Confounders of Early Processed EEG to Stratify Neurological Injury After Cardiac Arrest

Introduction: Electroencephalographic suppression ratio (SR) measured six hours after resuscitation from cardiac arrest (SR6) can stratify risk of neurological-etiology death (NED) after cardiac arrest. Confounders of this tool for risk stratification have not been described. Hypothesis: Misclassification of risk of NED by SR6 is explained by characteristics of the patient, arrest, or treatment. Methods: Comatose patients prospectively enrolled in an IRB-approved cardiac arrest database were treated by protocol. Patients with brain death or death after withdrawal of life support measures (WLST) due to poor neurological prognosis were considered NED; neuroprognostication follows European Resuscitation Council guidelines. We analyzed patients misclassified by the SR6; high SR that did not suffer NED, or low SR6 that had NED. We reviewed factors associated with misclassification in bivariate analysis, and performed chart review for potential confounders of SR6. Results: Among 364 patients resuscitated from cardiac arrest and with complete data, SR6 predicted NED with AUC 0.89, and an optimal cutoff value of 55.5%. Forty (11%) patients with high SR6 and 17 (5%) with low SR6 were misclassified. In bivariate analysis, factors associated with high SR misclassification were older age 62.6(±14.9) vs. 56.4 (±16) p=0.02, prior neuropathology [20% vs 9.1%, p=0.049], and longer no-flow time (7 [4-9] vs 5 [2-7] minutes, p=0.01). Those associated with low SR misclassification were heart failure [29.4% vs 11.4%, p=0.045] hypertension [82.4% vs 44.6%, P=0.003], obesity [29.4% vs 10.7%, p=0.04], seizures [47.1% vs 21.5%, p=0.03], and higher BMI 33.9 [26.5-36.1] vs 27.8 [23.7-31.3], p=0.04. In chart review, misclassification was associated with early WLST due to family preference, drug or alcohol overdose, seizures, death due to non-neurological cause, preexisting neurological disease, and “favorable” SR6 trajectory. Conclusion: Age, seizures, drug overdose, and non-neurological causes of death may confound accurate neurological risk stratification using processed EEG very early after cardiac arrest. In addition to these “fixed” confounders, a few patients have dynamic SR that improves after 6 hours, and deserve further study.