Bispectral index and suppression ratio are very early predictors of neurological outcome during therapeutic hypothermia after cardiac arrest

Introduction

The bispectral index (BIS) is calculated from frontotemporal electroencephalogram (EEG), and the suppression ratio (SR) estimates the percentage of EEG suppression. We monitored the BIS and SR during therapeutic hypothermia (TH) and compared them with neurological outcomes of encephalopathic survivors of out-of-hospital cardiac arrest (OHCA).

Methods

Thirty-two patients with anoxic encephalopathy after OHCA received 18 hours of TH at 32–34°C. BIS monitoring was initiated at the onset of TH, and neuromuscular blockade (NMB) was dosed in response to shivering. Blinded BIS and SR data were recorded after the first dose of NMB, and compared with the Cerebral Performance Category (CPC) at discharge and 6 months. CPC 1 or CPC 2 was considered a good outcome (GO).

Results

Fourteen out of 32 patients (44%) survived, 11 (34%) with GO. Five of the remaining 18 patients died before neurological evaluation at 72 hours, and one patient recovered neurological function but died of cardiogenic shock. No survivor recalled the period of NMB. First NMB was administered a median of 5 hours after cardiac arrest or 87 minutes after initiation of TH, at 35.6 ± 1.7°C. Patients with GO had a higher first post-NMB BIS (39 ± 6 vs 13 ± 14, P < 0.001) and a lower SR (10 ± 12 vs 69 ± 29, P < 0.001) than those with CPC 3–5. Initial NMB reduced frontotemporal electromyogram (EMG) power from 52 ± 8 to 27 ± 1 db, P < 0.001. In 17 of the patients with downloaded EEG data, an increase in EMG power of 17 dB (IQR 10–27) from baseline was associated with clinically detectable shivering. Epileptiform discharges were noted on the monitor during NMB in two patients, and seizure activity was confirmed by formal EEG in both.

Conclusion

In cardiac arrest survivors receiving TH, a higher post-NMB BIS score and a lower SR are very early predictors of neurological outcome. The potential benefits of monitoring BIS and SR, as well as EMG power for early recognition of shivering, and continuous frontotemporal EEG to detect seizures, warrant further study.