Passive cooling for initiation of therapeutic hypothermia in neonatal encephalopathy

Perinatal hypoxia–ischaemia accounts for a substantial proportion of admissions for neonatal intensive care. Therapeutic hypothermia improves survival and reduces the rate of disability at 18 months of age in infants who survive hypoxic–ischaemic encephalopathy.
Magnetic resonance studies suggested that therapeutic window after hypoxia-ischaemia lasts for about 6 hours and results from a trial indicates that infants treated within 4 hours from delivery tend to benefit the most from hypothermia. Though available data indicate that the earlier cooling is commenced the greater the potential for neutroprotection, cooling procedures are often delayed until the arrival of an infant at a cooling centre and the efficacy is likely reduced. To avoid this, a number of passive cooling and combined passive and active cooling strategies have been used during transport.
In the largest series of babies undergoing passive cooling, a significant proportion of infants were overcooled, thus suggesting the need for clear and validated passive cooling protocols.
On this basis, an UK group conducted a prospective study to develop a passive cooling protocol for initiation and maintenance of therapeutic hypothermia during neonatal transport and to assess whether skin or axilla temperature can be used as a proxy for core temperature.
Between January and October 2009, 39 infants were referred for therapeutic hypothermia. Once a cooling cot had been identified, passive cooling was initiated at the referring hospital in all cases. Attending teams were advised to commence continuous skin temperature monitoring, start passive cooling (switch off any active warming devices, open the portholes of the incubator, and nurse the baby naked apart from a nappy), and to document intermittent rectal temperature every 15 minutes. Despite indications to do so, no rectal temperature measurements were taken before arrival of the transfer team. Two infants were cooled to a temperature lower than 32ºC. Of the remaining 37 babies, 33 (89%) experienced a reduction in core (rectal) temperature with passive cooling alone. On arrival at the cooling centre, four babies had cooled to a temperature below 33ºC by passive cooling alone. The percentage of babies within the temperature range at referral, arrival of the transfer team and arrival at the cooling centre was 0%, 15% and 67%, respectively.
Initiation of passive cooling before and during transfer resulted in the therapy starting 4.6±1.8 hours earlier than if initiated on arrival at the cooling centre. Skin temperature measurements did not accurately reflect the core temperature in 91 cases; on the other hand, the axillary temperature correlated with the core temperature in 61 measurements (mean difference of 0.1ºC).
Overall, these findings suggest that passive cooling with appropriate monitoring is a simple and effective technique to initiate therapeutic hypothermia outside a cooling centre. The use of specific protocols can help to achieve therapeutic hypothermia while avoiding the risks of overcooling. Continuous rectal temperature monitoring is the monitoring of choice. As an alternative axillary temperature can provide a useful surrogate, while skin temperature should not be used as a proxy for core temperature in neonates undergoing therapeutic hypothermia.

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