Hypothermia results in slightly over one hundred deaths per year in Canada; typically as a result of primary hypothermia, frostbite and other cold exposure injuries (Toronto Public Health, 2014). In the United States, this is estimated at 1500 deaths annually (Baumgartner 2008). Both are likely underestimates of the scope of illness, as it only represents patients brought to a hospital, identified as hypothermic and where this was listed as the cause of death. A chart review of hypothermia cases in Finland over a ten year period showed the most common mechanism of hypothermia was cold water immersion (48%), cold environment exposure (39%), or submersion (13%) (Silfvast 2003).

In part one of a two-part series, we seek to review the scope and principles of hypothermia, how to appropriately measure temperature, discuss rewarming methods and review the current existing guidelines and evidence around hypothermia management.

Physiology

• Hypothermia is defined by a core temp< 35^oC
  • Primary hypothermia = body cannot match environmental stress
  • Secondary hypothermia = medical condition (eg. anorexia, hypoadrenalism)
• The normal response to cold is to increase metabolic rate, muscle tone, shiver, and peripheral vasoconstriction. For mild cold exposures, this is adequate.  However, behavioural responses (eg. seeking shelter or clothing) are our most effective methods. (Danzl 2012)
• Shivering (Bristow 1988, Tikuisis 1999)
  • 32-35 °C: shivering increases with decreases in skin temperature or core temperature to 32°C
  • 30-32 °C: shivering decreases
  • <30 °C: shivering ceases
  • Should be noted that variation in the onset or absence of shivering is seen, so should not be taken as absolute

• Metabolic rate (Wood 1991)
  • <32 °C: metabolic rate slows
  • 28 °C: oxygen consumption and pulse rate are about 50% of that of normothermia. Oxygen use decreases by about 6% per 1 °C drop.
  • 18 °C: the brain can tolerate cardiac arrest up to 10x as long

• Rescue collapse is defined as a cardiac arrest that is related to the extrication and transport of a patient with deep hypothermia (stage HT III). Rescue collapse has been attributed to circulatory collapse due to hypovolemia, cardiac arrhythmias triggered by interventions, afterdrop, and further cooling.
• Afterdrop is defined as continued core cooling after rescue. It is thought that ongoing heat loss (conductive loss from warm core to cool extremities, and convective loss to blood as flow is increased to the extremities and cool peripheral blood returns centrally).  It is further theorized that moving extremities can result in more blood return, and more afterdrop – this is part of the reason for avoiding excessive movement of a cold patient.  There is controversy on its clinical significance, or if it even exists. (Giesbrecht 1992, Romet 1988, Baumgartner 1992, Vassallo 2011)

Staging of Hypothermia

• Method: staging is based on core temperature
• Absence of shivering is suggestive of temperature < 30 ^oC, but can be variable and is not reliable enough for assessment
• AHA classification (Truhlar 2015):  based on core temperature, using 30 ^oC cutoff for severe. This is when the likelihood of VF is much higher. Mild (>34 C), moderate (30- 34 C), and severe (<30)
• Swiss system (Vanden Hoek 2010) – Developed to help rescuers estimate core temperature clinically. ERC and WMS prefer the Swiss system, as it includes clinical criteria. It is important to note that there may be variability in clinical presentations:
  • HT I: clear consciousness with shivering: 35 to 32C – mild
  • HT II: impaired consciousness without shivering: 32 to 28C – moderate
  • HT III: unconscious: 28 to 24C – severe
  • HT IV: apparent death: <24C – profound
  • HT V: death due to irreversible hypothermia: < 13.7C – irreversible? (has since been recover from a case 9 C)

Core temperature measurement

• Body temperature varies by site, perfusion state, and environmental temperature
• Core temperature is a more accurate reflection of the temperature of the heart and body physiology.
• Core temperature is a requirement for staging based on temperature.
• Additionally, probes must be properly calibrated and low temperature-sensing

Primary options: Esophageal, Rectal, Bladder, Tympanic membrane (epitympanic). Not acceptable: Oral, temporal, infrared cutaneous, aural (Vanden Hoek 2010, Zafren 2015)

• Esophageal
  • If intubated or comatose
  • Advance to lower 1/3 (~24cm below larynx)
  • Most reliable and accurate, but can be falsely elevated by heated air or gastric lavage
• Rectal
  • Reasonable option if not intubated, or if esophageal or tympanic not available
  • Advance to 15cm
  • Can lag behind true core temperature up to 1 hour
  • May be falsely elevated during colonic lavage
  • ERC and WMS de-emphasizes it
• Bladder
  • Reasonable option if not intubated, or if esophageal or tympanic not available
  • Unique catheter with temperature probe
  • Can lag behind true core temperature up to 1 hour, less reliable, ERC de-emphasizes it
  • May be falsely elevated during bladder or peritoneal lavage
  • ERC and WMS de-emphasizes it
• Tympanic membrane (epitympanic)
  • Must be in contact with TM
  • Theoretically reflects brain temperature, but need good access/contact
  • Probably as accurate as esophageal, but logistical challenges make it hard to use
  • When cardiac output is low, the epitympanic temperature is lower than core temperature

Rewarming Methods

• There is minimal data regarding a target rate of rewarming, and there are no specific recommendations
• Typical rates seen in literature with minimally invasive methods are 0.5-2.5 °C/hr , with higher rates in children. Typical rates with ECMO are 6-9 °C/hr. (Brown 2012)
• Suggested:
  • Without cardiac arrest –> target normothermia (>35 °C)
  • With cardiac arrest + ROSC –> targetted temperature management (32-36 °C, as per local protocols)
• A meta-analysis by Saczkowski et al. suggested that a slower rate of ECLS rewarming (6.0 °C/hr vs 7.8 °C/hr) may be independently associated with improved neurologic outcome.

Passive rewarming

• Blankets, removing dry clothes
• Shivering and increased metabolic rate can generate about 1-3°C/hr (Brown 2012)

Active external rewarming

• Warmed blankets, warm packs, forced-air heating (eg. BAIR hugger), fluid heating blankets (eg. Blanketrol), distal limb rewarming
• Whole-body bath/shower increases return of cold blood, and is NOT recommended (Zafren 2015)
• Most of these methods are insufficient on their own, and should be used as a bundle. Forced air heating and fluid heating blankets are the most effective of these measures. (Brown 2012)
• Rewarming rate is about 0.5-4 °C /hr (Paal 2016)
• Key baseline method in all hypothermia. Enough on its own for the majority of cases, unless the patient is in cardiac arrest or has insufficient perfusion.

Active internal rewarming

• Heated humidified oxygen, warm IV fluids, body cavity lavage (bladder, gastric, colon, peritoneal, or thoracic)
• Heated oxygen and IV fluids are often included with active external rewarming methods as “minimally invasive rewarming”
• Intravenous fluids should be warmed (38 to 42°C)
  • Very slow rewarming rate, but there is a benefit to not adding cool fluids
  • Recommended by AHA and ERC (Truhlar 2015, Vanden Hoek 2010, Zafren 2015)
• Gastric/Colonic/Bladder lavage
  • Usually seen in conjunction with other methods, without seeing a significant increase in rewarming rates
  • Minimal heat transfer dependent on conduction
  • Not recommended as they introduce complications and delay appropriate care in severe hypothermia or arrested patients (ie. ECMO) (Brown 2012)
  • Bladder lavage can be considered, as the risk profile is minimal
• Peritoneal lavage – rate 1-2 °C/hr, poor outcomes, higher complications rates, not recommended (Paal 2016)
• Thoracic lavage
  • According to case reports, thoracic lavage has cases of ROSC within 2 hours (Plaisier 2005). This is based on a small review of observational data (14 patients), despite reviewing articles 1966-2003.  There is clearly publication bias, and the data should be interpreted as such.
  • Reasonable, but high risk, alternative rewarming technique when ECMO or cardiopulmonary bypass is not available
  • Rate 1-2 °C/hr (Paal 2016)
  • Higher complication rates than typical rewarming scenarios (Danzl 2012, Taylor 2008)
  • Complications were mostly pulmonary, bleeding, or related to severe hypothermia

Extracorporeal rewarming

• Heating outside of the body
  • Without a pump – dialysis, VV-ECMO
  • With a pump – VA-ECMO, Cardiopulmonary bypass (CPB)
• Extracorporeal Life-Support
  • Extracorporeal life support (ECLS) is the supplementation or replacement of cardiopulmonary function. This is achieved through the use of machines (eg. heart-lung machine) for removal of blood, gas exchange, rewarming, and return of blood with pump function to maintain perfusion.  Unlike other rewarming methods, ECLS can provide rapid rewarming even if there is cardiac arrest.
  • Includes CPB and VA-ECMO methods
    • Both methods require advanced procedures and equipment, anticoagulation (heparin), and a dedicated perfusionist providing direct care
    • Cardiopulmonary bypass typically refers to a heart/lung machine used in the operating room.
    • VA-ECMO is similar in providing cardiac and respiratory support, but is a simplified version without a venous reserve. Extrathoracic cannulation is used, typically femoral-femoral or femoral-jugular.  Catheters are smaller, only partial anticoagulation is needed, and the system can be used for days rather than hours.
    • ECMO is starting to supplant CPB based on better outcomes evidence in avoiding late cardiovascular collapse (Ruttman 2007, Mazur 2019)
  • Indications:
    • Hemodynamic instability (e.g. systolic blood pressure <90 mmHg, ventricular rhythms)
    • Cardiac arrest or non-perfusing rhythms
    • Core temperature < 28°C
  • Efficacy
    • A recent analysis of 44 observational studies and 40 case reports by Saczkowski et al. 2018, comprising 658 patients with accidental hypothermia treated with ECLS, revealed an overall survival rate with a good neurologic outcome of 40.3%
    • Overall survival with good neurologic outcome among patients treated with ECLS is about 40.3%, based on a meta-analysis of observational data (Saczkowski et al., 2018). This is compared with roughly 10% survival without (Silfvast 2003), yielding an NNT of 3.
    • Prior estimates were about 47-63%, which is probably an overestimate based on the nature of observational data. (Brown, 2012)
    • Traditionally, CPB has been used for rewarming in low-perfusion states. However, overall survival is better with modern ECMO techniques (Ruttman 2007, Mazur 2019).

Resuscitation in Hypothermia

• The optimal hemodynamics and circulation for adequate perfusion in severe hypothermia (<28 °C) is not clear, as the brain can tolerate much lower perfusion states for a strikingly long time. (Lexow K 1991, Forti 2019)
• How long to check for a pulse?  – 60 sec
• Non-perfusing rhythm (VF, VT, asystole) vs. Perfusing rhythm (PEA)
• Arterial lines are helpful for pulse and hemodynamic monitoring in real-time
• End-tidal CO₂ may be helpful as a data point for the effectiveness of perfusion

Contrasting the Hypothermia guidelines

There are two major resuscitation guidelines (AHA and ERC), as well as the WMS society guidelines. The strength of evidence and/or recommendation is presented here as reported by the authors, or has been estimated where the authors did not report it.

Stay tuned for part 2 !