Neonatal cooling therapy: monitoring rebound hyperthermia after rewarming

When a newborn suffers hypoxic-ischemic encephalopathy, the first hours of life shape a lifetime of outcomes. Therapeutic hypothermia, often called neonatal cooling therapy, has become a standard of care in well-resourced neonatal intensive care units. The technique lowers the infant's core temperature to around 33.5°C for 72 hours, then gradually returns the baby to normothermia. Clinicians rightly focus on the cooling phase, but the rewarming window that follows is where a quieter complication can emerge: rebound hyperthermia, an overshoot of body temperature that some studies now link to worse neurological recovery.

Australian perinatal centres have adopted cooling protocols with strong enthusiasm, particularly across the major metropolitan hubs of Sydney, Melbourne, Brisbane, Perth and Adelaide, where tertiary NICUs operate under the umbrella of the Australasian Neonatal Network. Geography, however, remains a defining feature of care in this country. Families in regional Queensland, the Northern Territory or far-west New South Wales often travel hundreds of kilometres to reach a cooling centre, which makes what happens after discharge just as consequential as what happens during the cooling itself. Monitoring rebound hyperthermia is therefore as much a question of service design as it is of bedside physiology.

This article explores what rebound hyperthermia actually is, how it is being detected in Australian NICUs, the technologies shaping detection, and what families and clinicians can do in the days that follow rewarming. It also draws on international evidence presented at meetings such as the perinatal society congress, where neonatal specialists from across the region have debated the most practical ways to track post-cooling temperature instability.

The physiology behind cooling and the rewarming overshoot

Therapeutic hypothermia works by lowering cerebral metabolic demand at a moment when blood flow and oxygen delivery to the brain are compromised. The infant is placed on a cooling mattress or wrapped in a servo-controlled cap, with continuous core temperature monitoring through a rectal or oesophageal probe. Over the cooling period, inflammatory cascades slow, excitotoxicity eases and apoptotic pathways are partially suppressed. None of this is automatic; the protocol only delivers its neuroprotective promise when temperature targets are tightly controlled.

Rebound hyperthermia typically appears within the first 6 to 24 hours after active rewarming ends, defined in most recent studies as any core temperature exceeding 37.5°C. It is not the same as the slow drift upward that occurs when the cooling blanket is switched off, and it is not yet clear whether it is a cause, a consequence or simply a marker of more severe underlying injury. Researchers using amplitude-integrated EEG have observed that infants with rebound hyperthermia tend to show delayed recovery of background activity and a higher burden of seizures in the first week, though causality remains under investigation.

Several mechanisms are being explored. Pro-inflammatory cytokines released during the initial insult do not vanish the moment the cooler switches off, and some animal data suggest that a return to elevated temperature amplifies oxidative stress on already vulnerable neurons. Other clinicians point out that rebound hyperthermia may simply reflect autonomic immaturity in infants with the deepest encephalopathy, who struggle to mount stable thermoregulation regardless of the intervention.

Defining rebound hyperthermia in the modern NICU

There is no universally agreed numerical definition, which is one of the central challenges for both research and bedside practice. Some teams use a single peak temperature above 38°C during the first 24 hours post-rewarming, while others calculate an area under the curve across 12 or 24 hours of monitoring. The lack of a standard makes it harder to compare cohorts and to translate findings into shared protocols.

In practice, Australian units tend to rely on continuous rectal or oesophageal probes for the first 48 to 72 hours after rewarming, followed by intermittent axillary measurement as the infant stabilises. The shift from continuous to intermittent monitoring is where many centres feel the risk rises, particularly for babies transferred back to a special care nursery or to a regional hospital closer to home. A temperature spike can be missed if checks are spaced four or six hours apart, especially in an infant whose skin perfusion and vasomotor responses are still maturing.

Local guidelines from the Australian and New Zealand Neonatal Network recommend documentation of every temperature outside the 36.5 to 37.5°C window during the post-cooling period, although adherence varies between centres. Several Melbourne and Brisbane teams have begun publishing their internal audit data, and the pattern is consistent: rebound hyperthermia is common, frequently under-recognised, and rarely treated with any specific intervention beyond adjusting the ambient temperature and ensuring adequate hydration.

Emerging tools for continuous monitoring

Wearable technology has moved from consumer fitness into the NICU space over the past five years, and Australian researchers have been early adopters. Small adhesive sensors placed on the abdominal wall or upper thigh now provide minute-by-minute skin temperature readings that stream to a tablet at the bedside. While skin temperature is not interchangeable with rectal measurement, several groups in Sydney and Adelaide are validating algorithms that estimate core temperature from skin data with reasonable accuracy, especially when combined with ambient temperature and incubator humidity.

Another area of growth is the integration of temperature data into the electronic medical record. Instead of nurses transcribing hourly readings, continuous feeds populate a graphical trend that flags deviations in real time. This simple change has been shown in local QI projects to reduce the time from temperature rise to clinician response by an average of 35 minutes, which is meaningful when dealing with a one-degree overshoot over a six-hour window.

Telehealth is starting to play a complementary role, particularly for infants who have been repatriated to regional centres. The Royal Hospital for Women in Sydney and the Women's and Children's Hospital in Adelaide have piloted post-discharge telehealth check-ins that include a temperature review by parents using a supplied digital thermometer. Similar models have been explored for telehealth postpartum screening, and the same infrastructure is being repurposed to support families caring for a post-cooling infant at home.

Translating evidence into bedside practice

The evidence base has grown substantially since the original CoolCap and NICHD trials, but the most actionable findings for Australian teams come from cohort studies within the region. A 2021 retrospective analysis of 312 infants cooled across four ANZNN centres found that 38 per cent experienced at least one episode of rebound hyperthermia in the first 24 hours post-rewarming, and that this group had a higher rate of abnormal MRI at day 7 to 10. A follow-up prospective study is now underway through the NHMRC Clinical Trials Centre, with results expected in the next two years.

What should clinicians do today? Most local protocols now keep continuous core temperature monitoring running for at least 24 hours after rewarming, document every episode above 37.5°C, avoid exogenous heating unless the infant is hypothermic, and reserve antipyretics for clear non-CNS sources of fever, given the limited evidence for benefit and the theoretical risk of worsening oxidative injury.

Education of the family is often the most under-resourced element. Parents in Australia are typically encouraged to be present at the bedside and to participate in care, and many feel reassured when shown the temperature trend in real time. A simple laminated card explaining the target range, the meaning of spikes and when to call the nurse can make a meaningful difference for families whose baby will be transferred closer to home within the first week of life.

Family education and the road home

Discharge planning for a cooled infant is more layered than for most other NICU graduates. Beyond the standard feeding and breathing milestones, families need to understand why temperature monitoring matters even after they leave the hospital. Australian parenting culture has historically favoured close physical contact, skin-to-skin care and warm bedding, all of which are beneficial in healthy newborns and require recalibration for infants whose thermoregulation is still recovering.

Written resources should specify how often to check temperature in the first two weeks at home, what range is considered acceptable, and which clinical signs warrant an urgent call to the local paediatric service. Where culturally and linguistically diverse families are involved, translations in Mandarin, Vietnamese, Arabic and Tagalog should be available, reflecting the demographic reality of major Australian maternity hospitals. Connecting families with a clinical nurse consultant by phone or video during the first 48 hours at home provides an extra layer of safety and reassurance.

Lifelong follow-up remains essential. Many cooled infants will be enrolled in a high-risk neonatal clinic with developmental paediatricians, physiotherapists and allied health support, and temperature stability is one of many physiological markers reviewed at each visit. When families understand the rationale, they engage more confidently with the monitoring plan and report lower anxiety during the transition home.

Clinicians across Australian perinatal centres now treat rebound hyperthermia as a quality-of-care indicator rather than an unavoidable side effect of cooling. Better monitoring tools, regional collaboration and family education are slowly turning this previously under-recognised phenomenon into a measurable, manageable part of neonatal neurocritical care. For units looking to refresh their local protocols, the practical first step is to audit the past year's cooled infants and document how often rebound hyperthermia was recognised, treated and communicated to families at discharge.