When actively rewarming a frostbitten extremity in warm water you should change the water when the temperature falls below?

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The severely hypothermic patient must be handled and moved extremely carefully as the hypothermic myocardium is predisposed to ventricular fibrillation (VF) on patient handling. The management of hypothermia requires initial evaluation and support of the airway, breathing, and circulation; prevention of further heat loss; initiation of re-warming appropriate to the degree of hypothermia; and treatment of complications.

The appropriate intervention depends on the severity of the hypothermia, the available resource and the clinical state of the patient. As the severity of the hypothermia increases, so to the treatment of that hypothermia should become more aggressive. Other medical factors, such as alcohol intoxication, central nervous system disease, trauma and infection should be considered and treated concurrently. Consider Parenteral Thiamine for critically ill, malnourished, alcoholics and in those with unexplained lactic acidosis. Wet or cold clothes must be removed. The patient must be placed on a cardiac monitor, have intravenous access established and active re-warming measures initiated. The general treatment of any coagulopathies is by re-warming and not the administration of clotting factors. Re-warming strategies can be divided into active versus passive and invasive versus non-invasive.

Learning Bite

Supportive measures, with a focus on the Airway, Breathing, Circulation, Disability (including Dont Ever Forget Glucose) and the removal of wet, cold clothes is the cornerstone of management

Additional Diagnostic Measures

Laboratory investigations should be driven by the clinical situation. In most cases, it would be reasonable to obtain urea & electrolytes (U&E), a full blood count (FBC), a clotting screen, an arterial blood gas, a creatinine kinase level, a blood alcohol and a urine toxicology screen. These investigations should be repeated during the rewarming phase. Some of the physiological changes seen with hypothermia would include a shift of the oxyhaemoglobin dissociation curve to the left, an increased haematocrit due to the decrease in circulating plasma volume and a low to normal white blood cell count, even in the setting of infection [6]. Arterial blood gases should not be corrected for temperature. The blood gas analyser warms the blood to 37 C and the uncorrected values for pH and PaCO2 correspond almost perfectly to normal temperature values. An uncorrected pH of 7.4 and a PaCO2 of 5.3 kPa confirm acid base balance at all temperatures. The normal respiratory response to hypothermia is hyperventilation, resulting in a respiratory alkalosis. As the hypothermia worsens, respiratory depression predominates and this results in a respiratory acidosis. In a series of 135 hypothermic patients, 30% were acidotic and 25% were alkalotic. This reflects the unpredictability of the acid-base milieu in the hypothermic patient. Imaging studies should be geared toward the clinical situation.

Learning Bite

Dont correct for hypothermia when analysing the acid-base status normal values can be assumed to meet the needs of the hypothermic tissue

Temperature Monitoring

Core temperature monitoring is important in order to assess the efficacy of the rewarming measures. This should be accomplished by the use of a continuous or serial rectal or oesophageal thermometer. The rectal thermometer may lag behind the core temperature increases due to impaction into frozen faeces. The oesophageal temperature reading may be falsely elevated when concurrent heated, humidified oxygen is being administered. It also requires the patient to be intubated.

Fluid Resuscitation

In general, the hypothermic patient is dehydrated and fluid depleted. They should therefore be given a fluid challenge of warmed 0.9% saline or preferably Dextrose-Saline as they may also be hypoglycaemic. Hartmanns solution may also be used but since the hypothermic liver cannot metabolise lactate, it is best avoided. The patient should be monitored carefully for signs of fluid overload.

Passive Rewarming

This principle allows the patients own thermogenic mechanisms to rewarm them. They should be removed from the cold environment and have wet or cold clothes removed. They can then be covered by a blanket or sleeping bag and have their head covered to reduce heat loss. This technique can be used for the patient suffering from mild hypothermia that can still generate heat by shivering.

Active External Rewarming

This involves heat added to the patient from an external source. It is the treatment of choice in mild-moderate hypothermia patients whose own thermoregulatory mechanisms are impaired. Active external rewarming may be accomplished by a variety of methods including heat packs, heat lamps, blankets, warm water immersion, warmed blankets, and forced air systems. Patients must be monitored carefully for a presumptive aftedrop caused be the return of cool peripheral blood to the central circulation. Forced air systems are one of the most widely used means of active external rewarming in current use. Forced air systems are easy to apply, allow for patient monitoring, and seem to limit afterdrop. Rates of warming with these systems approach 1.0-2.5 C/hr [2-3]. The Bair Hugger is a well-known example.

Active Core Rewarming

Patients will moderate to severe hypothermia will require active core rewarming. This comprises a variety of techniques, from simple to more complex. The simplest example of active core rewarming is the use of warmed intravenous fluids and warmed, humidified oxygen.
Fluids can be warmed to 44 C and run through a large, short intravenous cannula. Warmed humidified air/oxygen should also be heated to 42-44 C. This may require special equipment or modification to the existing heating circuit. More invasive methods of active rewarming include cavity (gastric, bladder, peritoneal and pleural) lavage using warm fluids. These methods achieve the most rapid rewarming rates, but due to there invasiveness and complexity, should be reserved for the most severe and refractory of cases including hypothermic cardiac arrest, the failure of more conservative techniques, frozen extremities and evidence of rhabdomyolyis in conjunction with other electrolyte abnormalities [6].

Extracorporeal Blood Rewarming

This is used to aggressively rewarm blood in the severely hypothermic patient who has been refractory to all other methods of rewarming. There are several methods including haemodialysis, arteriovenous, veno-venous and cardio-pulmonary bypass. The biggest advantage of this method is the speed at which the patient can be rewarmed by directly warming their blood. Additional advantages include the continual delivery of oxygenated blood to the tissues despite the absence of mechanical cardiac activity (cardiopulmonary bypass). Extracorporeal rewarming should be considered for patients without perfusion who have no documented contraindications to resuscitation, patients with severe hypothermia, and those with completely frozen extremities [7]. There are no specific criteria for placing a patient on extracorporeal rewarming, but several centres reserve it for patients with a pH >6.5, a serum potassium <10mmol/L, and a core temperature > 12 C [8].

Hypothermic Cardiac Arrest

Both establishing and treating cardiac arrest in the severely hypothermic patient can be very difficult. The treatment of VF in the hypothermic patient is controversial. The European Resuscitation Council 2010 guidelines recommend up to three defibrillation attempts, to with- hold adrenaline until the core temperature exceeds 30 C, and to double the dose frequency until the temperature is above 35 C.2. The patient should be defibrillated, if VF persists, two further shocks should be given. If a total of three shocks have been given and the patient remains in VF, further defibrillation attempts should be withheld until the core temperature is above 30 C

Distinguishing between VF and asystole can be extremely challenging, particularly in the prehospital setting. Certain stable arrhythmias, such as atrial arrythmias, will respond to rewarming measures only and antiarrythmics should be avoided. The use of antiarrythmic medications in the hypothermic patient is highly controversial and in the small studies done so far, only bretylium has been shown to be of any benefit. Further studies are needed for validation. Once the diagnosis of cardiac arrest is established, it is unclear as to the rate and force needed for cardio-pulmonary resuscitation (CPR) in the severely hypothermic patient. The hypothermic chest wall and myocardium are stiffer and less compliant. CPR does seem to produce improved survival in the cardiac arrest patient, so the old adage should apply that no one is dead until they are warm and dead . CPR should continue until the patient is rewarmed to 30 – 32 C, at which point renewed attempts at defibrillation and resuscitation with Advanced Life Support medications are undertaken [9]. Cardiac Arrest secondary to Hypothermia in children may warrant prolonged resuscitation and expert consultation prior to termination of resuscitation as children may have an increased ability to tolerate and recover from hypoxic brain injury. The lowest recorded neurologically intact survival following hypothermic cardiac arrest (in this case associated with immersion) is 13.7 C (Reference 2005 American Heart Association Guidelines for Cardiopulmonary Resuscitation and Emergency Cardiovascular Care. Circulation 2005; 112:IV1)

Disposition

Patients with mild hypothermia who respond to passive rewarming are usually able to be discharged from the Emergency Department (ED). Patients with severe hypothermia need to be admitted to a HDU/ITU setting. Patients with moderate hypothermia are the most difficult. Their disposition will depend on the patients age, comorbid factors, social situation and response to ED treatment.

Conclusion

Hypothermia is frequently an easy diagnosis to make but it is a condition which must be considered in the setting of altered mental status, medical illness, multiple trauma patients, and poisoning cases. The treatment strategy will depend on the severity of the hypothermia. Mild hypothermia patients can be treated with passive rewarming, whereas moderate to severe hypothermia patients need for active and invasive methods to achieve rewarming. Active core rewarming should be reserved for those patients with severe and profound hypothermia who have not responded to less aggressive measures. The sequelae of hypothermia, including cardiac arrythmias should be treated with rewarming and antiarrthmic drugs are rarely needed.