TTM a major breakthrough
Prior to the introduction of TTM, comatose patients with ROSC after out-of-hospital cardiac arrest had a dreadful prognosis, with survival rates of 1%-10% in registry studies. In contrast, the survival rate in the landmark TTM clinical trials was 50%-60%. And while that’s a dramatic improvement, ROSC after cardiac arrest remains a high-mortality condition. Dr. Bohula was first author of a report by the Critical Care Cardiology Trials Network, composed of 16 tertiary cardiac intensive care units in the United States and Canada. Cardiac arrest was the primary indication for 8.7% of 3,049 consecutive admissions, and its 38% mortality rate was the highest of all cardiac critical care indications (JAMA Cardiol. 2019 Jul 24;4[9]:928-35).
TTM was developed in response to a recognition that two-thirds of deaths in patients who make it to the hospital after out-of-hospital cardiac arrest are neurologic – the result of brain anoxia – rather than being due to the myocardial ischemia that may have initially brought them to medical attention.
“Time is brain cells, the same way we think of time as cardiac muscle,” Dr. Bohula observed.
The main idea behind therapeutic hypothermia is that it lowers the cerebral metabolic rate of oxygen to reduce the consequences of ongoing anoxia. The brain doesn’t require as much perfusion when cooled.
TTM has other beneficial neurologic effects as well: It reduces cerebral blood volume via autoregulation, decreases intracranial pressure, and blunts the inflammatory response involved in the postcardiac arrest syndrome. In addition, TTM has anticonvulsant properties, an important effect because seizures and/or myoclonus occur in up to 15% of adults who achieve ROSC after cardiac arrest – and in even more of those who are comatose after doing so. And seizures increase the brain’s metabolic rate threefold, resulting in more cerebral ischemic injury, she explained.
Seizure activity can be difficult to distinguish from shivering in a patient on TTM. For this reason Dr. Bohula recommends putting patients on continuous EEG monitoring from the time of admission, as is the routine practice at the Brigham.
She reported serving as a consultant to Daiichi Sankyo, Servier, Lexicon, Kowa, Merck, Novartis, Novo Nordisk, and the National Institutes of Health. In addition, she generates institutional research grants provided by a half-dozen pharmaceutical companies.