Chapter 16 discussed the anatomy and physiology of the neuromuscular junction. In addition, Chapter 16 covered the clinical utility of neuromuscular blockade to induce patient paralysis, the medications used for this purpose, and the importance of monitoring the degree of neuromuscular blockade. This chapter focuses on how to use peripheral nerve stimulators to monitor neuromuscular blockade.

There are multiple peripheral nerve stimulators on the market today; however, they share many common features. Peripheral nerve stimulators work by delivering a small electric current through the skin to a peripheral nerve. This electric current stimulates a muscle innervated by that nerve to contract. Various controls on the stimulator allow delivery of different types of electrical stimuli to the muscles in order to assess the degree of blockade between the neuron and the muscle. The different types of stimuli include single twitch, tetanus, and automatic train of four. Each type will be discussed below. Other controls on the nerve stimulator include power on/off and a control to adjust the intensity of the electrical current. These functions are easily selected on the face of most nerve stimulators by touching the membrane buttons or operating dials. The control to adjust the current usually allows the operator to adjust the output current from between 0 milliamps (mA) and 7.0 mA. The amount of current (in mA) that is delivered to the patient is often displayed on a digital LCD screen (less sophisticated nerve stimulators only indicate the mA on a dial and do not have a display). In addition, many nerve stimulators have an LED light that flashes whenever a stimulus is sent. Others sound an audible beep each time a stimulus current is delivered (some systems do both). The loudness of the beep is often adjustable.

All stimulators have a pair of lead wires that need to be attached to the stimulator and to the patient. The polarity of the electrodes is found on the stimulator box. The black polarity slot represents the negative lead, while red represents the positive lead. The lead wires can be attached and detached from the stimulator unit at the polarity connectors (Fig. 40.1). The lead wires attach to the electrodes using alligator clips or standard snap-on connectors. Some stimulators have an attachment that allows direct application of metal ball electrodes to the patient and do not require lead wires or electrode pads. Peripheral nerve stimulators can be purchased through most medical supply companies. To illustrate nerve stimulator functionality, we describe two popular nerve stimulators here: the MicroStim III and MicroStim Plus (Fig. 40.2).

MicroStim III and MicroStim Plus have similar functionality. MicroStim Plus is smaller than MicroStim III and is easy to carry in a pocket. In addition, MicroStim Plus has metal probes that can be applied directly to the skin without the use of electrodes. The metallic probes attach to the nerve stimulator box at the polarity slots. The current is delivered through the probes. The current output is adjustable in 0.1-mA increments from 0 to 6.0 mA. The standby switch maintains power to the nerve stimulator without delivering a current. The twitch function delivers a pulse at 1 Hz (1 pulse per second) or 2 Hz (2 pulses per second). The tetanus function delivers 50 Hz (50 pulses per second) or 100 Hz (100 pulses per second). The twitch function
delivers one pulse at a time, whereas the tetanus function delivers a sustained or constant stimulus. The train of four function key delivers four pulses every 2 seconds and then repeats every 10 seconds.

Electrodes are applied to the skin of the patient over a target nerve (nerve targets will be discussed below). The electrodes have an adhesive to attach to the skin. The center of the electrode that touches the patient has a conductive jelly to improve the conduction of the stimuli to the patient. The conducting gel area is small, approximately 7-11 mm in diameter. The other side of the electrode has a metallic nipple to allow the attachment of the lead wires (ECG electrodes can be used). The skin should be cleansed properly by rubbing it with an abrasive or alcohol swab and then dried before applying the electrodes. The nerve stimulator lead wires are then attached to the metallic nipples on the electrodes using the alligator clip (or snap-on connectors) on the lead wire (Figs. 40.3