Therapeutic hypothermia is a treatment where a newborn’s body temperature is deliberately lowered to about 33.5°C (roughly 92.3°F) for 72 hours to protect the brain after oxygen deprivation during birth. It is the standard of care for infants born at or after 36 weeks of gestation who show signs of moderate to severe brain injury, known as hypoxic-ischemic encephalopathy (HIE). A meta-analysis of eight randomized trials involving 1,344 infants found that cooling reduced the combined risk of death or major neurodevelopmental disability at 18 months by 25%.
Why Oxygen Deprivation Damages the Brain in Stages
When a baby’s brain loses its oxygen supply during birth, the damage doesn’t happen all at once. It unfolds in distinct phases, and understanding this timing is key to understanding why cooling works.
The initial insult cuts off the brain’s fuel supply. Without oxygen, cells can’t produce energy, and some neurons die immediately. In moderate injuries, the brain tries to protect its most essential structures, like the brainstem, by redirecting blood flow away from less critical areas. This means the outer layers of the brain and deep structures like the thalamus and basal ganglia take the biggest hit.
Once oxygen is restored, a quiet window opens. This latent phase lasts roughly 6 to 12 hours. The brain partially recovers its internal chemistry, but inflammation is already building and a cascade of programmed cell death has been set in motion. This is the phase therapeutic hypothermia targets. After that window closes, a secondary wave of injury begins: toxic molecules called free radicals attack cells, energy-producing structures inside neurons fail, and seizures often develop. A third phase can stretch on for months, involving ongoing remodeling and late cell death.
How Cooling Protects the Brain
Lowering the baby’s core temperature during that critical latent phase slows nearly every destructive process unfolding in the brain. The cooled brain uses less energy, which buys damaged cells time to recover. Inflammation drops. The signals that trigger programmed cell death are suppressed. Harmful receptor activity that would otherwise amplify the injury is dialed down. None of these mechanisms alone would be enough, but together they significantly reduce the total amount of brain tissue lost to secondary injury.
Which Babies Qualify
Not every baby who has a difficult birth is a candidate. Eligibility requires two things: evidence of a significant event during delivery, and neurological signs that brain injury has already begun.
The first criterion looks at how severely the baby was affected at birth. This can include a very acidic blood gas taken from the umbilical cord or within the first hour of life, a low Apgar score (below 5) at 10 minutes, or the need for assisted breathing that continues for at least 10 minutes. The second criterion is neurological: the baby must show signs of moderate to severe encephalopathy, which can mean seizures, abnormal muscle tone, weak or absent reflexes, or problems with alertness and breathing patterns. Infants must be at least 36 weeks gestational age and weigh more than 1,800 grams (about 4 pounds).
The 6-Hour Window
Cooling must begin within 6 hours of birth to be effective. This narrow deadline reflects the biology of the injury: once the secondary phase of brain damage kicks in, lowering the temperature can no longer interrupt the cascade. Because of this tight timeline, hospitals that deliver high-risk babies are trained to recognize the signs of HIE quickly and either initiate cooling on-site or arrange rapid transfer to a facility with a dedicated cooling program. Some centers begin passive cooling, simply not actively warming the baby, during transport to buy time.
Whole Body vs. Selective Head Cooling
There are two main approaches. Whole body cooling lowers the baby’s entire core temperature to 33 to 34°C using a special mattress or blanket that circulates cooled water or gel. Selective head cooling wraps a cooling cap around the baby’s head, targeting the brain more directly while keeping the core temperature slightly warmer, around 34 to 35°C. Studies comparing the two methods have found no significant difference in side effects or short-term outcomes, and both are used in practice. Whole body cooling has become the more common method in most neonatal intensive care units.
What 72 Hours of Cooling Looks Like
Once cooling begins, the baby’s core temperature is brought down to a target of 33.5°C and held there for a full 72 hours. Throughout this period, the infant stays in a neonatal intensive care unit with continuous monitoring. Heart rate, blood pressure, oxygen levels, and temperature are tracked around the clock. The baby typically appears quiet and still, which is partly the effect of the lowered temperature slowing metabolism. Feeding is usually paused or minimal during cooling, with nutrition provided through an IV line.
Parents can generally still be at the bedside, touch their baby, and participate in care, though holding may be limited depending on the equipment being used. The 72-hour period can feel long and stressful, but the treatment itself is not painful for the baby.
The Rewarming Phase
After 72 hours, the baby is gradually warmed back to a normal body temperature of about 36.5°C. This happens slowly, at a rate of 0.5°C per hour, meaning the process takes roughly 6 to 8 hours. Rewarming too quickly can cause dangerous shifts in blood pressure, blood sugar, and brain activity, including a rebound risk of seizures. The medical team monitors the baby closely throughout this phase, and it’s considered one of the higher-risk portions of the entire treatment.
Potential Side Effects During Treatment
Cooling a newborn’s body intentionally does come with physiological consequences, most of which the medical team anticipates and manages. A slower heart rate is common and expected at lower body temperatures. Blood pressure can fluctuate. Platelet counts sometimes drop, which can affect blood clotting. The baby’s skin may appear mottled or pale. These effects are generally mild and resolve once rewarming is complete. The consistent finding across clinical trials is that the benefits of cooling for eligible babies far outweigh these temporary risks.
Long-Term Outcomes
Therapeutic hypothermia does not guarantee a normal outcome. It reduces the odds of death or serious disability, but some babies will still develop cerebral palsy, learning difficulties, or other neurodevelopmental challenges. The severity of the original brain injury matters enormously. Babies with moderate encephalopathy tend to respond better to cooling than those with severe injury.
Because the effects of HIE can take years to fully reveal themselves, babies who undergo cooling are typically followed by a developmental specialist well into childhood. Assessments at 18 to 24 months are standard, using tools that evaluate motor skills, language, and cognitive development. Some delays or difficulties only become apparent once a child reaches school age, so ongoing monitoring is important even when early assessments look promising.
Open Questions in Cooling Therapy
Current guidelines are well established for moderate to severe HIE, but several areas remain unsettled. One major question is whether babies with mild encephalopathy, who don’t currently meet the standard criteria, might also benefit from cooling. Another is how to best manage passive cooling during transport when a baby is born far from a cooling center. The 2025 American Heart Association and AAP guidelines specifically flag both of these as knowledge gaps needing further study.