Therapeutic hypothermia is a medical treatment that deliberately lowers a person’s body temperature to protect the brain from damage after a life-threatening event like cardiac arrest or oxygen deprivation at birth. By cooling the body a few degrees below its normal 37°C (98.6°F), the treatment slows harmful processes that would otherwise destroy brain cells in the hours following the injury. It’s now a standard part of care in hospitals worldwide for specific patients who meet certain criteria.
Why Cooling Protects the Brain
When the brain loses its blood supply, even briefly, a cascade of destructive events begins once blood flow returns. Cells release toxic levels of chemical messengers that overstimulate neighboring neurons. Free radicals build up, triggering oxidative damage. Inflammatory signals flood the area, and cells begin a programmed self-destruction process. All of this can continue for hours or days after the original event, meaning much of the brain damage happens after the crisis itself, not during it.
Lowering body temperature interrupts nearly every step in this cascade. Brain metabolism drops by roughly 5% to 10% for every degree Celsius of cooling, which reduces the brain’s demand for oxygen and energy at a time when both are scarce. Cooling also lowers the buildup of lactic acid from oxygen-starved metabolism, reduces the flood of excitatory chemical messengers like glutamate, cuts down free radical production, and suppresses the inflammatory response. Perhaps most importantly, it blocks multiple pathways of programmed cell death, giving damaged but still-viable neurons a chance to recover rather than self-destruct. It can even suppress seizure-like electrical activity that sometimes follows brain injury.
Cardiac Arrest in Adults
The most well-known use of therapeutic hypothermia is for adults who survive cardiac arrest but remain unconscious after their heartbeat is restored. Two landmark trials published in The New England Journal of Medicine in 2002 established that cooling these patients significantly improved their chances of neurological recovery. Those studies led to cooling becoming a standard recommendation in resuscitation guidelines.
Current American Heart Association guidelines recommend controlling the body temperature of unresponsive adult cardiac arrest survivors, targeting a range between 32°C and 36°C (roughly 90°F to 97°F) for at least 24 hours. More recent evidence from the large TTM2 trial has also shown that simply preventing fever by keeping the temperature below 37.5°C can be a reasonable approach for certain patients, particularly those with out-of-hospital cardiac arrest from a cardiac or unknown cause. This shift reflects ongoing refinement in how aggressively clinicians need to cool, though the core principle of preventing the brain from overheating after arrest remains firmly supported.
Newborns With Birth-Related Brain Injury
Therapeutic hypothermia is also the standard of care for newborns born at 36 weeks or later who experience moderate to severe brain injury from oxygen deprivation during or around delivery, a condition known as hypoxic-ischemic encephalopathy. Without treatment, about 62% of these infants will either die or develop moderate to severe disabilities by 18 to 22 months of age. Cooling reduces that rate to roughly 41%, representing a 26% relative reduction in mortality alone.
The treatment window is narrow. After the initial injury and restoration of blood flow, there is a latent period of about one to six hours during which the brain’s energy systems partially recover before a second wave of damage sets in. Cooling must begin within this window, ideally as soon as possible within the first six hours of life. The infant’s core temperature is lowered to between 33°C and 34°C (about 91°F to 93°F) and held there for 72 hours. Babies born before 35 weeks are generally not candidates, as the risks of cooling in very premature infants outweigh the known benefits.
How Cooling Is Delivered
Hospitals use several methods to lower and maintain body temperature, broadly divided into surface cooling and internal cooling. Surface methods include water-circulating cooling blankets, adhesive cooling pads placed on the skin, and ice packs positioned at major blood vessels in the groin, armpits, and along the neck. Cold intravenous fluids are sometimes used alongside these surface techniques to speed the initial temperature drop.
For more precise temperature control, some centers use endovascular cooling catheters, which are placed into a large vein and circulate cooled fluid in a closed loop inside the body. These systems allow tighter control over the target temperature but are more invasive. The choice of method depends on the clinical setting, the speed needed, and the resources available.
The Rewarming Phase
Rewarming is just as carefully managed as the cooling itself. Bringing the temperature back up too quickly can trigger a rebound of the same destructive processes the cooling was meant to prevent, including dangerous swings in blood chemistry and a surge in brain swelling. The standard approach is to raise the core temperature by about 0.5°C per hour, a pace slow enough that it takes roughly six to twelve hours to reach normal body temperature. During this period, patients are closely monitored for complications.
Risks and Side Effects
Cooling the body affects nearly every organ system, so the treatment comes with real risks that require active management. The heart rate typically slows, sometimes significantly. Blood sugar levels tend to rise. Potassium levels can shift in either direction, which matters because potassium imbalances can affect heart rhythm. The clotting system becomes less efficient at lower temperatures, raising the risk of bleeding, a particular concern for patients who already have bleeding issues or recent injuries.
Infection risk also increases during therapeutic hypothermia. A cooled body mounts a weaker immune response, and patients are often on ventilators and have multiple intravenous lines, each of which is a potential entry point for bacteria. Medical teams monitor for these complications continuously throughout the cooling and rewarming periods, adjusting treatment as needed to keep each one in check.
What the Experience Looks Like
Patients receiving therapeutic hypothermia are unconscious or heavily sedated throughout the process, so they don’t experience the cold the way you would stepping outside in winter. For families, the experience can feel unsettling. The patient may be wrapped in cooling blankets or covered in gel pads, connected to monitors tracking core temperature in real time, and sedated to prevent shivering, which would counteract the cooling and increase the brain’s energy demands.
The full timeline from initiation through rewarming typically spans about 30 to 84 hours depending on the protocol. For cardiac arrest patients, the cooling phase lasts at least 24 hours. For newborns, it lasts 72 hours, followed by the gradual rewarming period. After rewarming is complete, the medical team assesses neurological function over the following days to gauge the extent of recovery and plan next steps in care.