Mike: Hello, I'm Mike Clarke, podcast editor for the Cochrane Library. Cochrane has produced around 40 systematic reviews of interventions that might help people after cardiac arrest. In this podcast, review author Nikola Schütz talks with lead author Jasmin Arrich, a consultant working at the emergency department at the Medical University of Vienna in Austria, about the May 2023 update for one of these, looking at the effects of an intervention called therapeutic hypothermia or temperature control for patients who have been resuscitated after a cardiac arrest.

Nikola: Hello Jasmin, first of all, could you tell us a little about therapeutic hypothermia? What is it and what does it do?

Jasmin: Hello Nikola. Therapeutic hypothermia is a specialized treatment for people who have been resuscitated after cardiac arrest. In many cases, these people suffer severe brain damage and therapeutic hypothermia, which means cooling the body to a specific temperature and maintaining it for a period of time, has been suggested as a way to prevent or limit this brain damage.

Nikola: Why is it so important to have reviews of the effects of possible treatments for cardiac arrest?

Jasmin: The impact of sudden cardiac arrest on the people and their families can be devastating, so we need reliable evidence on what might be done to improve things and these therapies might include therapeutic hypothermia. Over time, numerous studies have yielded varying outcomes, some positive and others neutral, and each study has had limitations. Consequently, clinical guidelines following these studies have undergone changes, with different target temperatures being proposed, and discussions persist among experts in the field with different recommendations in different countries.
So, our aim was to provide an updated independent review of the scientific evidence concerning therapeutic hypothermia at 33°C for patients who have been successfully resuscitated after cardiac arrest. We wanted to help clinicians make well-informed decisions and to provide comprehensive information for patients, their families, and the general public.

Nikola: Did you find the evidence you needed, and what does it say about therapeutic hypothermia for treating neurologic damage after cardiac arrest?

Jasmin: We have focused was on a patient-oriented outcome known as "favourable neurologic outcome", which means that patients are able to lead independent lives, as opposed to experiencing more severe impairments. We have analysed 12 clinical trials, comprising nearly 4000 patients. These trials had compared therapeutic hypothermia with various standard treatments, including fever control, temperature control at 36°C, or no temperature control at all.

Nikola: And what do these trials tell us?

Jasmin: When we compared hypothermia at 33°C versus any type of standard treatment, we found that participants were 40% more likely to reach a favourable neurological outcome, which is similar to the finding in the previous version of our review in 2016. This result seemed robust against the limitations in the quality of the included studies. When we looked at the effects in subgroups of patients, there was a small number in these analyses but these was still some interesting results. For instance, the effect seemed to be larger in patients who have a worse prognosis, such as those who have an unwitnessed cardiac arrest or who take longer to be resuscitated. The effects was smaller if the start of therapy was delayed for several hours.

Nikola: What about safety? What evidence did you find about complications and adverse events?

Jasmin: It's important to consider the potential harms and closely monitor patients for adverse events, in particular those related to pneumonia, low potassium levels, and arrhythmias.

Nikola: Overall, what’s your take-home message about therapeutic hypothermia?

Jasmin: Despite the limitations with heterogeneity and the limitations of the included studies, therapeutic hypothermia at 33°C using conventional cooling methods may improve neurologic outcome after cardiac arrest. However, it is crucial to consider the limitations of the evidence and to have high-quality studies that employ timely application of hypothermia to establish clearer and more robust results; especially for patient groups that seem to benefit the most.

Nikola: Thanks, Jasmin. If people would like to read the review, how can they get hold of it?

Jasmin: Thanks Nikola. The review’s available online. If people go to Cochrane Library dot com and type 'hypothermia after cardiac arrest' in the search box, they will see a link to our review.