How Therapeutic Hypothermia is Achieved
Therapeutic hypothermia is achieved through a three-phase process (induction, maintenance, and rewarming) using multiple cooling modalities combined with sedation and neuromuscular blockade to prevent shivering, with continuous core temperature monitoring to maintain target temperatures of 32°C to 34°C. 1
Cooling Methods
External/Surface Cooling Techniques
- Servo-controlled cooling blankets positioned under or above the patient provide the primary method for surface cooling 1
- Ice packs applied to the groin, axillae, and neck offer simple but slower cooling 1
- Cold-water baths or wet towels combined with fanning can reduce core temperature 1
- Lowering room temperature serves as an adjunctive measure 1
- Cooling helmets represent another external option, though less commonly used 1
Surface cooling methods are simple to use but achieve slower cooling rates compared to invasive techniques 1. The "forced air" method using cooling blankets with cool air flow (10°C) has demonstrated feasibility in awake patients 2.
Intravenous Cooling
- Cold crystalloid infusion at 4°C administered as 30 mL/kg over 30 minutes significantly reduces core temperature without causing pulmonary edema 1
- Continuous infusions are relatively safe for lowering temperature 1
- Avoid intravenous push administration of cold saline due to risk of profound bradycardia 1
Invasive Cooling Methods
- Intravascular heat exchange devices enable rapid cooling and precise temperature control but are too invasive for prehospital or emergency department use 1
- Endovascular cooling catheters are not used in pediatrics due to large diameter and thrombosis risk, though may be considered for older children 1
- Peritoneal and pleural lavage with cold fluids is possible but not generally used 1
- Extracorporeal cooling methods are efficient but too invasive for most settings 1
Body surface area (BSA) is the most important determinant of effective cooling—larger patients are more difficult to cool and maintain in therapeutic range 3.
Temperature Monitoring
Continuous core temperature monitoring is mandatory during therapeutic hypothermia 1.
Monitoring Methods (in order of reliability):
- Bladder temperature probe - reliable for continuous monitoring 1
- Esophageal temperature probe - accurate core measurement 1
- Rectal temperature probe - reliable core measurement 1
- Pulmonary artery catheter - if already in situ 1
- Tympanic temperature measurements - less reliable, especially when intermittent 1
Temperature should be continuously monitored by the cooling device to enable active control and maintain stable temperature 1.
Shivering Prevention and Management
Shivering must be prevented as it leads to warming and increases oxygen consumption 1.
Pharmacological Management:
- Neuromuscular blockers combined with sedation represent the standard approach used in definitive trials 1
- Meperidine serves as the primary agent for shivering control 2, 3
- Buspirone (30-60 mg PO) can be added to the antishivering regimen 4
- Magnesium sulfate (4-6 g IV bolus followed by 1-3 g/hour infusion) increases cooling rate and improves comfort during surface cooling 4
- Surface counter-warming combined with pharmacological agents effectively controls shivering 3
The combination of meperidine, buspirone, and surface counter-warming minimally influences cooling effectiveness when properly implemented 3.
Three Phases of Temperature Management
Phase 1: Induction
- Target temperature should be achieved as quickly as possible, though optimal interval remains unknown 1
- Cooling should be initiated as soon as possible after ROSC but appears successful even if delayed 4-6 hours 1
- In clinical studies, the interval between ROSC and attainment of 32°C-34°C had an interquartile range of 4-16 hours 1
- Older patients cool more quickly than younger patients 3
Phase 2: Maintenance
- Maintain target temperature of 32°C-34°C for 12-24 hours in adults 1
- In pediatrics, maintain for 120 hours (5 days) based on THAPCA trials 1
- For normothermia/fever prevention, maintain temperature ≤37.5°C using pharmacological measures (acetaminophen), uncovering the patient, lowering ambient temperature, and cooling devices if temperature exceeds 37.7°C 1
Phase 3: Rewarming
- Normothermia should be restored slowly as rebound hyperthermia is common and must be avoided 1
- The rate of rewarming requires careful control to prevent complications 1
Metabolic and Physiological Monitoring
Electrolyte Management
- Hypokalemia, hypophosphatemia, hypomagnesemia, and hypocalcemia develop during hypothermia and may precipitate arrhythmias 1
- Close monitoring and treatment of electrolyte imbalances are required, especially during induction 1
Glucose Control
- Hypothermia decreases insulin sensitivity, placing patients at risk for hyperglycemia 1
- Tight glucose control is necessary during the cooling period 5
Hemodynamic Monitoring
- Bradycardia and hypotension have been observed during hypothermia 1
- Careful fluid balance monitoring is essential 5
Common Pitfalls and Complications
Avoid These Errors:
- Do not allow core temperature to fall considerably below 32°C as complications (arrhythmias, infection, coagulopathy) increase significantly 1
- Do not use rapid IV push of cold saline due to bradycardia risk 1
- Do not use endovascular catheters in young children due to thrombosis risk 1
- Do not allow rapid rewarming as rebound hyperthermia worsens outcomes 1