Hypothermic Cardiac Arrest Pt 2
This podcast is the second of a two part episode on hypothermic cardiac arrest. In the first podcast we discussed how hypothermic cardiac arrest happens and the physiology involved with it.
In this second podcast we’re going to discuss; the managment of the peri-arrest patient, management of a hypothermic cardiac arrest and the modifications to standard ALS therapy, how we can best warm patients pre-hospitaly, what our priorities are in managing these patients, in hospital treatments, and finally we’re going to discuss guidance around confirming deathing in hypothermia.
Hypothermia is in itself a reversible cause, however hypothermic cardiac arrest will bring with it other issues. Hypoxia from vasoconstriction and a potential VQ mismatch, hypoglycaemia, hypovolamia and potentially metabolic imbalances from cold diuresis (discussed in Ep 1).
Peri-arrest patient managment:
As discussed in the last episode, this group of patients will present with a variety of symptoms including reduced GCS and cardiac dysrhythmias. Theres no arbitrary figure at which a patient becomes "peri-arrest", however I would argue those with ECG abnormailities come under this category.
In managing them, like in all patients we will take a routine ABCDE approach, rapidly working our way down to E as we need to minimise exposure and start rewarming this patient group.
A:
We should be cautious with airway management, Igel or ETT may theoretically worsen bradycardia from vagalstimuation or precipitate VF due to a sensitive and easily agitated myocardium. Ensure a stepwise airway approach [1] [2], however advanced airways should obviously be used if required as the benefits of these out weigh the risk of arrest.
B:
O2: With regards to oxygen, there is every likelihood patients will present with low SPO2 readings. This is predominantly going to be due to shut down peripheries, however there is the chance patients may be hypoxemic. Whether it is due to significant vasoconstriction, reduced cardiac output, or a VQ mismatch due to physiological shunt either way this patients issue is not oxygenation… it is perfusion. Therefore increasing FiO2 by putting them on highflow oxygen is unlikely to help, in fact given we know oxygen is a vasoconstricter, you could argue it does the opposite. To further complicate oxygenation issues, hypothermic patients will likely have a slight leftwards shift of their oxygen dissociation curve, meaning haemoglobin has a higher affinity for it and will not release oxygen to ischemic tissues. If you’re really interested as to why this occurs (as I was) the answer can be found here [3] However I’m sure the majority of people will not want a brief lesson in thermodynamics.
My personal thoughts on the issue are give oxygen if you think it will make a difference however we should be trying to treat the perfusion issue first, and that involves rewarming.
C:
As discussed in Part One. These patients have a clear circulatory issue. The vasoconstriction creates artificial hypertension within the kidneys. This results in "cold diuresis" where the kidneys will excrete a greater ammount of water, potentially dehydrating the patient.
In addidtion to this they likely have cardiogenic issues from the decreased cardiac activity. As a result we should support these patients with fluid rehydration. We aren't able to warm fluids as effectively as in hospital, however we are able to warm fluids somewhat. I tend to keep a bag in the cab on particularly cold shifts, meaning we always have 500mls pre-warmed.
D:
There may be problems under D, confusion, reduced GCS generally these will be down to one of two things in a hypothermic patient. Poor cerebral perfusion/oxygenation ( treated by warming them up, see below) or hypoglycemia. Peri-arrest hypothermic patients will likely have been hypothermic for a long time, and as such used up a significant ammount of glucose stores trying to create and retain heat. Like in every patient: Dont Ever Forget Glucose!
E:
Exposure:
Our next issue centres around rewarming the patient. When thinking about warming or cooling our patient, it’s important to consider all aspects of heat loss.
Conduction, convection, evaporation and radiation. It’s important we manage all aspects to fully maximise our warming potential.
Evaporation: As discussed in the first episode the majority of patients will become hypothermic due to exposure to water. A significant degree of heat is lost through wet clothing and so removal and drying is a priority.
However, it is really important to be pragmatic about this. If we are in an isolated area where we aren't going to be able to strip patients down and get them in dry gear or blankets, then it might be appropriate to leave the wet clothes on and focus on minimising other areas of heat loss, particularly radiation. Remember the reason why we lose heat from wet clothes is because of water evaporating and taking heat away. If we stop that evaporation by placing enough barriers (read blankets) in the way, then the patinet will warm the water up and that in turn will help to keep them warm, much like the way a wet suit works.
Conduction: If the patient must be managed outside, give consideration to what they are lying on as heat is lost into the ground rapidly.
Convection: Ideally in the ambulance we will be able to provide convection warmth via the heater, but again if we have to manage the patient outside, ensuring we are broken from wind is a must. Often I have used the RRV as a wind breaker to good effect when waiting for a DCA.
Radiation: This pertains to our foil blankets. Remember these work by reflecting heat back into the patient and are primarily used to slow cooling after marathons etc. In a patient who is profoundly hypothermic, we have to ensure there is a heat source in with the patient to make the blanket effective.
Blanketing:
I’m a firm believer in maximising the potential of our equipment, as we don’t have the luxury of warm hospitals and barehuggers that our ED colleagues do. So whilst very simple and very boring I feel it’s impotant to cover blanketing the patient. Once wet clothes are removed (if that is appropriate) and the patient has been dried I tend to blanket them with a Fleece/Cloth blanket covering their skin, followed by an exothermic reaction blanket (pictured) and finally a foil blanket lightly on top to reflect maximal heat back toward the patient.
Worth noting with the warming blankets is that they take 30 minutes to warm up and should not be used directly on the skin. Often on very cold shifts on the RRV I will crack a blanket just after booking on. Once the reaction has started they stay warm for 10 hours and these means we have one readily available when we inevitably come across a cold patient. If you don’t it means you have something warm to cuddle if you miss your usual crew mate.
Something that is often taught in pre-hospital education is the fear of rapidly rewarming patients. There is theoretical concern about reperfusion injury, returning “cold blood” to the heart and causing arrhythmia, or causing massive “rewarming hypotension” by enervating global vasodilatation too quickly.
Non of the methods available to us pre-hospitally have any risk of doing this what so ever ( as found by Brown et al) so as long are you’re not doing peritoneal levage, crack on.
Whilst many of our pre-hospital warming options aren’t going to bring about massive changes quickly, we can prevent the patient getting colder and thus deteriorating. If we can start the warming process early and demonstrate good results, this will hopefully avoid more invasive in hospital methods.
Extrication:
Again as above we need to remember that this patient has a potentially easily agitated myocardium [1, 2 ], so if prei-arrest, minimal movement should be key.
Rather than talk about indepth ALS management we've instead listed the modifications to practice and then expanded on the thought process and evidence behind them:
-Pulse check CAN be extended for up to 60 seconds, as patients may be profoundly bradycardic. If in doubt start CPR.
-Rapid Rewarming is the highest priority outside of normal BLS. [1]
-Defibrilation can be attempted a maximum of 3 times but then should not be retried until temp is above 30degs [1]
-Ventilation Minute volume MAY need to be reduced. Capnography should be our guide *
-Cardiac Drugs should be withheld under 30 degrees and dose intervals doubled above this, until normothermia is approached / reached [1]
-Prioritise Auto-compressors if available and appropriate.
-Intra-arrest transport is appropriate.
Defibrillation:
Cold heart is not going to benefit from repeated defibrillation. If cardiac myocytes are not perfused and warm enough to return to a normal synchronised rhythm then there is little point continuing to defibrillate and risking damaging the myocardium.
What is difficult here is practically deciding when to begin shocking again, as discussed in Part 1, tympanic thermometers only measure as low as 32 degrees so we are left somewhat in the dark at these lower temperatures. Given an inability for more invasive temperature monitoring, I feel resuming normal defibrillation practice once we are doing all we can to rewarm is reasonable. Something else worth considering, as in all refractory VF/VT would be changing pad position to the AP position to see if a new vector can cardiovert. [4] (Reference for pad position change only, not DSD)
Cardiac arrest drugs:
In profoundly hypothermic patients the metabolisim of drugs is likely to be reduced. There is theoretical concern that IV meds could build up to toxic levels, as such ERC guidance is to withhold drugs below 30 degrees Celsius and above 30 degrees to double the dosage intervals.
Auto compressors:
Hypothermia is likely to result in muscular rigidity, particularly of the chest wall. This could result in difficult ventilations and chest compressions. In what is likely to be a lengthy and prolongued resuscitation we should aim to get resources with an autocompression device to scene. This will free members of the team up, prevent fatigue and facilitate effective and safe intra-arrest transport. We should of course not delay on scene waiting for this resource.
Intra-arrest transport:
As above, this will preferably be done with an autocompressor in situ, as manual CPR is neither safe nor effective during transport.
However if this is not possible, we will still need to transport these patients. Hypothermic arrest is not something we can pronounce on scene, as there is significant inhospital support required to rewarm the patient.
Recognition of Life extinct:
Whilst there are some factors in the literature that allow rescuers to confirm death in hypothermia ( Crushed in an avalanche, frozen tissues) many of these are not relevant to UK paramedic practice + are potentially difficult to accurately identify.
The literature is pretty clear in when you can call a hypothermic arrest and that is the old adage of “You’re not dead until you’re warm and dead”. Essentially, we will not be calling a cardiac arrest pre-hospitally, as our rewarming methods are too slow and ineffective.
Transportation:
Something we should be considering is an early pre-alert and discussion with the receiving hospital. A patient who is in cardiac arrest due to hypothermia will require some form of extracorporeal warming (ECMO or cardiac bypass). As a result we need to ensure our receiving hospital can both facilitate this and we give them enough time to find the equipment, blow the dust of and prepare it.
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