FEATURE
ImagIng In anaesthesIa
allows one to visualize neural structures (plexus and peripheral nerves) and the surrounding structures (e.g., blood vessels and pleura), navigate the needle toward the target nerves, and visualize the pattern of local anesthetic spread.
ULTRASOUND IN REGIONAL ANAESTHESIA Ultrasound equipment Ultrasonography has many applications in clinical anaesthesia. With appropriate probes, vascular imaging, echocardiography, and nerve imaging can be performed with the same unit. Compound imaging is an advanced feature in some of the cart-based units. The resolution of nerve images is enhanced with compound imaging when multiple lines of crystals on the transducer (as opposed to a single line) emit and receive ultrasound in multiple planes before final display of the image that is electronically reconstructed. Colour Doppler is another useful feature that differentiates vascular from nonvascular structures (e.g., nerves). Compact portable units currently available with many of the sophisticated features are also suitable for peripheral nerve imaging.
Transducers (probes) Ultrasound scanning of deep abdominal organs such as liver, gallbladder, and kidneys requires low-frequency probes (3-5 MHz). Scanning superficial structures such as the brachial plexus, on the other hand, requires high-frequency probes (10-15 MHz) that provide high axial resolution however, beam penetration is limited to 3-4 cm. A lower-frequency probe (4-7 MHz) is suited for scanning deeper structures, such as the brachial plexus in the infraclavicular region and the sciatic nerve in adults.
PERIPHERAL NERVE IMAGING Probe orientation It is advisable to follow the tradition of pointing the premarked end of the probe towards the head when scanning in a sagital or parasagital plane, and pointing towards the patient’s right when scanning in an axial plane, so that saved images will be correctly interpreted at a later time. Scanning technique Patient positioning for each block is
essentially the same as is used for standard, non-image-guided peripheral nerve blocks. Sterile technique should be followed, especially when a continuous catheter technique is performed, in which case a long sterile sheath covering the probe and the cord and sterile conducting gel are recommended. Transverse and longitudinal
views are most commonly used for nerve imaging. When the probe is perpendicular to the long axis of the nerve, the transverse (short axis, cross-sectional) view shows nerves in round to oval shape with internal hypoechoic nerve fascicles surrounded by the hyperechoic epineurium. When the probe is parallel to the long axis, nerves in longitudinal view
Ultrasound-guided techniques may improve the accuracy, success, and safety of regional anaesthesia
appear tubular with linear hypoechoic fascicular components mixed with hyperechoic bands corresponding to the interfascicular epineurium. Nerves have different degrees of echogenicity. For example, nerve roots and trunks of the brachial plexus in the interscalene and supraclavicular regions appear mostly hypoechoic, while peripheral branches of the brachial plexus and the sciatic nerve are largely hyperechoic.
GENERAL PRINCIPLES OF ULTRASOUND- GUIDED NERVE BLOCK TECHNIQUES Ultrasound guided-blocks for peripheral nerves follow several general principles The quality of ultrasonographic nerve images captured is dependent on the quality of the ultrasound machine and transducers, proper transducer selection (e.g., frequency) for each nerve location, the anesthesiologist’s familiarity and interpretation of sonographic anatomy pertinent to the block, and good eye-hand coordination to track needle movement during advancement Optimal patient positioning and sterile technique are encouraged. This is particularly important for the continuous catheter technique, when it is necessary to use sterile conducting gel and a sterile plastic sheath to fully cover the entire transducer Nerve localization by ultrasound can be combined with nerve stimulation. Both tools are valuable and complementary, not mutually exclusive. Ultrasonography provides anatomic information, while a motor response to nerve stimulation provides functional information about the nerve in question Observing local anesthetic spread is another valuable feature of ultrasound in addition to real-time visual guidance to navigate the needle toward the target nerve Two approaches are generally available to block peripheral nerves. The first approach aims to align and move the
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