The evoked potential waveform

All four EP techniques involve the application of a stimulus that generates a neuronal response with measurement of that response. Typical recordings are expressed as a graph of time (in milliseconds) on the abscissa (i.e., x-axis) and voltage (mV) as the ordinate (i.e., y-axis) (Figure 20-1). The responses are very low voltage and require signal averaging to enhance their quality, that is, recorded waveforms are a composite of 50 to 100 or more measurements following multiple stimulation measurement cycles that serve to “subtract out” higher-voltage interference (e.g., electrocardiogram, electroencephalogram, and electrical noise within the operative suite). Peak voltages in the measured waveform refer to positive or negative deflections, designated by a P or N, respectively.

Two major characteristics of the measured waveform are usually described: amplitude and latency. Amplitude refers to the voltage difference between either a successive peak or a designated reference voltage. Latency refers to the length of time following stimulation for a specific peak to appear and is usually designated as a subscript of the positively or negatively deflected peak (e.g., N₂₀ is a negatively deflected peak occurring 20 ms after stimulation). Interpeak latency refers to the time difference (in ms) between two different peaks.

Many factors influence the recorded waveform. Monitoring variables may include displacement of monitoring leads or electrical impedance, and improper patient positioning may compress a nerve, which can then interfere with conduction, even if the surgical site is remote (e.g., ulnar nerve compression in the prone position during spine operations).

Anesthetic agents have variable effects, depending on the EP modality and the anesthetic drug. Surgical factors (e.g., injury to a neural pathway from compression, reduced perfusion, or transection) are the reasons to intraoperatively monitor EPs. Physiologic variables include decreased O₂ delivery to the neural pathway being monitored, which can occur with hypotension, anemia, and hypoxia. Hypothermia can also reduce the rate of neural conduction and impact recordings.

Brainstem auditory evoked responses

BAERs allow monitoring of the integrity of the auditory pathway both peripherally and centrally. Stimuli are loud, repetitive clicks produced by a device placed over or in the auditory canal or canals. Measurement of the response is from electrodes placed on the scalp or external ears to record contralateral and ipsilateral signals that have and have not decussated, respectively. BAER monitoring allows assessment of the acoustic transduction system of the middle and inner ear, the cochlear nerve (i.e., cranial nerve VIII), and the entire central auditory pathway rostrally to the primary auditory cortex located in the temporal lobe of the brain (Figure 20-2). Some anesthetic agents may cause minor changes in amplitude or latency of recorded waveforms; however, these changes are usually very small, even with large changes in anesthetic dose. Therefore, significant intraoperative BAER changes are usually indicative of a surgical trespass.