What is therapeutic hypothermia?

Therapeutic Hypothermia: Definition and Clinical Applications

Therapeutic hypothermia is the intentional reduction of core body temperature to 32°C to 34°C for neuroprotective purposes, primarily used in comatose survivors of cardiac arrest to improve neurological outcomes and reduce mortality. 1

Definition and Mechanism of Action

Therapeutic hypothermia (TH), also known as targeted temperature management (TTM), involves the controlled lowering of a patient's core body temperature to achieve neuroprotection. This therapeutic approach works through several mechanisms:

• Reducing brain metabolism (approximately 6% reduction for every 1°C decrease in temperature)
• Lowering intracranial pressure
• Suppressing chemical reactions associated with reperfusion injury
• Reducing free radical production and excitatory amino acid release
• Decreasing calcium shifts that lead to mitochondrial damage
• Inhibiting apoptosis (programmed cell death)
• Limiting inflammatory responses in brain tissue 1

Clinical Applications

Primary Indication: Post-Cardiac Arrest

The strongest evidence supports the use of therapeutic hypothermia in:

• Unconscious adult patients with spontaneous circulation after out-of-hospital cardiac arrest when the initial rhythm was ventricular fibrillation (VF) 1

In these patients, cooling to 32°C-34°C for 12-24 hours has demonstrated:

• Improved neurological outcomes (55% favorable outcome vs. 39% in normothermia group)
• Reduced mortality (41% vs. 55% in normothermia group) 1

Other Potential Applications

While less established, therapeutic hypothermia may also be beneficial in:

• Other cardiac arrest rhythms
• In-hospital cardiac arrest
• Traumatic brain injury
• Stroke
• Subarachnoid hemorrhage
• Neonatal peripartum encephalopathy 2, 3

Implementation Protocol

1. Patient Selection:

   • Primarily comatose survivors of out-of-hospital VF cardiac arrest
   • Consider for other rhythms or in-hospital arrest based on clinical judgment

2. Cooling Methods:

   • External cooling: specialized cooling mattresses, ice packs to head and torso
   • Target temperature: 32°C-34°C
   • Aim to reach target within 4 hours of return of spontaneous circulation (ROSC)

3. Maintenance:

   • Maintain target temperature for 12-24 hours
   • Monitor core temperature via bladder probe or pulmonary artery catheter
   • Provide sedation and often neuromuscular blockade to prevent shivering

4. Rewarming:

   • Passive or active rewarming
   • Slow rewarming is essential (typically starting at 18-24 hours)
   • Avoid rapid rewarming which can cause hemodynamic instability 1

Potential Complications

Therapeutic hypothermia is associated with several potential adverse effects:

• Increased risk of pneumonia (NNH 12)
• Bleeding complications (NNH 14)
• Sepsis (NNH 16)
• Arrhythmias
• Electrolyte disturbances
• Hyperglycemia
• Coagulopathy
• Decreased cardiac index and increased systemic vascular resistance 1

Important Considerations

• Patient Selection: The evidence is strongest for a specific patient population (out-of-hospital VF arrest), and benefits must be weighed against risks for other indications
• Timing is Critical: The sooner cooling begins after ROSC, the better the outcomes
• Avoid Hyperthermia: Even when full therapeutic hypothermia is not implemented, avoiding fever in post-arrest patients is important
• Monitoring: Close monitoring for complications is essential during the cooling and rewarming phases

Pitfalls to Avoid

1. Inappropriate Patient Selection: Not all post-arrest patients benefit equally; consider comorbidities and arrest characteristics
2. Delayed Implementation: Efficacy decreases with time; initiate cooling as soon as possible
3. Inadequate Temperature Control: Precise temperature management requires dedicated equipment and protocols
4. Rapid Rewarming: Can cause hemodynamic instability and negate benefits
5. Failure to Anticipate Complications: Proactive management of potential side effects is essential

Therapeutic hypothermia represents an important neuroprotective strategy with proven benefits in specific clinical scenarios, particularly following cardiac arrest with ventricular fibrillation. When properly implemented with attention to patient selection, timing, and careful monitoring, it can significantly improve neurological outcomes and survival.