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Chapter 13. Electrical Nerve Stimulators and Localization of Peripheral Nerves

All of the following answers are correct except one. Choose the answer that is not correct. Motor response to nerve stimulation of the femoral nerve is absent:

A. Under general anesthesia with sevoflurane, rocuronium, and a continuous infusion of remifentanil

B. Under spinal anesthesia

C. In a patient who already received two injections of local anesthetics

D. When the saphenous nerve is the target

B is correct. Reliability of peripheral nerve stimulation is not affected by the presence of spinal or epidural anesthesia; therefore, motor response to stimulation of the femoral nerve will be present in this case.

Which setting (electrical repetition rate) of the peripheral nerve stimulator should be used to detect muscular twitches, when the current is set at 0.5- to 1-mA intensity and 0.1 ms duration?

A. 2 Hz

B. 50 Hz

C. 100 Hz

D. 200 Hz

A is correct. 2 Hz is ideal to detect motor responses. See Figure 13–1.

Figure 13–1

An algorithm for use of nerve stimulation with ultrasound-guided nerve blocks. Please note that the nerve stimulator here is used primarily as a safety-monitoring tool, rather than a nerve localization tool. The stimulator is set at 0.5 mA (0.1 ms), and the current is rarely manipulated. Instead, a motor response is obtained; extra caution is exercised as this indicates an intimate needle-nerve relationship. Instead of adjusting the current intensity to determine at which current the motor response extinguishes, the needle is slightly withdrawn to abolish the response and distance the needle tip from the nerve. A small amount of local anesthetic is then injected to determine the needle tip location while avoiding an opening pressure greater than 15 psi. LA, local anesthetic; OIP, opening injection pressure.

Regarding action potential, which of the following statements is correct?

A. Pain fibers have a short chronaxy.

B. It takes a shorter time to depolarize the membrane of pain fibers compared to motor fibers.

C. Motor fibers have a long chronaxy.

D. Motor fibers have a short chronaxy (only the area of the nodes of Ranvier count).

D is correct. Pain fibers have a long ...

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