Amplitudes of SSEP and outcome in cardiac arrest survivors: A prospective cohort study

Abstract

Objective: To investigate the relationship between somatosensory evoked potential (SSEP) amplitudes and neurologic outcome after cardiac arrest.

Methods: We prospectively studied SSEPs, recorded 24 hours to 4 days after cardiac arrest, in patients with targeted temperature management. SSEP amplitude was defined pragmatically as the highest short-latency amplitude of 4 cortical recordings (2 per side, CP3/CP4 vs Fz) at least 4.5 ms after the spinal SSEP. Cerebral performance category (CPC) was determined upon intensive care unit discharge. CPC 1-3 was defined as good, CPC 4-5 as poor outcome.

Results: Of 318 patients, 25 had incomplete recordings, no reproducible spinal SSEP, or high noise level. Of the remaining 293 patients, 137 (47%) had poor and 156 (53%) good outcome. The lowest amplitude in a survivor with good outcome was 0.62 μV. All 78 patients with lower amplitudes had poor outcome. None of 27 patients with CPC 4 (unresponsive wakefulness) had amplitudes above 2.5 μV. In the majority of 24 patients who died despite amplitudes above 2.5 μV, clinical course and other prognostic parameters argued against severe hypoxic encephalopathy.

Conclusions: The prognostic value of SSEPs extends beyond an absent/present dichotomy. Absent and very low amplitude SSEPs appear to be highly predictive of poor outcome after cardiac arrest. Prospective external validation of the lower threshold found in our study is necessary. SSEP recordings should not be used for prognostication if noise could mask potentials with critically low amplitudes. High SSEP amplitudes argue against severe hypoxic encephalopathy.