Elevated preoperative blood pressure predicts the intraoperative loss of SSEP neuromonitoring signals during spinal surgery

Abstract

Intraoperative neuromonitoring of somatosensory evoked potentials (SSEPs) can allow identification of evolving neurologic deficit. However, SSEP deterioration is not always associated with postoperative deficit. Transient physiologic changes, including a decrease in blood pressure (BP), can result in signal deterioration, defined as a decrease in waveform amplitude of >50 % seen without neurologic deficit. This study examines the relationship between intraoperative BP decrease and SSEP neuromonitoring to determine whether hypertensive patients are more prone to decreases in BP and if such BP declines are associated with signal loss. We conducted a retrospective review of 43 lumbar laminectomy patients at Mount Sinai. Patients were categorized based on whether they had a previous hypertension diagnosis and if they presented with a first systolic BP of greater than 140 mmHg in the admission area on the morning of surgery, two groups that were not mutually exclusive. We measured BP drop by calculating fractional mean arterial pressure (fMAP, lowest MAP/baseline MAP) and change in BP. We identified patients’ SSEP tracings in which signal amplitude decreased >50 %. After dividing patients’ recording times into 5-min epochs, we calculated median MAP and whether SSEPs deteriorated in each epoch. We compared the likelihood of signal loss in hypertensives to patients presenting with elevated BP, calculating the odds ratio. Elevated BP prior to surgery is associated with lower fMAP (p = 0.007) and a larger intraoperative decrease in BP (p < 0.001). A diagnosis of hypertension is not associated with lower fMAP or BP drop. Lower epoch fMAP is associated with signal loss (p = 0.0026). While the presence of preoperative elevated BP predicts SSEP abnormality (p = 0.0039), a diagnosis of hypertension does not. Elevated BP, not a hypertension diagnosis, is associated with intraoperative loss of SSEP signals. This effect of elevated BP on SSEPs may be due to the larger associated intraoperative BP decline.