Staying Cool: An Overview of Therapeutic Hypothermia

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    Staying Cool: An Overview of Therapeutic Hypothermia

    If you’ve ever been confused about why cooling matters in post-cardiac arrest, when to do it, how to order it, or what temperature (34C? 36C?) is best, read on.
    Why do we do this?
    To protect the brain from the effects of poor profusion due to cardiac arrest. There is good evidence that hypothermia slows cerebral metabolism (decreases O2 consumption by 6% for each degree in body temperature reduction), limits cerebral cell death, and lessens cerebral edema. This protective effect of hypothermia is very time-sensitive, however. One study showed that poor neurological outcome increased by 8% with each 5 min delay in initiating hypothermia, and by 17% for every 30 min delay in time to target temperature.
    Who should get it?
    • Witnessed cardiac arrest with ROSC (regardless of initial rhythm)
    • ROSC < 30 minutes from EMS arrival
    • ROSC < 6 hours from time of hypothermia protocol initiation
    • Comatose (does not follow commands)
    • MAP > 65 mmHg (with no more than 1 vasopressor)
    • Initial temp > 30C
    Who definitely shouldn’t get it?
    • Pt is DNR, has poor baseline status or terminal disease with poor prognosis
    • Acute or severe ICH
    • Active bleeding or high risk for bleeding (known bleeding diathesis and/or major surgery in the past 14 days)*
    • Traumatic etiology of cardiac arrest
    • History of cryoglobulinemia
    • Age < 18 years

    *Hypothermia slows coagulation.

    Who probably shouldn’t get it?
    • Pregnant (consider Gyn consult first)
    • Relatively recent major surgery (> 14 days)
    • Septic etiology of cardiac arrest*

    *Hypothermia depresses the immune system and slightly increases the risk of infection.

    For how long?
    Patients should be rapidly cooled to the target temperature (32-34C, or 32-36C, depending on your institution’s protocol), then tightly maintained at that temperature for 12-24 hours, then gradually rewarmed.

    Why the debate about 34C vs 36C?
    Back in 2002, two large trials demonstrated improved survival and brain function when post-cardiac arrest patients were cooled to 32-34C. Based on this, the American Heart Association came up with therapeutic hypothermia guidelines in 2005 that targeted 32-24C, which many hospitals around the country adopted in the 2000s. However, in 2013, a large European trial found that a target temperature of 36°C had similar outcomes. This was confirmed by a meta-analysis in 2015. This prompted the AHA to revise its guidelines in 2015 to target temperatures of 32-26C, which only some institutions have adopted so far. Others are still catching up and sticking with the original 32-24C target temperatures. Still others are targeting 32-34C, but have a separate 36C protocol for some patients who may be excluded by the original AHA criteria from 2005 (for instance, see the University of Pennsylvania protocol).

    Where do these patients go after the ED?
     The ICU. Which ICU depends on the patient’s underlying disease processes (cause of arrest) and bed availability, but because of the rigorous monitoring involved, these patients will always need ICU-level care.
    What do I order?
     Every institution has a slightly different protocol, so check with your department first. The Mount Sinai therapeutic hypothermia protocol divides the process into 3 phases: Induction, Maintenance, and Rewarming.
    INDUCTION
    • First, insert a Foley catheter with a temperature probe. If the patient cannot receive a Foley, consider an esophageal or rectal temperature probe instead. A combination of surface and internal cooling is then initiated.
    • Surface cooling is achieved with  the Arctic Sun device (or other temperature management device) with a set target temperature of 32-36C on Automatic Mode.
    • Inter cooling is achieved with a cold Normal Saline infusion, which is stopped once the target temperature is reached OR once 30ml/kg total IV bolus has been given. Cold NS is stored at 4.4C in the med room refrigerator.

    MAINTENANCE

    • Tightly control the patient’s temperature for 12-24 hours using a combination of the Arctic Sun, cold NS infusion, or a cooling blanket (if the Arctic Sun or other similar device is not available).
    • Watch out for hypotension, hypokalemia, and shivering. 
    • Stop cooling and start rewarming immediately if the patient develops dysrhythmia or severe bleeding.
    • Repeat vital signs (including core temperature) every hour.
    • Hypothermia tends to cause hyperglycemia, so perform serial fingersticks and maintain the blood glucose at <140.

    REWARMING

    • Unlike cooling, rewarming should be gradual and take place over 8-24 hours. If the patient is stable, simply reset the Arctic Sun at 37C with a rewarming rate of 0.1C/hr. If the patient develops hemodynamic instability, dysrhythmia, or severe bleeding while being cooled, start rewarming to 37C immediately at a rate of 0.3C/hr.
    What if the patient starts shivering?
    Shivering generates heat and is therefore counterproductive to targeted temperature management. You can consider giving medications to stop shivering. These include:
    • Meperidine (50 mg IV q 6 hrs)
    • Buspirone (30 mg po q 8 hrs)
    • Sedation (midazolam, fentanyl, propofol, lorazepam)
    • Neuromuscular blocker (vecuronium 0.1 mg/kg bolus; cisatracurium infusion 0.15 mg/kg bolus followed by 1-10 mcg/kg/min infusion)

    Keep in mind that using a neuromuscular blocker can hide seizure activity, so use your best judgement. Rapid on/off sedation medications are preferable to permit serial neuro checks.

    Thank you to Dr. Greg Fernandez for inspiring this Pearl. Stay cool. 

     

    Jean Sun

    Jean Sun

    PGY2 Resident

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