Therapeutic Hypothermia after Cardiac Arrest: Just Not Cool Anymore?

Anyone who follows #FOAMed on Twitter will have noticed that there has been a bit of a melt-down when it comes to Therapeutic Hypothermia (TH) for Out-of-hospital Cardiac Arrest (OOHCA) in the last day or so.  So what is getting everyone hot under the collar?

The thing that is raising our temperatures is the publication of research that threatens to overturn the accepted knowledge of how to manage the comatose post cardiac arrest patient:

Mild Therapeutic Hypothermia to Improve the Neurologic Outcome after Cardiac Arrest in the NEJM is the main cause of the furore, although a second article may pique the interest of pre-hospital providers as well: Effect of Prehospital Induction of Mild Hypothermia on Survival and Neurological Status Among Adults With Cardiac Arrest in JAMA (Both articles are available in full-text for free)

So what is the big deal?  Well, for quite some time now we have been fairly convinced that cooling patients in the post cardiac arrest phase is a good thing as it helps manage some of the deleterious effects of post-cardiac arrest syndrome and leads to better mortality, but more importantly, better neurological outcomes.  You can read the Cochrane Review here, and listen to my podcast for a simple run down of post-arrest care (as I understand it)

We thought that we had a number needed to treat (NNT) of 6 for neurologically intact survival using TH

However the NEJM article seems to trash this idea entirely.  It is a reasonably large (larger than the previous studies that gave us cooling) international multi-center trial.  It is  pragmatic in that it enrolled patients resuscitated from non-shockable rhythms as well as VF/VT, which reflects common practice in the real world.

The study enrolled patients to have temperature maintained at either 33 degrees Celsius (basically what we aim for with TH) or 36 degrees.  Primary outcome was death (a nice dichotomous outcome that is hard to argue with) and secondary outcome was neurological status at 180 days.  They report that they found no difference: 50% dead when cooled to 33, 48% when cooled to 36.  No difference between either group, and no difference between the sub-group of VF/VT.  (Check out The Intensive Care Network for a better summary than this)

Bugger.

So this seems to be a large, well designed, pragmatic study that completely refutes what we have known and been practicing for some time.  This makes us sad, because we were sure we had something to offer patients in the face of what had previously been an almost inevitably devastating condition (I guess this is how neurologists feel about tPA)

But, maybe we are being too hasty throwing the baby out with the iced water.
For one thing, 36 degrees is not really normothermia; it is still preventing fever which is something we still think is probably beneficial.

Simon Carly from St Emlyns has run the numbers through his scary-smart scone, and it appears that the study may in fact be under-powered to detect the difference they think they have.  The results have a wide confidence interval, which in a nutshell means we can’t trust the results too much.  The statistics make my eyeballs bleed, so I will not try to break them down here.  Please read Simon’s post over at St Emlyns for his take on this study, as it is all a little too much for me to handle.  The upshot though, is this may not be the world shattering game changer we thought initially, although it probably still asks some important questions about the state of the science.

So does this leave us all out in the cold when it comes to therapeutic hypothermia?  For most of us it may be a moot point.  We are still held to the observation of the sacraments of the clinical guidelines and may not be allowed to play things the way we want anyway.  However I don’t feel bad about continuing to cool post arrest patients.  I don’t think these new studies completely overturn our practice yet, so it is probably still reasonable to cool OOHCA survivors.  The underlying rationales for using cold fluid also makes sense to me, and that helps salve my conscience as well.  As it stands, if (when?) I have my cardiac arrest, I want to be cooled.

In the meantime we will continue enrolling patients to intra-arrest cooling under the auspices of the RINSE trial and see where that takes us (if anywhere) alongside the trials for cooling head injured patients and the upcoming trial of cooling spinal cord injuries.  So we won’t be throwing out our fridges and icepacks just yet!

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5 Responses to Therapeutic Hypothermia after Cardiac Arrest: Just Not Cool Anymore?

  1. Sensible Robbie, sensible.

    But if you ever need a use for all that ice…make mine a cold one (drink, that is)!

    • The only confirmed benefit of pre-hospital therapeutic hypothermia was the installation of refrigerators in our ambulance. After doing CPR on a 42 degree day there is nothing better than a bottle of ice-cold (4 degrees C) water that happened to find it’s way in next to the saline…

  2. Pingback: Should I Cool the Cardiac Arrest Patient? - KI Doc

  3. Pingback: All in a lather over TTM - LITFL

  4. Ben Hoffman says:

    There is no doubt we should be cooling people who have had a cardiac arrest.

    The questions is when to start i.e. out of hospital or in-hospital and if we are doing it out-of-hospital what sort of cooling do we use? Infusing 4° NaCl 0.9% has not shown to be of significant benefit so is there something else that is feasible for use in the out of hospital environment? perhaps a pericutaneous ice blanket in addition to (or instead of) cold fluid?

    If we’re going to start pre-hospital then I reckon we need to be making these people much colder than at the moment otherwise I don’t think its worth it.

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