Peripheral Nerve Blocks and Ultrasound Guidance for Regional Anesthesia Flashcards
(1 cards)
How the peripheral nerve stimulation technique for localizing a nerve is done?
Peripheral nerve stimulators deliver small pulses of electric current to the end of a block needle to cause depolarization and muscle contraction when the tip of the needle is near a neural structure. This technique allows for localization of
a specific peripheral nerve without requiring the elicitation of a paresthesia, thus allowing patients to be more sedated during block placement. It is necessary to attach the cathode (negative terminal) to the stimulating needle and the anode (positive terminal) to the surface of the patient because cathodal stimulation is about twice as efficient as anodal stimulation. The location of the anode on the body surface does not influence nerve stimulation characteristics. Most current-stimulating needles are coated with a thin layer of electrical insulation along the needle shaft except for the tip. This allows for higher current density at the tip of the needle. Higher current output (>1.5mA) is more likely to stimulate neural structures through tissue or fascial planes and can be associated with painful, vigorous muscle contractions. After localization of the correct motor response, the current is gradually decreased to a current of 0.5mA or less. A motor response at a current of approximately 0.5mA is appropriate when used to facilitate the location for injection of local anesthetic or catheter.
When a motor response occurs at a low current output, such as 0.2mA or lower, one cannot reliably discern if the needle tip is abutting the nerve or is subepineural based on nerve stimulation alone.
Immediately following injection of local anesthetic or saline (ionic solutions), the current density at the needle tip will rapidly dissipate and the evoked motor response
is eliminated (the Raj test). The stimulating pulse of current can be modified to produce a sensory response. The short-duration impulse commonly used (0.1ms) is effective in stimulating motor fibers, but a longer-duration pulse (0.3ms) will also stimulate sensory fibers, a useful feature if a pure sensory nerve is being sought
