THERAPEUTIC HYPOTHERMIA FOLLOWING CARDIAC ARREST
Introduction
Therapeutic hypothermia has been shown to reduce mortality and improve neurological outcomes for patients who remain comatose following out of hospital cardiac arrest caused by primary VF or pulseless VT. 1,2
Mild induced hypothermia therapy is now recommended as standard treatment in these cases.3, 4
Such cooling may also be beneficial for other rhythms or following in-hospital cardiac arrest.
Inclusion Criteria
Patients who have suffered a primary VF or pulseless VT cardiac arrest and:
●Return of a spontaneous circulation within 60 minutes.
●The patient remains unconscious and ventilated.
●There is no other apparent cause for coma.
Exclusion Criteria
●Cardiac arrest of a non primary cardiac cause.
●Limitation of medical treatment or advanced care plan that precludes further resuscitation.
Although original studies excluded a range of situations including, children, pregnant women, patients with coagulopathy and ongoing arrhythmias current expert opinion would not exclude these scenarios from therapeutic hypothermia management.
Original studies further stated that there should be a return of spontaneous circulation within 60 minutes of cardiac arrest before therapeutic hypothermia management could be considered. Current expert opinion would now consider this a relative contraindication.
Patients in cardiogenic shock were similarly excluded in original studies; however subsequent studies have shown benefit from therapeutic hypothermia management in these cases as well. 5
Hypothermia Induction
Early induction of hypothermia methods relied on body surface cooling via ice packs, however this proved a relatively slow means by which to lower core body temperature, (about 10 C per hour). Cooling should occur as soon as possible after cardiac arrest in order to improve outcomes. Safe and rapid cooling can be achieved by the rapid infusion of a large volume of ice cold crystalloid fluid (at 40 C). 6 In order to maintain mild hypothermia the normal physiological responses to hypothermia (such as shivering) must be suppressed. This is achieved by a combination of muscular paralysis, ongoing sedation and mechanical ventilation.
Hypothermia induction is thus achieved by:
●Muscle paralysis with Vecuronium 20 mg IV with ongoing mechanical ventilation. This is a large initial dose which is still safe and will provide around one hour of initial paralysis.
●Infusion of 40 mls/kg of cold (40 C) crystalloid (normal saline or Hartmann’s solution) as a rapid bolus over 20-30 minutes.
Cooled Hartmann’s and saline solutions should be kept in an Emergency department fridge at all times.
●The placement of body surface ice packs. These are best placed over regions of large superficial venous complexes, such as the neck, axillae and groin regions. Ice packs should not be placed directly on the skin, (to avoid freezing injury to tissues). Packs should be wrapped first in towels or pillowcases.
Alternatively specifically designed cooling blankets may be used if available instead of ice packs.
Hypothermia management
1.Once induction has occurred hypothermia should be maintained for a period of 24 hours.
2.The temperature range to be maintained is 32.5 0 C to 33.5 0 C, (ideally 33 0 C)
3.Ongoing sedation with IV midazolam infusion
4.Maintenance of paralysis with vecuronium.
5.Monitoring
All patients undergoing induced hypothermia must have:
●Ongoing core temperature monitoring. Indwelling urinary catheters with temperature probes are recommended.
●Continuous ECG monitoring.
●Central venous pressure monitoring is required for all these patients. A femoral line is a good option; they are easy to place and avoid possible cardiac irritation in hypothermic patients.
●A radial artery arterial line should be placed. If this is not possible a femoral arterial line is an alternative.
●Careful monitoring of potassium levels, as these may fall during induction. Levels will need to be checked 1-2 hours initially. Be aware that during rewarming potassium levels will tend to rise.
●Note that the production of CO2 is decreased by up to 30 % when core temperature is 33 0 C, therefore the ventilator rate may need to be decreased down to only 6-8 breaths per minute. This should be guided by ABG analysis, (ETCO2 recordings may be misleading in cases of low or variable cardiac output as well as hypothermia)
6.Temperature management
Should the core temperature fall below 32.5 0 C:
●Remove ice packs.
●Withhold any neuromuscular blocking agents.
●More active measures such as warming blanket devices should not be necessary unless hypothermia becomes severe.
Should the core temperature rise above33.5 0 C:
●Reapply ice packs (if not already on)
●Administer further neuromuscular blocking agent, if shivering occurs.
●Increase sedation.
Complications of Induced Hypothermia
Possible complications of induced hypothermia include:
1.Hypokalemia.
2.Arrhythmias
●Most arrhythmias in mild hypothermia will be benign and will require no specific treatment. Slow AF and sinus bradycardia will be the most common.
3.Excessive hypothermia, (less than 32.5 0 C)
4.Coagulopathies
●This is not usually clinically significant and many patients will require anticoagulation as a necessary part of the management of associated myocardial ischemia
If the patient has evidence of myocardial infarction
●Thrombolysis is not contra-indicated during cooling.
●Aspirin may be administered via a nasogastric tube.
5.Glucose
●Both hypoglycaemia and hyperglycaemia may be seen in hypothermia. Both are detrimental in the setting of anoxic brain injury. If the glucose level is > 10 mmol/L commence an insulin infusion.
6.Sepsis: if sepsis is suspected take appropriate cultures (tracheal aspirate, urine and blood) and commence board spectrum antibiotics. This is not usually a problem however for mild hypothermia in periods of less than 24 hours.
Rewarming
Active rewarming should commence after 24 hours of induced hypothermia.
Rewarming should occur slowly to avoid rebound hyperthermia, which should be avoided.
The recommended rate for rewarming is 3 degrees over 12 hours. Rewarming can be achieved by:
●Cessation of paralysis
●Removal of ice packs
If rewarming occurs too quickly, ice packs can be temporarily reapplied. Midazolam sedation can be used to suppress shivering that may be causing the patient to rewarm too quickly.
If rewarming is occurring too slowly more active measures such as warmed and humidified oxygen and warming blanket devices may be used.
References
1.Bernard S, et al “Treatment of Comatose Survivors of Out of Hospital Cardiac Arrest with Induced Hypothermia”, NEJM, vol 346, no.8 February 21, 2002, p557-563.
2.The Hypothermia after Cardiac Arrest Study Group, “Mild Therapeutic Hypothermia to Improve the Neurologic Outcome after Cardiac Arrest”, NEJM, NEJM, vol 346, no.8 February 21, 2002, p. 549-556.
3.Therapeutic Hypothermia After Cardiac Arrest, An Advisory Statement by the Advanced Life Support Task Force of the International Liaison Committee on Resuscitation (ILCOR). Circulation July 8: 2003, p.118-121.
4.Australian Resuscitation Council Guideline 11.9 February 2006.
5.Oddo M et al. From evidence to clinical practice: Effective implementation of therapeutic hypothermia to improve patient outcome after cardiac arrest. Crit Care Med 2006 vol 34 no7: 1865-1873
6.Bernard S et al “Induced Hypothermia Using Large Volume, Ice Cold Intravenous Fluid in Comatose Survivors of Out of Hospital Cardiac Arrest: a preliminary report”, Resuscitation, 56 (1): 9-13, January 2003
Dr J. Hayes / Dr Peter Papadopoulos.
Dr G. Duke, Director Critical Care Services
Dr J. Briedis, Director Anaesthetics Department
Julie Considine, Clinical Nurse Educator, Emergency Department.
Reviewed 2 April 2007