What Are the Risks During HIE Rewarming Therapy for Babies?

After therapeutic hypothermia (cooling therapy), the medical team will gradually warm your baby back to a normal body temperature. This transition—called the rewarming phase—is carefully controlled over several hours. Because changes in temperature can affect brain activity and circulation, there is a risk of seizures and shifts in blood pressure during this time ^[1][2]. To keep your baby safe, the Neonatal Intensive Care Unit (NICU) team will continuously monitor their brain waves, heart rate, and other vital signs to manage these changes carefully ^[3][4].

A Slow and Steady Process

Rewarming is a gradual process designed to reduce abrupt changes in the body’s circulation and metabolism. Most NICU protocols slowly raise a baby’s core temperature (their internal body temperature) over a 6- to 12-hour period, often aiming for an increase of about 0.5°C (roughly 0.9°F) per hour ^[3][5].

This schedule can vary. The exact pace depends on your baby’s condition and the specific protocols of your NICU. Once your baby reaches a normal temperature, the team continues to monitor them closely to ensure they do not become too warm, as avoiding a fever (hyperthermia) is very important for brain healing after a hypoxic-ischemic injury ^[3].

Understanding the Risk of Seizures

During cooling therapy, your baby’s brain metabolism and need for oxygen were intentionally lowered ^[6]. As the body warms, the brain’s metabolic rate increases ^[6]. This transition can expose the areas of the brain injured by HIE to changing conditions, which can sometimes trigger abnormal electrical activity, or seizures ^[1].

Research shows that babies who already experienced seizures during the cooling phase are at a higher risk of having them again as they are rewarmed ^[7]. For babies who had no seizures during cooling, the risk is lower—which is reassuring—but it does not completely eliminate the possibility ^[7].

Watching the Brain: Continuous EEG

Because newborn seizures are often “silent”—meaning they happen as electrical sparks in the brain without causing visible shaking or twitching—the NICU team relies on an EEG (electroencephalogram) ^[4]. An EEG uses small, painless stickers on the scalp to continuously track brain waves.

The medical team will usually keep the EEG on throughout the entire rewarming phase to watch for these hidden seizures ^[4]. How long the EEG stays in place after rewarming depends on the baby and the hospital’s guidelines. Many protocols continue monitoring for 24 hours or more after the baby reaches a normal temperature, or for a set time after the last detected seizure ^[8][9]. If seizures are detected, the team will promptly evaluate your baby’s breathing, circulation, and lab results to choose the safest anti-seizure treatment ^[8].

Managing Vital Signs and Lab Results

As your baby’s body warms, their circulatory system adapts. You may notice changing numbers or alarms on the monitors by your baby’s bed. It is expected that a baby’s heart rate and cardiac output (the amount of blood the heart pumps) will increase, while their blood pressure may decrease ^[2][10].

While these changes are possible and anticipated, the medical team evaluates every shift to ensure it is safe for your baby, closely managing:

• Blood pressure and heart rate: The team may adjust IV fluids or medications to support healthy blood flow ^[3][2].
• Breathing (respiratory status): Some babies may temporarily need more breathing support as their oxygen needs change ^[3][11].
• Blood sugar and electrolytes: Blood tests will check that glucose (sugar) and electrolytes (essential body salts like sodium and potassium) stay within safe ranges ^[3].
• Urine output: Kidney function and fluid balance can change during rewarming. The team will carefully track your baby’s urine output, often by weighing diapers or using a urinary catheter ^[12].

Please do not try to interpret the monitor numbers on your own. Ask your bedside nurse what trends they are watching for, and lean on the medical team to manage this complex transition.