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pair), or peripheral nerves. Just as monitoring specific pathways with EP may be more sensitive than simply monitoring the EEG, monitoring the motor pathway may be better than monitoring sensory pathways.
Branston originally demonstrated a threshold relationship between the amplitude of SSEPs and cerebral blood flow (CBF). The threshold of focal cortical blood flow reduction below which SSEP amplitude (N2O/P25) is significantly reduced measures approximately 15 mL/lOO g/min. The cortical SSEP amplitude is also reduced when systemic blood pressure falls below 30 to 40 mm Hg. For global cerebral monitoring, which is important in cases with multiple stenotic lesions of the supra-aortic arteries, we place the nerve stimulator at both wrists over the median nerve. The placement of the recording electrodes (steel EEC needles) is performed in positions C3 2 cm posterior to C3 and F3 or C4/F4, respectively, according to the 10-20 International System (International 10-20 EEC positions).
During carotid surgery, SSEPs are registered y the stimulation of the contralateral median nerve at the wrist with a 0.2-millisecond rectangular electrical stimulus. The stimulus frequency is set at 3 Hz.. The intraoperative SSEPs are performed at intervals of 2 to 10 minutes according to the phases of the operation. lntraoperatively, the SSEP amplitude is recorded three times or more before and 60 to 80 seconds after test clamping of the internal carotid artery. The first registration after test clamping is compared to the baseline recording prior to clamping to determine the need for an intraluminal shunt. The SSEP amplitude (N20/P25) is graded as abnormal when the N20/P25 amplitude (SSEP amplitude) decreases by more than 50%. Independently of the use of shunt, the SSEP amplitude is recorded at 5-minute intervals until the end of surgery.
INTRAOPERATIVE SSEPs
The feasibility, reliability, and the degree of amplitude variations were determined in a prospective study of 1295 patients in which 1411 carotid endarterectomies were performed between 1990 and 1993 (Table 1). The analysis of SSEPs was not reliable in 7.8% of the cases. In 65.7% of the cases, no amplitude variation was observed. A reduction of less than 50% occurred in 10.4% of the cases. An abnormal N20/P25 amplitude reduction (more than 50%) was registered in 11% of the cases, a total loss of the amplitude occurred in 2.3%, and a further 2.8% later showed a pathologic SSEP amplitude alteration. The reasons for the poor reliability of some SSEP curves may have been due to the use of narcotics such as nitrous oxide, which can lead to a reduction and, later, to an inability to analyze the amplitudes in a dose-dependent manner. The dose-dependent effect of etomidate on the SSEP amplitude is clearly demonstrated. Neuromuscular junction blockade during general anesthesia is helpful to eliminate background muscle contraction artifacts on the SSEPs. Major medical equipment with strong electrical fields within the vicinity of the operating room can (Continued on page 111)
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