Quarterly Newsletter

2014-August_IVG Newsletter-Toxicology-Interventional Analgesia

The IVG Hospitals Quarterly Veterinary Newsletter features articles of interest to the veterinary medical community, written by veterinarians and veterinary specialists at our four locations.

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| 6 | Neurostimulation location requires an inexpensive nerve stimulator, specialized sheathed needles, and catheters designed to deliver a low enough current to assure proximity to nerves, nerve roots, plexi, or ganglia. To treat the intended issue, and avoid toxicity, extraneural structural damage and intraneural injections, the goal of any local or regional anesthetic technique should be to use the lowest appropriate volume of local anesthetic agent as close as possible to the nerve of question (but not within this nerve). One way to avoid intraneural injection is to locate with electrostimulation. Peripheral nerve stimulators used for nerve location should have short pulse durations such that motor responses can be elicited without causing discomfort or pain (in human pain management, these are used in awake patients without causing discomfort). The lower the intensity needed to stimulate the nerve, the closer the needle tip will need to be, and hence the more accurate and effective the blockade. Insulated needles should be used when performing nerve stimulation-guided blocks. These needles typically have a Teflon coating along the shaft of the needle with an exposed tip. Neurostimulation location occurs only when current intensity is applied by the needle electrode and the tip is sufficiently close the nerve; consequently, a muscle contraction becomes evident. ULTRASOUND GUIDED LOCATION Utilizing ultrasound, one must interrogate the proposed area using both grayscale and Doppler imaging; vessels should be noted and avoided. A general site of approach is then approximated and bony landmarks/skin may be marked if desired. Needle and syringe selection is joint-, tendon- and nerve-specific and should be prepared prior to injection. The beam should be directed obliquely at the needle and advancement is directly visualized. The tip of the needle is directed underneath the ultrasound transducer. The needle may be started cranial or caudal to the transducer. The needle is advanced through skin and subcutaneous structures into the given area, and sampling may occur prior to injection. During the injection, circumferential spread of drug around a nerve or tendon, ligament, or other pathologic structure confirms appropriate placement. A special ultrasound visible needle (echogenic) may be used to improve visualization, although scoring the shaft of the distal needle with a scalpel blade has a similar effect. Most practitioners trained in ultrasound can visualize soft tissue structures well due to obvious structure, shape, and echogenicity of various problem areas. In longitudinal view, muscles appear as relatively hypoechoic structures with fine, oblique echogenic striations representing fascia. Likewise, tendons are hyperechoic and composed of multiple organized parallel lines Fig. 1: Ultrasound guided femoral nerve block being performed via inguinal approach using ultrasound, in a patient undergoing cruciate reparative surgery. Fig. 2: Ultrasound guided femoral nerve block. Arrow shows sheathed needle penetrating vastus medialis muscle (outlined by red line). Small white circles show femoral nerve (larger white circle) and saphenous nerve (smaller white circle). Blue circle outlines deposition of injectate.

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