Locating the epidural space in obstetric patients—ultrasound a useful tool: Continuing Professional Development

0 M. Balki, MD (&) Department of Anesthesia and Pain Management , Mount Sinai Hospital, University of Toronto, University Avenue , Room 1514, Toronto , ON M5G 1X5, Canada Purpose The objective of this continuing professional development module is to describe the role of ultrasound for spine demarcation before placement of neuraxial blocks and to provide a comprehensive systematic approach towards ultrasound scanning in obstetric patients. Principal findings Ultrasound imaging of the lumbar spine has been shown to be effective in improving the success rate of neuraxial blocks in obstetric patients. The success rate of ultrasound-guided epidural insertions at first attempt was found to be 30-60% greater when compared with the traditional epidural method. Pre-puncture ultrasound scanning can provide reliable and accurate information on several critical aspects needed for successful epidural placement, such as the interspace level, the midline of the spine, the optimal puncture point, the optimal angle for needle insertion, and the depth to the epidural space. Ultrasound scanning of the lumbar spine can generate images in two distinct patterns one in the transverse plane represented by a ''flying bat'' and the other in the paramedian longitudinal plane represented by a ''saw-tooth'' configuration. Both approaches allow the visualization of the ligamentum flavum and the dura mater in addition to other adjacent bony landmarks. A stepwise scanning approach can serve as a useful guide to facilitate the successful placement of epidural needles in patients with both normal and abnormal spinal anatomy. Conclusions Incorporating the use of lumbar spine ultrasound scanning into day-to-day clinical practice may improve the ease of performing epidurals as well as add to patient safety and comfort. 1. Understand the principles of ultrasound imaging and the interpretation of sonoanatomy of the lumbar spine in obstetric patients; 2. Identify normal ultrasound image patterns in various scanning planes and determine their applications in facilitating spinal and epidural techniques; 3. Recognize sonoanatomy patterns in patients with spine abnormalities and morbid obesity; and 4. Discuss the potential impact of the ultrasound-guided technique in improving the quality and safety of neuraxial blocks. Ultrasound has several applications in the practice of medicine. In anesthesia, this technology has been associated with substantial benefits in the placement of peripheral nerve blocks and central venous cannulations; likewise, its routine use in clinical practice for guiding neuraxial blocks can greatly improve their precision and effectiveness. In 2008, the National Institute for Health and Clinical Excellence (NICE) in the United Kingdom already issued guidance regarding the use of ultrasound for placement of epidural catheters.1 The traditional method to perform neuraxial blocks involves a blind approach based only on anatomical landmarks. However, these landmarks may not always be evident or even accurate. This situation becomes even more complex and the technique more tentative when the anatomical landmarks are difficult to palpate due to obesity and/or spine abnormalities. This twofold problema blind approach and variable patient anatomymay account for many of the complications that arise during neuraxial block insertions. Such complications include multiple puncture attempts, difficult placement, trauma to various vital structures, and unintentional dural puncture. Ultrasound technology available today has the potential to add to the safety and efficiency of performing neuraxial blocks by un-blinding the target and providing crucial information on the structure of the spine in different planes. However, the successful and safe use of ultrasound depends on the appropriate understanding of its basic concepts and an accurate interpretation of ultrasound images based on relevant anatomy. The purpose of this module is to provide guidance on the systematic assessment of the lumbar spine using ultrasound and to illustrate an evidence-based approach for its use. Basic concepts of spinal ultrasound The first step in the systematic lumbar scanning process is to select an appropriate transducer to obtain the best possible image. A high frequency (715 MHz) linear transducer is preferred for peripheral nerve blocks and central venous cannulations where the structures to be visualized are located superficially. This transducer produces a high-resolution image; however, it has poor beam penetration. Since the spinal structures are located deeper, a transducer with deep penetration of sound waves is appropriate for lumbar ultrasound. This can be achieved by a low frequency (25 MHz or 36 MHz) curved array transducer, although one of its limitations is image resolution.2 The curved surface of the transducer provides a wide footprint of the image, which allows the visualization of anatomical structures that are located more laterally, such as the transverse processes. It is recommended that depth settings of 914 cm be utilized in order to view all of the relevant structures with optimal resolution. The ultrasound beam is only 1 mm thick and passes through the centre of the transducer; therefore, an angle of its incidence perpendicular to the structures of interest is likely to provide the optimum quality image. The 90 angle is most appropriate because the ultrasound waves are reflected back from the structures and returned to the transducer to the maximum extent at this angle. Spinal ultrasound is particularly challenging because the structures of interest for neuraxial blocks are not only deep but they are also shielded by a complex bony vertebral cage. The ultrasound waves do not pass through bone because of its high acoustic impedance, and this hindrance impedes visualization of deeper structures. Bone appears intensely hyperechoic (bright) on the ultrasound image and produces a hypoechoic (dark) shadow underneath, termed an acoustic shadow. Hence, while scanning through bony structures, it is necessary to find an acoustic window, which is a gap between bony areas through which ultrasound waves can be transmitted to visualize the deeper structures.3 Two such acoustic windows can be obtained while performing lumbar ultrasound scanningone through a transverse or axial plane in the interspinous space and another through a paramedian longitudinal or sagittal plane in the interlaminar space.4,5 Sonographic assessment of the lumbar spine Lumbar spine ultrasound scanning can be done either as a pre-puncture procedure to demarcate the landmarks or as real-time scanning to provide dynamic images during epidural placement. Currently, the pre-puncture method is preferred, as it is simple and easy to perform and provides reliable information.6 It is important to ensure that the scanning is performed in the same patient position as for the epidural needle placement. The following information can be obt (...truncated)