Continuous EEG During Therapeutic Hypothermia After Cardiac Arrest — Is it Useful?

Abstract & Commentary

By Eric C. Walter, MD, MSc, Pulmonary and Critical Care Medicine, Northwest Permanente and Kaiser Sunnyside Medical Center, Portland. Dr. Walter reports no financial relationships relevant to this field of study.

Synopsis: Moderate or severe EEG abnormalities are frequently seen in patients during therapeutic hypothermia following cardiac arrest and these findings are associated with poor outcomes.

Source: Crepeau AZ, et al. Continuous EEG in therapeutic hypothermia after cardiac arrest. Neurology 2013;80:339-344.

Electroencephalograms (EEGs) have been recommended to monitor for seizure activity in patients undergoing therapeutic hypothermia following cardiac arrest.1 Data supporting this recommendation are limited. In 2009, the Mayo Clinic instituted continuous EEG (cEEG) monitoring among all patients undergoing therapeutic hypothermia following out-of-hospital cardiac arrest (ventricular fibrilation, witnessed asystolic arrest, or pulseless electrical activity). In this article, Crepeau and colleagues retrospectively reviewed EEG data from 54 consecutive patients to determine the prognostic significance of EEG abnormalities and develop an EEG grading system. They then sought to validate this grading system by correlating it with neurologic outcomes. EEG findings were graded as mild, moderate, or severe. Neurologic outcomes were graded according to the Cerebral Performance Category. Outcomes were dichotomized as good (normal or slight disability or moderate disability but awake and alert) or poor (conscious patients with a severe disability, or persistent vegetative state, or dead/brain dead).

During hypothermia, 67% of patients had moderate (56%) or severe (11%) EEG findings. Similar results were seen during rewarming and normothermia. Seizures occurred in four patients during hypothermia and one patient during rewarming (total of 9%). All seizures were generalized and occurred in patients with a severe EEG background prior to the seizure. Good neurologic outcomes were seen in a surprisingly high 33 (61%) patients. Of the 21 patients with a poor outcome, 19 died. Having a severe EEG finding was strongly associated with a poor outcome. A severe EEG was seen in 16 of 21 (76%) patients with a poor outcome. Despite treatment, no patients with seizures had a good outcome. Six patients had only mild EEG findings and all had a good neurologic outcome.


Prognostication after out-of-hospital cardiac arrest has challenged and frustrated critical care physicians for decades. Recently, therapeutic hypothermia has been shown to improve neurologic outcomes, and our previous strategies for estimating neurologic prognosis may no longer be adequate in this era. To this end, Crepeau and colleagues provide this retrospective report. While the study was retrospective, they enrolled nearly all patients who underwent therapeutic hypothermia with cEEG monitoring. They developed a relatively simple EEG grading system and this system had reasonably good correlation with prognosis. That should not be overly surprising. One might expect that those patients with more severe brain injury would be more likely to have a severe EEG and would be more likely to have a poor prognosis. Indeed, 76% of patients with a severe EEG had a poor outcome. While this is helpful it is by no means definitive, as the remaining 24% of patients with a severe EEG had a good outcome. The number of patients with a mild EEG was small (six), but notably all had a good outcome.

The major limitation of this study was that the treatment team was aware of EEG findings. It is probable that this information influenced both clinicians and families — a severe EEG leads to estimation of a poor prognosis, which leads to potential limitation of care, thus creating a self-fulfilling prophecy.

Is cEEG useful during therapeutic hypothermia following cardiac arrest? The jury is still out. The goals of cEEG are to detect potentially treatable conditions such as seizures and to improve prognostication. This study adds to previous studies showing that seizures and malignant EEG patterns are associated with poor neurologic outcomes.2,3 However, seizures may just be a marker of a severely injured brain. Treatment of seizures in this study and others2,3 has not been shown to improve neurologic outcomes. It is also too early to advocate for cEEG as a prognostic tool. The “self-fulfilling prophecy” is a serious limitation in observation studies such as this one. In addition, cEEG is extremely labor intensive and expensive. We do not know how it compares to other tools used currently to estimate prognosis. Perhaps the old fashioned bedside exam may still be useful.


1. Peberdy MA, et al. Part 9: post-cardiac arrest care: 2010 American Heart Association Guidelines for Cardiopulmonary Resuscitation and Emergency Cardiovascular Care. Circulation 2010;122:S768-786.

2. Rittenberger JC, et al. Frequency and timing of nonconvulsive status epilepticus in comatose post-cardiac arrest subjects treated with hypothermia. Neurocrit Care 2012;16:114-122.

3. Mani R, et al. The frequency and timing of epileptiform activity on continuous electroencephalogram in comatose post-cardiac arrest syndrome patients treated with therapeutic hypothermia. Resuscitation 2012;83:840-847.