Hypothermic Rewarming Strategies

Educational Objectives

The goal of this program is to improve rewarming in patients with hypothermia. After hearing and assimilating this program, the clinician will be better able to:

1. Use extracorporeal membrane oxygenation as a rewarming strategy in patients with hypothermia.
2. Cite factors that guide the decision to terminate rewarming efforts in patients with hypothermia.
3. List causes of death while rewarming patients with severe hypothermia.

Summary

Epidemiology of hypothermia: primary hypothermia (PH) primarily affects individuals who work, live, or occasionally recreate outdoors (eg, skiers, snowboarders); PH disproportionately impacts people experiencing homelessness; cases of PH often peak during periods of rapid temperature drops rather than during the coldest parts of winter; in the United States, PH accounts for ≈1500 deaths annually; patients vulnerable to PH include the injured, intoxicated, and those exposed to sudden drops in temperature

Assessment of hypothermia: the severity of hypothermia is often best assessed by evaluating the mental status, but intoxication or trauma can complicate this evaluation; severe hypothermia (SH), characterized by core temperatures dropping <28°C, is marked by unconsciousness and the absence of shivering; the Swiss system categorizes hypothermia by core temperature and clinical signs (eg, shivering, consciousness levels); the term irreversible hypothermia lacks a definitive temperature threshold; some patients have survived extremely low core temperatures of ≈14°C; the duration of cold exposure does not necessarily dictate outcomes, as documented cases have shown patients surviving ≈6 hr of hypothermia

Physiology of hypothermia: accidental hypothermia primarily occurs through the skin and lungs via convective heat loss to cold air and conductive heat loss to water; risk factors include homelessness, psychiatric conditions, advanced age, malnutrition, chronic diseases, and certain medications; elderly individuals are particularly susceptible because of reduced physiologic reserves and social isolation; alcohol or medications can impair awareness of the cold and the ability to respond to it; physiologically, the hypothalamus attempts to combat cold stress through mechanisms that stimulate heat production (eg, shivering, increased thyroid and adrenal activity) sympathetic-mediated vasoconstriction conserves heat by redirecting blood flow to the core; as core temperatures drop <32°C, metabolism, ventilation, and cardiac output decline; shivering becomes less effective, leading to a rapid fall in core temperature

Physiology of SH: the central nervous system is relatively protected during cooling, which contributes to favorable neurologic outcomes in survivors; patients with hypothermia exhibit bradycardia <32°C, and medications are ineffective in raising heart rates; in SH, atrial fibrillation and more severe arrhythmias (eg, ventricular fibrillation [VF], ventricular tachycardia) occur; cardiac output and systemic vascular resistance markedly decrease; pulmonary vascular resistance increases, leading to potential right ventricular failure and pulmonary edema during rewarming; respiratory system challenges include rigidity, trismus, and unreliable end-tidal carbon dioxide measurements, which complicate intubation; the renal effects of SH include vasoconstriction-induced cold diuresis; hematologic changes involve hemoconcentration; dysfunctional clotting factors and impaired platelets increase the risk for bleeding in trauma cases; endocrine effects include hyperglycemia, but insulin is ineffective during hypothermia; SH can also deplete glucose stores, leading to hypoglycemia

Management of hypothermia: mild cases can be managed with passive rewarming by removing wet clothes, using warm blankets, and applying ambient heat; moderate cases benefit from active external rewarming using forced air devices and heating pads; rewarming should focus on the core to avoid rapid drops in core temperature caused by peripheral vasodilation; SH necessitates invasive methods (eg, warm intravenous fluids, humidified and warmed oxygen); cavity rewarming via the lungs and peritoneal cavity is an option; the bladder is too small for effective rewarming, and rewarming via the stomach poses risks for aspiration and electrolyte disturbances; endovascular warming devices can provide slow but controlled rewarming for patients with vital signs

Management of SH: for patients without signs of life, extracorporeal membrane oxygenation (ECMO) or cardiopulmonary bypass may be used; ECMO provides both circulatory and ventilatory support and has demonstrated significantly higher survival rates and favorable neurological outcomes compared with cardiopulmonary resuscitation (CPR) alone

Basics of ECMO: initiation of ECMO requires a potassium <10 mmol/L; venoarterial ECMO is usually used to provide perfusion during rewarming; anticoagulation is generally not needed; when on ECMO, a pulse oximetry reading in the 70s to low 80s is typically sufficient, assuming normal flow through the ECMO circuit and adequate hemoglobin levels; the pulse oximeter and blood pressure monitor, preferably invasive, should be placed on the right side to avoid artificially high readings caused by ECMO return flow on the left; monitoring on the right also confirms blood reaching the brachiocephalic artery; during ECMO, the pulmonary vasculature sees no flow, so there is no end-tidal carbon dioxide; oxygenation and ventilation are typically managed by the perfusionist or respiratory therapist; ventilation settings are minimal, ≈4 mL/kg tidal volumes with a low respiratory rate to maintain lung movement; a low fraction of inspired oxygen is used since oxygenation is primarily managed by ECMO

Using ECMO for SH: rewarming patients with SH on ECMO involves rapid temperature increases, ≈1°C every 10 min; the goal is to achieve euthermia and restore a perfusing rhythm as quickly as possible; rewarming continues to ≈37°C before decannulation, as temperatures can drop after ECMO; an esophageal temperature probe placed in the lower third of the esophagus is ideal for monitoring temperature, as it correlates well with pulmonary artery readings; bladder and rectal temperatures are less reliable; obtaining arterial lines can be challenging because of cold-induced stiffness and vasoconstriction of the extremities; therefore, ultrasonography guidance and proximal access (eg, the axillary artery) are often required

Coagulation: coagulation factors and platelets function poorly <33°C; warming is prioritized over administering fresh frozen plasma or platelets; persistent coagulopathy after rewarming suggests disseminated intravascular coagulation

Volume management: patients are hypovolemic because of cold diuresis and third spacing; while initial crystalloid boluses before ECMO are helpful, over-resuscitation poses risks for complications; once on ECMO, fluid administration is minimized, and vasopressors (eg, norepinephrine) are preferred to address systemic vascular resistance; even after the return of spontaneous circulation (ROSC), blood pressure often requires support through ECMO or pressors

Cardiac management: persistent bradycardia warrants low-dose epinephrine infusions; arrhythmias are common, especially atrial arrhythmias or VF; defibrillation is generally safe, with ≤3 attempts recommended; if unsuccessful, rewarming to ≥30°C is necessary before initiating further advanced cardiac life support (ACLS) measures (eg, epinephrine, amiodarone) that are ineffective below this threshold; epinephrine dosing intervals are extended to 6 to 10 min when the patient is cold; pacing may be required; arrhythmias may persist after rewarming

Terminating ECMO: once ROSC is achieved, lung-protective ventilation strategies are implemented while weaning off ECMO, ensuring that temperatures remain stable; if ROSC is unattainable after rewarming to >32°C to 35°C and following ACLS measures, care is deemed futile

Other aspects: drug metabolism is altered during hypothermia, resulting in decreased clearance, while ECMO increases drug volume distribution; lipophilic drugs can be sequestered in the ECMO circuit; amnestic agents (eg, ketamine 50 mg/hr) are recommended after reestablishing cerebral perfusion, typically ≈28°C; inhaled anesthetics can be considered once ROSC restores significant lung blood flow; however, they should not be used as sole agents because of minimal ventilator settings and lung bypass during ECMO; certain criteria (eg, pH <6.5, potassium <13 mmol/L, intravascular thrombosis, unsurvivable trauma) guide decisions to forego ECMO or rewarming efforts; reflexes, pupillary findings, response to stimuli, electrocardiography findings, apparent rigor mortis, and duration of CPR cannot be used as criteria to stop resuscitation in patients with SH

Prognosis of SH: pulmonary edema is the leading cause of death during rewarming, exacerbated by trauma from CPR; the survival rate for SH is ≈60% at the speaker’s institution, with better outcomes associated with lower lactate levels, higher pH, and younger patients

Readings

Jeunesse E, O’Malley P, Petrus N, et al. A rare case of a patient being alert and communicative despite severe hypothermia. Cureus. 2024;16(3):e56293. doi:10.7759/cureus.56293; Morley D, Yamane K, O'Malley R, et al. Rewarming for accidental hypothermia in an urban medical center using extracorporeal membrane oxygenation. Am J Case Rep. 2013;14:6-9. doi:10.12659/AJCR.883728; Prekker ME, Rischall M, Carlson M, et al. Extracorporeal membrane oxygenation versus conventional rewarming for severe hypothermia in an urban emergency department. Acad Emerg Med. 2023;30(1):6-15. doi:10.1111/acem.14585; Tveita T, Sieck GC. Physiological impact of hypothermia: the good, the bad, and the ugly. Physiology (Bethesda). 2022;37(2):69-87. doi:10.1152/physiol.00025.2021; Wan L, Huang H, Zhang F, et al. Is pericapsular nerve group block superior to other regional analgesia techniques following total hip arthroplasty? A systematic review and network meta-analysis. Perioper Med (Lond). 2024;13(1):96. doi:10.1186/s13741-024-00455-y.

Disclosures

For this program, members of the faculty and the planning committee reported nothing relevant to disclose.

Acknowledgements

Dr. Benish was recorded at TASCON 2024, held on October 18, 2024, in Philadelphia, PA, and presented by the Trauma Anesthesiology Society and Sidney Kimmel Medical College at Thomas Jefferson University. For information about upcoming CME activities from this presenter, please visit https://www.tashq.org. Audio Digest thanks the speakers and Trauma Anesthesiology Society and Sidney Kimmel Medical College at Thomas Jefferson University for their cooperation in the production of this program.

CME/CE INFO

Accreditation:

The Audio- Digest Foundation is accredited by the Accreditation Council for Continuing Medical Education to provide continuing medical education for physicians.

The Audio- Digest Foundation designates this enduring material for a maximum of 1.00 AMA PRA Category 1 Credits™. Physicians should claim only the credit commensurate with the extent of their participation in the activity.

Audio Digest Foundation is accredited as a provider of continuing nursing education by the American Nurses Credentialing Center's (ANCC's) Commission on Accreditation. Audio Digest Foundation designates this activity for 1.00 CE contact hours.

Lecture ID:

AN671001

Expiration:

This CME course qualifies for AMA PRA Category 1 Credits™ for 3 years from the date of publication.

Instructions:

To earn CME/CE credit for this course, you must complete all the following components in the order recommended: (1) Review introductory course content, including Educational Objectives and Faculty/Planner Disclosures; (2) Listen to the audio program and review accompanying learning materials; (3) Complete posttest (only after completing Step 2) and earn a passing score of at least 80%. Taking the course Pretest and completing the Evaluation Survey are strongly recommended (but not mandatory) components of completing this CME/CE course.

Estimated time to complete this CME/CE course:

Approximately 2x the length of the recorded lecture to account for time spent studying accompanying learning materials and completing tests.