Ultrasound-Assisted Nerve Blocks M. A. Moniem, MD Consultant Anesthetist
CONTENTS•Anatomy•Rationale•US In Regional Anesthesia – US Principles – US Equipments – Transducers (Probes)•Peripheral nerve imaging – Probe Orientation – Scanning Techniques•Imaging Of Brachial Plexus – The Intercsalene Region – The Supraclavicular Region – The Infraclavicular Region – The Axillary Region•Lumbar Plexus – Femoral Nerve – Sciatic Nerve – Obturator Nerve Block.
Rationale: In recent years there has been a growing interest in the practice of regional anesthesia and, in particular, PNB for surgical anesthesia and postoperative analgesia. Peripheral nerve blocks have been found to be superior to general anesthesia: (1) Effective analgesia with few side effects (2) Hasten patient recovery
• Imaging guidance for nerve localization holds the promise of improving block success and decreasing complications.• Essentially "blind" procedures, since they both rely on indirect evidence of needle-to-nerve contact, Seeking nerves by trial and error and random needle movement can cause complications.• US seems to be the one most suitable for regional anesthesia: By Provide anatomic examination of the area of interest. Visualize neural and the surrounding structures. Navigate the needle toward the target nerves. Visualize the pattern of local anesthetic spread.
US PrinciplesDepending on the amount of wave returned, anatomic structures take on different degrees of echogenicity.Structures with high water content, such as blood vessels and cysts, appear Hypoechoic (black or dark), because ultrasound waves are transmitted through the structures easily with little reflection.On the other hand, bone and tendons block ultrasound wave transmission and the strong signal returned to the transducer gives these structures a Hyperechoic appearance (bright, white) on the screen
Transducers (probes)Deep organs scanning such as liver, gallbladder, and kidneys requires low-frequency probes (3-5 MHz).Superficial structures such as the brachial plexus,, requires high-frequency probes (10-15 MHz) that provide high axial resolution BUT Beam penetration is limited to 3 to 4 cm.
Probe orientationIt 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.Pointing towards the patients right when scanning in an axial plane.
Scanning techniquePatient 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.
General Principles of USGNB TechniquesThe quality of US 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 anesthesiologists 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.Nerve localization by US can be combined with nerve stimulation. Both tools are valuable and complementary.
Two approaches are available to block peripheral nerves: The first approach aims to align and move the block needle inline with the long axis of the US transducer, so the needle stays within the path of the US beam. In this manner, needle shaft and tip can be clearly visualized. This is preferred when it is important to track the needle tip at all times (supraclavicular block to minimize inadvertent pleural puncture). The second approach places the needle perpendicular to the probe, in this case, the ultrasound image captures a transverse view of the needle, which is shown as a Hyperechoic "dot" on the screen.Accurate moment-to-moment tracking of the needle tip location can be difficult, and needle tip position is often inferred indirectly by tissue movement. D5W
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