injuries poisoning
How To Do a Median Nerve Block
Step by Step Description of Procedure
Check sensation and motor function of the median nerve. Wear gloves and use appropriate barrier precautions. Locate the flexor carpi radialis and palmaris longus tendons, which become prominent when the patient flexes the wrist against resistance. The palmaris longus tendon is usually the more prominent of the two tendons. Needle-entry site:The needle will be inserted adjacent to the radial (lateral) border of the palmaris longus tendon just proximal to the proximal wrist crease. If the palmaris longus tendon is absent, the needle-entry site is about 1 cm ulnar to the flexor carpi radialis tendon. Cleanse the site with antiseptic solution. Place a skin wheal of anesthetic, if one is being used, at the needle-entry site. Insert the needle perpendicularly through the skin and advance it slowly until a slight pop is felt as the needle penetrates the flexor retinaculum. When paresthesia in the distribution of the median nerve confirms proper needle placement, withdraw the needle 1 to 2 mm. Aspirate to exclude intravascular placement and then slowly (ie, over 30 to 60 seconds) inject about 3 mL of anesthetic. If the patient does not feel paresthesia, redirect the needle in an ulnar direction, under the palmaris longus tendon. If paresthesia is still not felt, slowly inject 3 to 5 mL of anesthetic in the proximity of the nerve 1 cm deep to the tendon. Allow about 5 to 10 minutes for the anesthetic to take effect. Set the ultrasound machine to 2-D mode or B mode. Adjust the screen settings and probe position if needed to attain an accurate left-right orientation. This almost always means orienting the side-mark on the probe to the operator's left side (corresponding to the left-sided marker dot/symbol on the ultrasound screen). Cleanse the sides and volar surface of the wrist with antiseptic solution. Cover the probe tip with a layer of gel, then cover the tip with a sterile transparent dressing tightly (to eliminate air bubbles underneath). Apply sterile lubricant to the covered tip. Place the probe tip transversely (short-axis cross-sectional view) on the proximal wrist crease. Adjust the gain on the console so that the blood vessels are hypoechoic (appear black on the ultrasound screen) and the surrounding tissues are gray. Nerves appear as an echogenic (white), honeycombed, triangular shape, often adjacent to an artery (the median nerve, however, is not accompanied by an artery). Adjust ultrasound maximum depth to about 3 cm. Identify the median nerve about 1 cm deep to the radial border of the palmaris longus tendon. Slide the probe slightly up the wrist to more clearly see the nerve. Do not move the probe from this spot. Insert the needle and slightly tilt/rotate the probe to view the needle on the ultrasound screen (an in-plane, longitudinal image). Maintaining the entire longitudinal needle image on the screen, advance the needle tip close to the nerve. Inject a small test dose of anesthetic (about 0.25 mL) to see whether it spreads around the nerve. If not, move the needle closer to the nerve and inject another test dose. When the needle tip is properly positioned, inject 1 to 2 mL of anesthetic solution to further surround the nerve. If necessary, reposition the needle tip and inject more small amounts; however, the donut sign—nerve completely surrounded by anesthetic—is not required. 