Techniques for therapeutic hypothermia during transport and in hospital for perinatal asphyxial encephalopathy
Introduction
Within the last decade, therapeutic hypothermia for infants with perinatal asphyxial encephalopathy has been studied in preclinical models1, 2, 3 and several major randomised clinical trials in the developed world.4, 5, 6, 7 Despite the clinical heterogeneity of perinatal asphyxia and the use of different cooling methods, there are consistent findings that hypothermia reduces the extent of neurological damage and improves survival without disability.8 Therapeutic hypothermia is now widely offered to moderately or severely asphyxiated infants in countries and centres which participated in the trials.9 Anecdotally, other countries are also adopting cooling therapy. At present, however, the safety and efficacy of therapeutic hypothermia in low-resource and transitional-care countries has not been proven.10, 11, 12 ‘Low-tech’, simple methods of cooling are now being developed that could allow therapeutic hypothermia to be provided in an economical and sustainable fashion in future trials of cooling in such settings.13
The extent of neuroprotection by hypothermia depends on the time of initiation of cooling and its duration – prolonged cooling within the ‘therapeutic window’ (defined in the current neonatal trials as within 6 h of birth) is essential for long-term, robust protection.14, 15 Although the precise brain temperature that provides optimal neuroprotection following perinatal asphyxia is unknown,15, 16 there is increasing evidence from the adult literature that the depth of cooling, within the mild hypothermia range (32–35 °C; see below), may be a less important factor. As reviewed by Sarkar and Barks in this issue (‘Systemic complications and hypothermia’), at least potentially, there may be greater risk of systemic complications when cooling is moderate or deep (<30 °C) which might counteract the potential benefit of cooling. This is an area of active research in preclinical models of perinatal asphyxia. The effect of temperature stability on outcome is also unknown; however, rapid swings in temperature will impact on many enzyme and physiological functions and therefore at present it is recommended that it should be avoided if possible.
This review outlines some of the unique factors that need consideration when cooling infants, such as the recent transition from fetal to extrauterine life, the effect of body size, the occurrence of natural hypothermia, and the site of temperature measurement. Different cooling methods are reviewed and recent experience in the UK, Europe and USA of cooling during transport to cooling centres is described.
Section snippets
Mechanisms of heat loss in the newborn
Mammals are homeotherms – organisms capable of maintaining body temperature within a narrow range, usually above that of the surroundings despite large variations in environmental temperature. In the human, a highly effective thermoregulatory system controls core body temperature to near 37 °C (this is set slightly higher in some mammals such as the piglet and sheep).17, 18
The temperature of the fetus is 0.3–0.5 °C higher than that of the mother; heat is transferred via the placenta and the
‘Natural’ hypothermia in infants with perinatal asphyxial encephalopathy
The phenomenon of exaggerated cooling in term babies after birth with perinatal asphyxia was first described more than 50 years ago.29 The authors compared babies with ‘asphyxia’ (defined as a failure to establish respiration within 3 min of birth) with non-asphyxiated babies. The rectal temperature of the asphyxiated infants was ∼34.5 °C within 2 h of birth compared with control babies who were ∼2 °C warmer (Figure 1a). We have recently observed the same phenomenon of ‘natural’ hypothermia in
Temperature measurement
The human body can be divided into two thermal compartments: a ‘core’ compartment consisting of the trunk and head and a ‘peripheral’ compartment consisting of the skin and extremities. In mammals, the core body temperature is maintained within a narrow range to facilitate optimal functioning of physiological processes. Core temperature measurements are typically slightly higher than oral measurements, and oral measurements are higher than skin temperature. The commonly accepted adult average
Cooling methods and phases of cooling
An ideal cooling device should induce cooling rapidly to the desired core temperature, maintain the core temperature tightly within the target range for the desired duration (usually 72 h) and should allow rewarming in a slow and controlled manner at a set rate (at present, usually 0.2–0.5 °C/h).44 The device should be easy to use, require minimal nursing input, should not interfere with access to the baby and should not cause undesirable side-effects such as shivering. When evaluating the
Sedation during cooling
In awake patients shivering can result in 40–100% increase in oxygen consumption and is an undesirable situation in post-hypoxic brain injury. In post-surgical adult patients, shivering is associated with increased morbidity.63 Shivering is not commonly seen in infants (see discussion above on brown adipose tissue). Studies of cooling using ice, frozen gel packs and fans have reported apparently higher incidences of shivering59, 60, 61 than more recent studies using active conductive cooling.
Cooling during transport
In order to initiate therapeutic hypothermia as early as possible after birth, and hence maximise the neuroprotective effect in encephalopathic infants, initiation of therapeutic hypothermia at the referring centre and during transport to the cooling centre remains an attractive proposition.66, 67, 68, 69 Cooling of neonates on transport presents a particular challenge over and above that of cooling at the receiving centre. The babies transferred are often sick, and require significant
Conflict of interest statement
None declared.
Funding sources
This work was undertaken at UCH/UCL who received a proportion of funding from the UK Department of Health's NIHR Biomedical Research Centers Funding Scheme.
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