Position Statement of the Hong Kong Society of Critical Care Medicine
The Use of Hypothermia After Cardiac Arrest
Dr LEUNG Kit Hung, Anne (Consultant, Queen Elizabeth Hospital, Hong Kong)
On behalf of the Hong Kong Society of Critical Care Medicine
This Position Statement was endorsed by the HKSCCM Council in the 20th Council Meeting on 19th July 2011

Recommendations on the use of therapeutic hypothermia (32-34 oC) after cardiac arrest
1. Cardiac arrest patients who present with ventricular fibrillation or nonperfusiong ventricular tachycardia, are resuscitated to hemodynamic stability, but remain unresponsive should receive therapeutic hypothermia (32oC to 34oC for 12-24 hours)
2. Therapeutic hypothermia (32oC to 34oC for 12-24 hours) should be considered for rhythms other than VF ie cardiac arrest patients who present with asystole or pulseless electrical activity; felt to be of cardiac origin and are resuscitated to hemodynamic stability, but remain unresponsive.
3. Therapeutic hypothermia should be initiated as soon as possible

4. Patient who are successfully cooled within eight or more hours of return of spontaneous circulation may still benefit from this therapy
5. Shivering should be prevented by use of neuromuscular blocker and sedation.
6. Patient undergoing therapeutic hypothermia should have their core temperature continuously or frequently monitored by rectal, esophageal, bladder and pulmonary artery temperature. The device must be designed to measure temperatures in the hypothermic range.
7. Normothermia should be restored slowly and rebound hyperthermia should be avoided
8. Until further data are available, therapeutic hypothermia should not be used for patients with severe cardiogenic shock or life-threatening arrhythmias or primary coagulopathy
9. Patient under 18 years of age and pregnant women may benefit from this therapy, but its role is unproven. Consideration should be on a case-by-case basis (Ref 1)

Mechanism of Action of Therapeutic Hypothermia
Induced hypothermia decreases the metabolic rate by 6-7% for every 1 oC decrease in temperature, hence improved the oxygen supply and reduces oxygen consumption in the ischaemic brain. There are three phases of cerebral injury after hypoxic insult: early, intermediate and late. Besides, therapeutic hypothermia is considered to be neuroprotective by acting at each of the three stages of injury.

Cardiac arrest immediately decreases cerebral blood flow despite ongoing consumption of oxygen, ATP and glucose. In this early stage, hypothermia decrease energy utilization, consumption of oxygen, and glucose

The intermediate or latent phase occurs in the hours post-arrest. Excitatory amino acids and glutamate are released in the brain, activating cytotoxic cascades including free radicals and nitric oxide. Hypothermia decrease the release of excitatory amino acids and other neurotoxic mediators. Cooling lessens nitric oxide production, and delays the peak of nitric oxide.

The latent phase of cerebral injury can occur up to 24 hours after cardiac arrest. At this stage, the blood-brain barrier breaks down and cerebral edema worsens; seizures and neuronal death may occur. Hypothermia slows the deterioration of the blood-brain barrier, and decrease cerebral edema

Background and scientific evidence 
-  Induction of moderate hypothermia (28-32oC) has been successfully used since 1950s to protect the brain against global ischemia that occurs during some open heart surgeries
- In animal studies, induction of hypothermia after return of spontaneous circulation (ROSC) has been associated with improved functional recovery and reduced cerebral histological deficits.
- In 2002, two prospective randomized clinical trials were published and provide evidence to support its use in selected groups of patients. In summary, therapeutic hypothermia reduced mortality by 41% and increase functional survivors by 55%
- A European study found that 75 of the 136 patients (55%) in the hypothermia group had a favourable neurological outcome (able to live independently and work at least part of the time) at 6 months compared with 54 of 137 (39%) in the normothermia group
- An Australian study found that 21 of 43 patients (49%) treated with hypothermia had good neurological function at discharge (to home or rehabilitation facility) compared with 9 of 34% (26%) in the normothermia group.
- In 2003, the International Liaison Committee on Resuscitation recommended treatment with therapeutic hypothermia
- In 2005 the American Heart Association recommended that unconscious adult patients with spontaneous circulation after an out-of-hospital cardiac arrest should be cooled to 32-34oC for 12-24 hours when the initial rhythm was Ventricular Fibrillation (VF)

Who is going to benefit from hypothermia?
- Age above 18
- Cardiac arrest with return of spontaneous circulation. Initial rhythm preferably ventricular fibrillation or pulseless ventricular tachycardia
- Remained unresponsive with Glasgow Coma Scale <10 or not responsive to verbral commands after return of spontaneous circulation ( ROSC)
- Patient should have no refractory shock (MAP<60 mmHg or SBP<90mmHg) or persistent hypoxia ( oxygen saturation <85%)

Who should not get hypothermia?
- Coma unrelated to arrest eg head trauma, hypovolemic shock, stroke or sepsis
- Patient’s condition not suitable for therapeutic hypothermia eg Severe cardiogenic shock , active bleeding or pre-existing coagulopathy
- Received CPR for more than 45 minutes (Ref 4)
- Do Not Resuscitate status
- Pregnancy ( may need to discuss on case-to-case basis)

How to apply therapeutic hypothermia?
- Methods: Combination of methods could be used to achieve the goal. Example: Ice Packs, Cooling Mattresses, Cooling Blanket, Cooling Catheter ( closed loop), Ice cold IV Saline
- Timing and Depth of Cooling: As soon as possible, down to 32-34 for 12-24 hrs
- Monitoring: Routine monitoring plus continuous temperature monitoring and monitor for shivering. Neuromsucular blockade may be used if shivering continued

What are the potential side effects during therapeutic hypothermia?
- Arrhythmia
- Infection
- Coagulopathy
- Hyperglycemia

1. Canadian Association of Emergency Physicians. Positional statement and Guidelines For the use of Hypothermia after cardiac arrest May 2005
2. Heat and Stroke Foundation of Canada Positional statement on Therapeutic hypothermia after cardiac arrest Dec 2004
3. Part 9: Post-cardiac Arrest Care of 2-1- American Heart Association Guidelines for Cardiopulmonary Resuscitation and Emergency Cardiovascular care. Circulation 2010”122[Suppl 3]:S768-S786
4. SPARC (Strategies in Post Arrest Resuscitation Care) Network Project of the PreHospital and Transport Medicine Research Program, University of Toronto
5. The Hypothermia after Cardiac Arrest study Group. Mild therapeutic hypothermia to improve neurologic outcome after cardiac arrest. N Engl J Med 2002:346:549-556
6. Treatment of comatose survivors of out-of-hospital cardiac arrest with induced hypothermia. N Engl J Med 2002;346:557-63