Cardiac Dysrhythmias and Cardiac Arrest
Hypothermia-induced dysrhythmias are notoriously difficult to treat as cardiac pacing, defibrillation and standard medications are generally ineffective at low core body temperatures. To support airway and breathing, early intubation may be necessary in obtunded patients, those with respiratory failure, altered mental status, or a decreased cough reflex.1 Standard medications may be used to perform rapid sequence intubation, at which time continuous ECG monitoring, CPR-preparedness, and placement of defibrillation-pads are recommended.6
Risk of cardiac dysrhythmias and cardiac arrest begins to develop at a core temperature of 30°C and ventricular fibrillation susceptibility is greatest at <22° C.4 Clinicians must carefully assess for and confirm the absence of pulse before beginning CPR. In severe hypothermia, the heart rate may be only 2 to 3 beats per minute, which due to severely decreased metabolic demands, may satisfy the body’s circulatory needs. When available, it is best to check for a central pulse for up to 1 minute using a Doppler.1 Additionally, a bedside echocardiogram can be performed to identify cardiac contractions.1
Chest compressions should not be performed in patients who manifest an organized rhythm on a cardiac monitor even if they have no palpable pulses.1 It is suggested that such rhythms may reflect successful perfusion that could be disrupted by chest compressions, that instituting cardiac compressions could lead to life-threatening arrhythmias, and that any pulseless electrical activity is likely to be transient.1 There is little downside to withholding CPR briefly in these hypothermic patients. Should pulseless electrical activity become asystole, chest compressions should be started immediately.
Given the lack of research, it is reasonable to treat patients with ventricular fibrillation according to ACLS guidelines, including defibrillation and administration of vasopressors.1 Defibrillation may be ineffective at core temperatures less than 30o C.1,4 If core temperature is <30oC, initiate and continue extended CPR, but withhold intravenous medication and limit shocks to a maximum of 3 defibrillations (with single attempts made with every 1 to 2° C increase in core temperature) and then wait for rewarming to greater than 30oC before further attempts.6 Active rewarming may correct the dysrhythmia or increase the chance of successful defibrillation. Studies reveal vasopressors had a higher association with return of spontaneous circulation (ROSC) than antiarrhythmic medications.1 It is recommended to give intravenous medications as indicated once the core body temperature greater than 30oC, but spaced at longer than standard intervals.6 If ventricular fibrillation persists despite rewarming, the Advanced Cardiac Life Support guidelines recommend administration of amiodarone.8
Bradycardia may be physiologic in severe hypothermia, and transcutaneous cardiac pacing is not required unless bradycardia persists despite rewarming to 32 to 35° C.1 Atrial arrhythmias may develop during rewarming, but are generally benign and do not require treatment except in rare cases of rapid ventricular response.1 Atrial fibrillation is common when the core temperature is <32° C and is not worrisome in the absence of other signs of cardiac instability.2
As patients with moderate to severe hypothermia are warmed, they may experience hypotension due to volume loss from cold diuresis, vasodilation, and fluid shifts necessitating fluid resuscitation, which will require 2 large-bore intravenous catheters.2 The use of warmed (42° C) isotonic crystalloid is critical, as infusion of room-temperature fluids can worsen hypothermia.1 If a central venous catheter is placed, the femoral location is preferred to avoid precipitating an arrhythmia from right atrial irritation.1 Studies support the use of low-dose dopamine (2-5 mcg/min) as a vasopressor.1 Clinicians must use caution administering inotropic agents due to possible cardiac stimulation and ectopy, and risk of peripheral tissue malperfusion.2,4
Complications and Prognosis of Hypothermia
Factors associated with poor prognosis in moderate to severe hypothermia include prehospital cardiac arrest, low or absent blood pressure on presentation, elevated blood urea nitrogen levels, asphyxia by drowning or avalanche burial, and the need for endotracheal intubation.1 Almost all healthy patients who develop accidental hypothermia and are hemodynamically stable at presentation survive neurologically intact (mortality rate less than 5%).1
A multicenter survey found a 21% mortality rate for patients with moderate hypothermia (28-32° C).1 Mortality rates for patients with severe hypothermia, especially with pre-existing illness, may exceed 50%.1 A survey of patients with hypothermia IV at 1 center showed that organ failure was common 24 hours after admission, and among fatal cases of organ failure, the most common cause of death was pulmonary edema.2
Rhabdomyolysis, pulmonary, renal and neurologic complications are common after rewarming. Patients with primary hypothermia and cardiac stability who have been treated with active external and minimally invasive rewarming have a rate of neurologically intact survival of approximately 100%, whereas for patients with cardiac arrest treated with ECLS, the rate is approximately 50%.2
Based on the understanding of clinical manifestations attributed to core body temperature, clinicians should beware of vital signs and neurological manifestations that are inconsistent with the degree of hypothermia that may suggest alternative or concurrent diagnoses. A relative tachycardia inconsistent with core temperature suggests hypoglycemia, hypovolemia, or overdose. Relative hyperventilation implies an underlying organic acidosis, since CO2 production should be decreased in moderate or severe hypothermia. In elderly patients, sepsis can manifest as hypothermia.
Empiric, broad spectrum antibiotics should be administered if there is unexplained hypothermia, a probable source of infection, aspiration, failure to rewarm, or other signs of sepsis.1 If level of consciousness is not proportional to the degree of hypothermia, suspect head injury, neurologic infection, stroke, or intoxication or overdose. Do not assume that areflexia or paralysis is due to hypothermia until spinal injury has been ruled out.
All clinicians should be aware of the epidemiology and contexts for accidental hypothermia. Anticipatory guidance should be provided to patients as relevant. Appropriate assessment of hypothermic patients requires an understanding of its etiology and risk factors, and pathophysiologic changes related to the stages of hypothermia. The neuroprotective effects of hypothermia and the treatments currently available for rewarming give patients an excellent chance of neurologically intact survival despite prolonged cardiac arrest times.
- Zafren K, Mechem CC. Accidental hypothermia in adults. UpToDate website. https://www.uptodate.com/contents/accidental-hypothermia-in-adults. Published March 12, 2018. Accessed January 9, 2019.
- Brown DJA, Brugger H, Boyd J, Paal P. Accidental hypothermia. N Engl J Med. 2012;367:1930-1938.
- Vassal T, Benoit-Gonin B, Garrat F, Guidet B, Maury E, Offenstadt G. Severe accidental hypothermia treating in the ICU: prognosis and outcome. Chest. 2001:120(6):1998
- Li J, Silverberg MA, Decker W, Edelstein JA. Hypothermia. Medscape website. https://emedicine.medscape.com/article/770542-overview. Published November 9, 2018. Accessed January 9, 2019.
- Berko J, Ingram D, Saha S, Parker JD. Deaths attributed to heat, cold, and other weather events in the United States, 2006-2010. Natl Health Stat Report. 2014;(76)1-15.
- Paal P, Gordon L, Strapazzon G, et al. Accidental hypothermia — an update: the content of this review is endorsed by the International Commission for Mountain Emergency Medicine (ICAR MEDCOM). Scand J Trauma Resusc Med. 2016;24(1):111.
- Brugger H, Durrer B, Elsensohn F, et al. Resuscitation of avalanche victims: Evidence-based guidelines of the international commission for mountain emergency medicine (ICAR MEDCOM) Intended for physicians and other advanced life support personnel. Resuscitation. 2013;84(5):539-546.
- Algorithms for Advanced Cardiac Life Support 2020. ACLS Training Center website. https://www.acls.net/aclsalg.htm. Accessed August 12, 2020.