guided central vein cannulation. Int J Obstet Anesth. 1997; 6:69.
2. Lo WK, Chong JL. Neck haematoma and airway obstruction in a pre-eclamptic patient: A complication of internal jugular vein cannulation. Anaesth Intensive Care. 1997; 25:423-425.
3. Gray AT. Ultrasound-guided regional anesthesia: Current state of the art. Anesthesiology. 2006; 104:368-373.
4. Chan VWS. Ultrasound imaging for nerve block: A standard practice for the future? ASA Newsl. May 2005; 69(5):8-9.
5. Cork RC, Kryc JJ, Vaughan RW. Ultrasonic localization of the lumbar epidural space. Anesthesiology. 1980; 52:513-516.
6. Currie JM. Measurement of the depth to the extradural space using ultrasound. Br J Anaesth. 1984; 56:345-347.
7. Yeo ST, French R. Combined spinal-epidural in the obstetric patient with Harrington rods assisted by ultrasonography. Br J Anaesth. 1999; 83:670-672.
8. Coley BD, Murakami JW, Koch BL, et al. Diagnostic and interventional ultrasound of the pediatric spine. Pediatr Radiol. 2001; 31:775-785.
9. Grau T, Leipold RW, Horter J, et al. The lumbar epidural space in pregnancy: Visualization by ultrasonography. Br J Anaesth. 2001; 86:798-804.
10. Wallace DH, Currie JM, Gilstrap LC, Santos R. Indirect sonographic guidance for epidural anesthesia in obese pregnant patients. Reg Anesth. 1992; 17:233-236.

Andrew T. Gray, M.D., Ph.D., is Associate Professor, Department of Anesthesia and Perioperative Care, University of California, San Francisco General Hospital, San Francisco, California.