Therapeutic hypothermia
| Therapeutic hypothermia |
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Associate Editor: Cafer Zorkun, M.D., Ph.D. [2]
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Overview
Brain temperature during the first 24 hours after resuscitation from cardiac arrest has a significant effect on survival and neurological recovery. Fever (Tmax) during the first 48 hours is associated with a decreased chance of good neurological recovery (OR 2.26 [1.24, 4.12] for each 1°C over 37°C)[1]. Cooling to 32-34°C for 24 hours decreases chance of death (OR 0.74 [0.58, 0.95]) and increases chance of good neurological recovery (OR 1.40 [1.08,1.81])[2]. Cooling to 32-34°C for 12 hours increases chance of good neurological recovery (OR 2.65 [1.02, 6.88])[3].
Mechanism of Effects
- Hypothermia activates the sympathetic nervous system causing vasoconstriction and shivering. Shivering increases O2 consumption by 40-100%. Sedatives, opiates, and neuromuscular blockers can counteract these responses and enhance the effectiveness of active cooling. However, initiating paralysis in a patient that is already hypothermic should be avoided because it can result in a precipitous drop in core body temperature. Elderly patients will cool more quickly than younger or obese patients. [4]
- Hypothermia shifts the oxyhemoglobin curve to the left may result in decreased O2 delivery. However, the metabolic rate is also lowered, decreasing O2 consumption / CO2 production, cardiac output and cerebral blood flow. Ventilator settings may need to be adjusted due to decreased CO2 production, using temperature corrected blood gases. [5]
- Hypothermia initially causes sinus tachycardia, then bradycardia. With temp <30º C there is an increased risk for arrhythmias. With temp <28º C there is an increased risk for ventricular fibrillation. The severely hypothermic myocardium (<30°C) is less responsive to defibrillation and medications. Therefore it is extremely important to keep temp >30ºC.
- Hypothermia can induce coagulopathy which is treatable with platelets and FFP.
- Hypothermia-induced diuresis is to be expected and should be treated aggressively with fluid and electrolyte repletion. Magnesium, phosphorus and potassium should be monitored closely and maintained in the normal (because it will rebound to very high) range.
- Decreased insulin secretion and sensitivity leads to hyperglycemia, which should be treated aggressively.
- Re-warming too rapidly can cause vasodilation, hypotension, and rapid electrolyte shifts.
Therapeutic goals
Therapeutic hypothermia is practiced in many tertiary hospitals. The treatment goals followed in most of them are:
- Goal temperature- 32-34 degree celsius
- Active cooling for 24 hours.
- Rapid achievement of cooling temperature (within 3-4 hours)
- Rewarming
- Slow rewarming (may take 8-10 hours) is preferred over rapid rewarming as the latter may cause dilatation of the vessel and result in hypotension
- It can be achieved by discontinuation of methods used for cooling like cooling blankets, ice and drugs
- Continuous blood pressure monitoring should be done to avoid development of hypotension
- It should be started 24 hours after the time of initiation of cooling
- Slow rewarming @ 0.3-0.5ºC/hr
- Target rewarming temperature 36°C
Cooling techniques
- Ice packs
- Head, neck, axilla, and groin are preferred sites (these areas have good heat exchange)
- Advantages
- Inexpensive
- Disadvantage
- Difficulties in achieving and maintaining rapid rates of cooling
- Rate of cooling varies between patients
- Blankets or surface heat-exchange device and ice
- Similar problems of achieving and maintaining temperature is there as seen with ice packs.
- During cooling phases both blankets, the cooling device's heat-exchange pads and ice packs are applied. Ice packs are removed once target temperature is achieved. The temperature is maintained with blanket or heat exchange pads
- Surface cooling helmet
- The helmet has a solution of aqueous glycerol that helps in achieving hypothermia.
- Disadvantage - Slow attainment of target temperature
- Catheter based internal cooling methods / Endovascular heat-exchange catheters
- Devices available: Celsius Control System and Cool Line System
- Advantage
- Both endovascular cooling and rewarming are faster
- Good at maintaining target temperature
- Placed in femoral vein
- Low rate of complications
- Minimization of shivering and need for chemical paralysis of the patient
- Infusion of cold fluids
- Advantage - Rapid cooling
- Disadvantage - rate of cooling is unpredictable
- Achieved by infusion of normal saline or lactated Ringer solution
Eligibility Criteria for Post-Cardiac Arrest Therapeutic Hypothermia
- Meets eligibility criteria for Post-Cardiac Arrest Care Pathway
- Comatose at enrollment with a Glasgow Coma Motor Score <6 pre-sedation (i.e., patient doesn’t follow commands)
- No other obvious reasons for coma
- No uncontrolled bleeding
- Hemodynamically stable with no evidence of:
- Uncontrollable dysrhythmias
- Severe cardiogenic shock
- Refractory hypotension (MAP <60 mm Hg) despite preload optimization and use of vasoactive medications
- No existing, multi-organ dysfunction syndrome, severe sepsis, or comorbidities with minimal chance of meaningful survival independent of neurological status
Relative Contraindications for Therapeutic Hypothermia:
- Prolonged arrest time (> 60 minutes)
- Thrombocytopenia or other coagulopathies (hypothermia may impair the clotting factors)
- Pregnancy (Therapeutic hypothermia can potentially be performed on pregnant female in consultation with OB/Gyn)
- Major surgery within 14 days (it may increase risk of infections and bleeding in this group of patients)
- Sepsis and other systemic infections (decreased immune function due to hypothermia)
- Coma from other causes like drug intoxication
- Patients who have signed a 'Do not resuscitate'(DNR)
References
- ↑ Zeiner A, Holzer M, Sterz F, et al. Hyperthermia after cardiac arrest is associated with an unfavorable neurologic outcome. Arch Intern Med. Sep 10 2001;161(16):2007-2012.
- ↑ Hypothermia after Cardiac Arrest Study G. Mild therapeutic hypothermia to improve the neurologic outcome after cardiac arrest.[see comment][erratum appears in N Engl J Med 2002 May 30;346(22):1756]. New England Journal of Medicine. Feb 21 2002;346(8): 549-556.
- ↑ Bernard SA, Gray TW, Buist MD, et al. Treatment of comatose survivors of out-ofhospital cardiac arrest with induced hypothermia.[see comment]. New England Journal of Medicine. Feb 21 2002;346(8):557-563.
- ↑ Sunde K, Pytte M, Jacobsen D, Mangschau A, Jensen LP, Smedsrud C, et al. Implementation of a standardised treatment protocol for post resuscitation care after out-of-hospital cardiac arrest. Resuscitation 2007;73:29-39
- ↑ Kim F, Olsufka M, Longstreth WT Jr, Maynard C, Carlbom D, Deem S, et al. Pilot randomized clinical trial of prehospital induction of mild hypothermia in out-of-hospital cardiac arrest patients with a rapid infusion of 4 degrees C normal saline. Circulation 2007;115:3064-70