Therapeutic hypothermia is a medical treatment in which a patient’s body temperature is deliberately lowered to protect the brain after a life-threatening event, most commonly cardiac arrest. The technique is now formally called targeted temperature management (TTM), a broader term that reflects how clinicians may cool a patient below normal, maintain normal temperature, or simply prevent fever, depending on the situation. The core goal is the same: control body temperature precisely to give the brain its best chance of recovery.
How Cooling Protects the Brain
When the heart stops and blood flow to the brain is interrupted, brain cells lose their primary energy supply within minutes. Even after circulation is restored, the damage isn’t over. Roughly 8 to 24 hours later, a second wave of injury begins. Toxic byproducts accumulate inside cells: excess calcium, free radicals, and a surge of excitatory chemical messengers like glutamate. This cascade, called secondary energy failure, can destroy neurons that survived the initial event.
Cooling the body slows this process at several levels. Research published in the Journal of Cerebral Blood Flow & Metabolism found that hypothermia reduced concentrations of excitatory brain chemicals by 11 to 35 percent and boosted the brain’s emergency energy reserves by about 20 percent compared to measurements taken after rewarming. Because signaling between neurons accounts for roughly one-third to one-half of the brain’s total energy use, dialing down that activity dramatically cuts the brain’s energy demand at exactly the moment when energy is scarce. The result is less cell death, less swelling, and a lower likelihood of seizures during the critical hours after injury.
Who Receives This Treatment
The most common use is in adults who remain unconscious after their heart is restarted following cardiac arrest. If a patient regains a pulse but doesn’t respond to verbal commands, temperature control is a standard part of post-resuscitation care. The treatment is also used in newborns who experience oxygen deprivation during birth, a condition called hypoxic-ischemic encephalopathy. In both populations, the logic is the same: the brain is vulnerable to that secondary wave of damage, and cooling buys time for cells to stabilize.
Target Temperatures and Duration
Earlier protocols called for cooling patients to between 32°C and 34°C (about 90°F to 93°F). More recent trials, including those reviewed in the 2025 American Heart Association guidelines, compared that range to milder targets near 36°C (96.8°F) and found no clear difference in survival between the two. Importantly, none of the trials found that colder temperatures produced worse outcomes, so the confidence intervals still leave room for a possible benefit of deeper cooling.
What the guidelines do emphasize is duration. Temperature control should be maintained for at least 36 hours in adults who remain unresponsive after cardiac arrest. One trial compared 36 total hours of temperature control to 72 hours and found no difference in outcomes, suggesting 36 hours represents the minimum effective window without strong evidence that longer is better.
How Cooling Is Done
There are two broad approaches. Surface cooling uses water-circulating blankets, gel pads, or even ice packs placed on the skin. Some surface devices include automated temperature feedback, constantly measuring the patient’s core temperature and adjusting the cooling output; simpler setups rely on manual monitoring. Intravascular cooling uses a catheter placed in a large vein that circulates cold saline internally. These devices almost always include automated feedback, which helps maintain a steady target and avoid unintentional overcooling, a recognized problem with basic ice-pack methods.
Both approaches can achieve and maintain the target temperature. The choice often depends on what’s available at the hospital and whether the clinical team needs especially precise control.
The Rewarming Phase
Bringing a patient back to normal temperature is just as important as the cooling itself, and rushing it can undo the benefit. Current guidelines recommend rewarming at a rate of 0.25°C to 0.5°C per hour. At that pace, a patient cooled to 33°C would take roughly 6 to 12 hours to return to 37°C.
Animal and human studies have consistently shown that rapid rewarming raises intracranial pressure, increases the brain’s metabolic rate before it’s ready, and worsens cognitive outcomes. One study of cardiac arrest patients found a trend toward poor neurologic outcomes when rewarming exceeded 0.5°C per hour. Slower rates allow cerebral blood flow and oxygen delivery to readjust gradually, protecting tissue that is still fragile from the original injury.
Risks and Side Effects
Cooling the entire body affects more than the brain. The most common side effects include:
- Shivering. The body’s natural defense against cold. Shivering raises metabolic demand and oxygen consumption, which directly opposes the purpose of cooling. Managing it with sedation or other medications is a routine part of the protocol.
- Electrolyte shifts. Potassium, magnesium, and calcium levels can fluctuate as the body cools, requiring close monitoring and correction.
- Blood sugar elevation. Hypothermia causes insulin resistance, which drives blood sugar up. High blood sugar, in turn, raises infection risk.
- Suppressed immune response. Cooling dampens the body’s inflammatory system. While that helps limit brain swelling, it also makes the patient more vulnerable to infections like pneumonia.
- Heart rhythm changes. Arrhythmias can occur, particularly at lower target temperatures, and coagulation problems may develop because cold slows the clotting process.
These complications are well understood and actively managed in intensive care. For most patients who meet the criteria for TTM, the brain-protective benefits outweigh the risks of controlled cooling.
What the Patient Experience Looks Like
Patients receiving TTM are sedated and on a ventilator throughout the process. They are not awake or aware during cooling. Family members will typically see cooling pads or blankets covering the patient’s body, along with standard ICU monitors. The full cycle, from induction through maintenance and controlled rewarming, spans roughly two to three days. After rewarming, the medical team gradually reduces sedation and begins neurological assessments to evaluate how much brain function has been preserved. Recovery timelines from that point vary widely depending on how long the brain went without oxygen and the patient’s overall health before the event.