What Are the Side Effects of Cooling for Neonatal HIE?

Seeing your baby kept cool in the Neonatal Intensive Care Unit (NICU) for hypoxic-ischemic encephalopathy (HIE) can be frightening. However, therapeutic hypothermia is a carefully controlled medical treatment, not dangerously uncontrolled cooling ^[1]. While the therapy does have side effects, many are manageable. The NICU team monitors your baby continuously during the entire 72 hours, watching for both the effects of the cooling and the effects of the HIE itself ^[2].

Expected Changes and Common Side Effects

Because cooling slows down the body’s systems to protect the brain, it causes changes in how the heart, blood, and metabolism work.

• Slower heart rate (bradycardia): It is very common for a baby’s heart rate to slow down while they are cooled ^[3]. This is often expected as long as the rhythm, blood pressure, oxygenation, and circulation remain adequate. Medications used for pain and comfort can also lower the heart rate ^[4]. The team watches the heart monitor continuously, as irregular heartbeats (arrhythmias) can occasionally happen ^[5].
• Low blood platelets and clotting changes: Cooling can cause a drop in platelets (the blood cells that help form clots) or alter other blood clotting factors ^[5]. This increases the risk of bleeding. If bleeding occurs or lab levels drop significantly, the team may give a transfusion of platelets (to increase platelet count) or plasma (to replace clotting factors), depending on your baby’s specific needs ^[6].
• Changes in blood pressure (hypotension): Low blood pressure can occur during cooling or from the underlying HIE ^[3]. If this happens, doctors can give intravenous (IV) fluids or vasopressors (medicines that help the heart pump strongly and raise blood pressure) ^[7].
• Breathing and kidney changes: The team also monitors your baby’s breathing and oxygen levels, as pulmonary hypertension (high blood pressure in the lungs) can occur ^[8]. Urine output is tracked closely because HIE and cooling can affect how well the kidneys function ^[2]. Furthermore, the cooling process changes how the body breaks down medications, so drug doses are carefully managed.

Less Common Complications

• Metabolic and blood sugar changes: Fluctuations in blood sugar and electrolytes (essential body minerals like calcium and magnesium) can occur ^[9][10]. The medical team regularly checks these levels and will adjust IV fluids or provide supplements to correct them as needed ^[9].
• Skin changes: Rarely, babies may develop firm, red, or purplish bumps under the skin—a condition called subcutaneous fat necrosis ^[11]. This happens because the fat under the skin reacts to the cold and pressure ^[12]. While the skin spots usually heal on their own, the condition can cause a dangerous rise in blood calcium (hypercalcemia) that can persist or appear weeks or months later, even after discharge ^[11]. If your baby develops these spots, they will need follow-up calcium tests. Symptoms of high calcium at home can include poor feeding, vomiting, unusual sleepiness, or constipation.

Rewarming

After the 72 hours of cooling, your baby will go through a gradual rewarming process. This phase is also strictly monitored because temperature, blood pressure, blood sugar, and the risk of seizures can shift as the body safely returns to a normal temperature.

How Your Baby is Kept Safe

Therapeutic hypothermia uses a highly precise cooling blanket or cap to keep your baby’s core body temperature strictly controlled—often targeted between 33°C to 34.5°C (about 91.4°F to 94.1°F), depending on the hospital’s specific protocol ^[13]. A temperature probe (usually placed gently in the rectum or esophagus) ensures the temperature stays within this safe range ^[13].

During this time, the NICU team uses continuous heart monitors, frequent or continuous blood pressure and oxygen monitors, and regular blood tests to quickly spot and treat any complications. Safety Note: If you notice new bleeding, oozing, unusual bruising, swelling, or sudden changes in your baby’s comfort or breathing, alert your nurse immediately.

While cooling therapy has side effects and cannot undo brain injury that has already occurred, research shows that for eligible newborns with moderate-to-severe HIE, this treatment significantly reduces the combined risk of death or moderate-to-severe long-term neurodevelopmental impairment ^[1].