Active Electrical Examination (Somatosensory Evoked Potentials)

A muscle twitch is elicited by generating a 0.2- to 0.3-msec pulse of current to depolarize a motor nerve. This motor nerve then conducts the impulse to the muscle, where a twitch is generated. We can understand the "in-use" failings of this device by following the path of the signal. Clearly, we must start with an adequate power source (no power = no twitch). Aside from intrinsic mechanical failure, if the coupling of the electrodes to the patient is poor, that is, if the electrodes are dry or good skin contact is not made, or both, the circuit has high resistance, very little current will flow (see earlier discussion and Equation 8), and a diminished twitch is generated. In summary, the simplest way to be certain that this monitor is functioning properly is to perform both a positive (see the desired response, i.e., thumb twitch, before the chosen drug is given) control and a negative control (see the twitch disappear in response to administered drug) (see Chapter 39 ).

Evoked potential (evoked response) monitors can determine the status of multiple parts of the sensory nervous system by measuring the central nervous system response to a discrete sensory stimulus. The stimulus can be auditory, optical, or peripheral sensory. The amplitude of the evoked response measured at the skin can be very small—5 µV in the case of some cortical potentials. This very small signal is in a "sea" of spontaneous EEG (100-µV amplitude) signals. We therefore resort to a signal enhancement technique called "ensemble averaging." Rather than trying to measure the response to a single stimulus, we average the responses from hundreds (or thousands) of stimuli. Because the evoked responses occur consistently at the same time after the stimulus, this averaging process reinforces the signal from the evoked

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