LEARNING OUTCOMES
After completing this module the paramedic will be able to:
Paramedics play a vital role in maintaining a patient's body temperature—especially in the prehospital environment and trauma situations. It is important that paramedics are able to perform accurate temperature measurements and initiate timely intervention, as evidence shows that this can reduce hypothermia-associated mortality (Kirkegaard et al, 2017).
According to the National Institute for Health Research (NIHR) (Galvao et al, 2010), the issue of keeping patients warm during a stay in hospital is an important one, but one that can pose a challenge. Preventing hypothermia—that is, a core body temperature below 35°C—and providing sufficient warming devices in order to prevent changes in a patient's body temperature requires careful assessment, monitoring and training. This issue is particularly important in patients who are admitted to the intensive care unit (ICU) or the emergency department (ED).
Exposure to cold environments and impaired thermoregulation caused by alcohol, drug ingestion, trauma, myocardial infarction, extremes of age or comorbid illness, for example, can all cause hypothermia (Paal et al, 2016). The clinical dangers of accidental hypothermia include cardiac arrhythmias, renal hypoperfusion and changes to the oxygen-dissociation curve (Bersten, 2018). Further, accidental hypothermia may also aggravate haemorrhaging and coagulation problems. Severe hypothermia (<33°C) has adverse effects on almost every organ system. Within the ED and ICU environments, trauma and cardiac patients are especially at risk. Research demonstrates that accidental hypothermia (also referred to as spontaneous hypothermia) in the ED is frequently observed in critically ill patients and is associated with poor physiological and mortality outcomes (Rubiano et al, 2013; Balvers et al, 2016).
Clinical scenarios
Many patients attended by paramedics experience some form of hypothermia. This is especially true if the patient has been outside for any length of time. It is necessary when treating the patient with hypothermia that a full history is taken. In addition, copresenting illnesses, alongside patient exposure and duration of exposure, is also required, as is the ambient temperature. Past medical and drug history also need to be determined as all can influence overall patient treatment and management.
The following clinical scenarios present three common circumstances which often result in hypothermia:
Physiology of hypothermia
Hypothermia is associated with a high mortality rate among older people and occurs when heat loss exceeds heat production, and can be determined by cold exposure and/or impaired thermoregulation, caused by a range of conditions (Table 1).
| Diseases | Conditions |
|---|---|
| Metabolic | Hypothyroidism, hypoglycaemia, diabetic ketoacidosis |
| Central nervous system | Stroke, head trauma, spinal cord injury, Alzheimer's, Parkinson's |
| Drugs | Alcohol, drugs such as opiates, benzodiazepines, tricyclic antidepressants |
| Miscellaneous | Sepsis, burns, trauma, malnutrition, anorexia nervosa, pancreatitis, mental illness and extremes of age |
Source: Mitchell and Medzon, 2005
Hypothermia is defined as a core body temperature of less than 35°, where temperatures of 35–32.2° are defined as mild; moderate at 32.2°–28° and severe at <28° (Sholl, 2005; Joint Royal Colleges Ambulance Liaison Committee (JRCALC), 2016). However, in trauma patients, a temperature of <36° is considered hypothermia, becoming severe at <32° (Thorsen et al, 2011).
Hypothermia can further be divided into three categories: primary, secondary and induced/therapeutic hypoterhmia, now referred to as targeted temperature management..
Primary hypothermia
Primary hypothermia is caused by exposure to cold temperatures where heat is lost through conduction, radiation, evaporation and convection (National Association of Emergency Medical Technicians, 2019). The normal bodily physiological processes will attempt to increase heat production through shivering, release of epinephrine, norepinephrine and thyroxine (Porth, 2014). However, decreased muscle mass and less heat generated through shivering is seen in the elderly, whereas children and infants have an amplified risk of heat loss because of their increased body surface area.
Secondary hypothermia
Secondary hypothermia is often seen in trauma, intoxications and some disease states, and tends to result from underlying conditions that predispose a patient to increased heat loss, decreased heat production or altering the body's ability to thermoregulate.
Targeted temperature management
Moreover, targeted temperature management (induced/therapeutic hypothermia) is most commonly seen post ventricular fibrillation cardiac arrest or during specific surgeries such as valve replacement or coronary artery bypass grafting (CABG) (Polderman and Herold, 2009).
The anterior hypothalamus regulates body temperature and, when exposed to cold, the body will need to conserve heat. The hypothalamic heat-producing centres respond to thermoreceptor impulses leading to peripheral vasoconstriction. This reduces blood flow to the superficial skin vessels, resulting in less heat loss (Porth, 2014).
Cellular hypoperfusion also plays a role in loss of body heat as cellular effects result in a switch from aerobic cellular to anaerobic respiration with the production of ADP rather than ATP. ATP is far more efficient at generating heat than ADP, thus impairing heat production (Kheirbek et al, 2009). As the core body temperature decreases, so does the mental status of the patient. Errors in judgment, dysarthria, confusion and decreased fine motor skills are seen in mild hypothermia. However, in moderate hypothermia, gross motor skills are lost, and further confusion, slow reflexes and altered pupil responses are seen. In severe hypothermia, loss of consciousness and coma occur.
Hypothermia and the ECG
Hypothermic patients are at increased risk of cardiac arrhythmias or ischaemic changes (Dolan and Holt, 2013). An ECG is vital to determine the rhythm of the heart and identify abnormalities as, in hypothermia, the ECG may show J-waves (otherwise known as Osborn waves). J-waves occur at the junction of the QRS complex and ST segment. Harash et al (2017) identified that patients with J-waves had an increased risk of mortality.
Consequences of hypothermia
Prolonged, prehospital cardiac resuscitation is often associated with hypothermia (Brown et al, 2012). Indeed, according to Den-Hartog et al (2010) and Reynolds et al (2012) many patients admitted to the ICU via the ED (following cardiac arrest) have clinical hypothermia.
This is significant because hypothermia upon ICU admission is a strong predictor of an unfavourable neurological outcome in patients resuscitated for primary cardiac arrest in either the prehospital or ED environment.
Trauma is also associated with accidental hypothermia, especially if the patient has been outdoors for any length of time. Obviously, the stabilisation efforts for the trauma patient, within the ED, takes precedence over other considerations; it is therefore often the case that hypothermia ensues (Bukur et al, 2012; Balvers et al, 2016). However, as Balvers et al (2016) discovered, hypothermia is not only common in critically ill trauma patients, but it is also one of the most important physiological predictors for early and late mortality in critically ill ICU trauma patients. Clearly, the management of body temperature should form part of both trauma management protocols and cardiac resuscitation procedures.
In addition to cardiac and trauma patients, patients admitted to hospital with a traumatic brain injury also have a significant increase in the risk of mortality if they develop spontaneous hypothermia. When one considers traumatic brain injury, accidental hypothermia is associated with a significant increase in mortality when compared with normothermic patients. This is in spite of certain claims that the application of therapeutic hypothermia in the management of the severe TBI has a beneficial outcome. As Rubiano et al (2013) maintain:
‘Whilst therapeutic and spontaneous hypothermia share the same homeostatic end-point, the pathophysiological mechanisms and conditions through which they occur are distinctly separate and thus the prognostic outcomes in studies focusing on therapeutic hypothermia cannot be accurately assumed to apply in spontaneous hypothermia cases.’
Thus, it is vitally important that the distinction between therapeutic and spontaneous hypothermia is made and understood. While it may be the case that targeted temperature management is beneficial in certain clinical situations—the evidence is clear that accidental/spontaneous hypothermia is associated with increased mortality. There is, therefore, an obvious need to maintain/re-establish normothermia in critically ill patients. Fortunately, the management of accidental hypothermia has made substantial progress over the last two decades. However, it needs to be stressed that the delivery of any rewarming method requires an initial, accurate core body temperature measurement. Further, attempts should not be made to rewarm frostbitten extremities until a patient's core body temperature is >34°C (Freer and Imray, 2012).
Rewarming techniques
According to Paal et al (2016) Rewarming methods can be classified as passive (protection from further heat loss while the patient raises their own body temperature), active external (delivery of heat to the surface of the body) and active internal (delivery of heat to the interior of the body). The different types of rewarming methods are summarised in Table 2.
| Rewarming technique | Rewarming rate | Notes and controversies | Rewarming complications |
|---|---|---|---|
| PASSIVE REWARMING | |||
| Passive rewarming | 0.5–4 °C hr-1 (dependent upon patient's thermoregulatory function and metabolic reserves) | Protect from further heat loss and allow patient to self-rewarm. Minimal controversy for mild hypothermia | Negligible in isolated mild hypothermia |
| Passive rewarming with active movement | 1–5 °C hr-1 | Exercise has been shown to increase afterdrop in physiology studies from ~0.3 °C in controls to ~1 °C in exercised subjects | No reported complications |
| ACTIVE EXTERNAL REWARMING | |||
| Active rewarming e.g. forced air surface using Arctic Sun® | 0.5-4 °C hr-1 | Protect from further heat loss, deliver external heat and (if required) warmed IV fluids. Minimal controversies | Similar to passive rewarming |
| ACTIVE INTERNAL REWARMING | |||
| Bladder lavage | Variable. Adds ~0.5–1 °C hr-1 | Helpful if rewarming rate is slow. Minimal controversies | Negligible unless difficult catheterisation |
| Gastric lavage | Adds ~0.5–1 °C hr-1 | Not commonly used due to risk vs. benefit ratio | Potential for aspiration, fluid and electrolyte shifts |
| Intravascular catheter rewarming e.g. Icy® catheter (CoolGuard®) | Device specific (adds ~0.5–2.5 °C hr-1) | Uncertain indication for use, potential for benefit exists in colder and sicker comorbid patients with stable circulation | Potential for haemorrhage or thrombosis, potentially worsening hypotension in unstable patients. |
| Thoracic] or Peritoneal lavage | Adds ~1–2 °C hr-1 | Not commonly used unless patient is unstable | Potential for haemorrhage, lung or bowel trauma, fluid and electrolyte shifts. Thoracic lavage has the potential to impair CPR quality |
| Extra-corporeal life support (VA-ECMO or CPB) | ~4–10 °C hr-1 | Preferred rewarming method for patients in cardiac arrest. CPB can use femoral route avoiding need for sternotomy | Potential for haemorrhage, thrombosis, haemolysis, etc (as with all intravascular devices) |
Source: Freer and Imray, 2012; Paal et al, 2016
As highlighted in Table 2, different clinical conditions require different rewarming rates. In most cases of mild hypothermia (over 32°C), passive external rewarming is the method of choice, with a rewarming rate of between 1.5–2°C per hour being optimal (Danzl, 2012).
Evidence also suggests that the use of extracorporeal-assisted rewarming in the management of accidental hypothermia is proving particularly beneficial (Brown et al, 2012; Dunne et al, 2014; Svendsen et al, 2017). Within the ICU and ED environments, the National Institute for Health and Care Excellence (NICE) (2008) recommends that patients should be actively warmed using forced-air warming blankets (NICE, 2008; Galvao et al, 2010).
Rewarming and the paramedic
Advances in rewarming techniques can improve the prognosis for patients with hypothermia, especially those on ICU or those admitted via the ED by paramedic/emergency ambulance.
Acknowledging this, paramedics should accurately record the core body temperature of the seriously ill and initiate necessary and appropriate rewarming techniques as soon as possible. According to Li (2018), because patients developing hypothermia from coldwater immersion are at high risk of ventricular fibrillation, in order to prevent cardiac dysrhythmia with continued hypothermia, paramedics should attempt prehospital rewarming. It needs to be acknowledged that both cardiac pacing and atropine are generally ineffective for hypothermia-induced bradyarrhythmia and Lidocaine is ineffective in preventing hypothermia-induced ventricular dysrhythmias. The only anti-arrhythmic agent of any use in the hypothermia-induced dysrhythmia situation is Bretylium (Mallet, 2002).
Anecdotal reports of sudden cardiac death associated with tracheal intubation (in patients who are hypothermic) however appear to be exaggerated, particularly if a patient is adequately preoxygenated (Li, 2018). Paramedics need to first stabilise a patient and attempt to prevent further heat loss. Once this has been achieved, attempts at rewarming can be made.
The assessment and management of hypothermic patients in the prehospital environment should always include an ABCDE assessment, correcting any airway or breathing/circulation problems and continue the management of the patient en route to definitive care. A pre-alert should be given.
Preventing further heat loss will involve the removal of any wet clothing if the incident is protracted. However, if removal to a warm environment can be achieved quickly, the removal of wet clothing is of limited effectiveness (Henriksson et al, 2015). Patients should not be wrapped directly in a foil blanket. Rather, a blanket should be placed between the patient and the foil blanket. If the patient is conscious, provide a hot drink/food if available. The paramedic should not rub the patient's skin as this may increase heat loss and cause tissue damage.
Conclusion
Spontaneous hypothermia is associated with a significant increase in mortality when compared with normothermic patients. Therefore, prevention of hypothermia in the seriously/critical ill patient is an important clinical consideration for all paramedics and other emergency professionals. With the implementation of timely and effective rewarming techniques, it is possible to mitigate against hypothermia.