Every effort
should be made
to reduce the
secondary
reperfusion
injury to the
brain – during
this ischaemia–
reperfusion
time, it is
suspected that
ATP molecules
are reduced
in the brain
and anaerobic
glycolysis occurs
morbidity and mortality for OHCA. However once
primary neurological insult has occurred, there
should be every effort made to reduce the
secondary reperfusion injury to the brain.
The mechanism of this injury is thought
to be governed by free radical formation and
inflammatory mediators such as TNFα and
interleukin-1. 10 During this ischaemia–reperfusion
time, it is suspected that ATP molecules are
reduced in the brain and anaerobic glycolysis
occurs, causing intracellular levels of phosphate,
hydrogen and lactate to rise; this, in turn, leads to
intra- and extracellular acidosis and subsequently
cell apoptosis and death. 10
It is known in animals that there is reduction
in cerebral metabolic rate for oxygen (CMRO 2 )
of 6% for every 1-degree reduction in brain
temperature (>28°). 11 It is theorised that the
injured brain could be therefore be protected
in this critical post-resuscitation period. There is
also evidence that a 4–5-fold increase in oxygen
radicals may be present in hyperthermia. 12
Subsequently this is suggested to be a factor in
why hyperthermic patients suffering OHCA have
an increased mortality rate and incidence of
neurological sequlae. 13
Complications
While inducing hypothermia for TTM seems to
convey protection, it is not without risk. The
documented side effects for this treatment range
from diuresis causing electrolyte disturbance,
shivering (causing need for muscle relaxation,
thus masking seizure activity) to cardiac
dysrhythmia, bleeding and immune suppression.
A review listed cardiovascular complications as
the most likely to occur, but deemed this due
to pre-existing ischaemic heart disease (IHD)
exacerbated by the haemodynamic imbalance
caused by TTM. 14
One key area of complication with TTM is
glucose homeostasis, specifically with insulin
resistance and reduced insulin secretion leading
to hyperglycaemia; this causes an increase in the
rate of infections, neuropathy and renal failure.
Overall morbidity and mortality has been shown
to be higher with hyperglycaemia in the critically
ill population and therefore close monitoring and
appropriate management of hyperglycaemia
should be undertaken with TTM. 14 In our hospital,
we use a concentrated dextrose infusion with
variable rate insulin regime to control blood
glucose levels within a targeted range.
Techniques
The reason TTM has replaced MTH or TH is to
place the importance on defining a temperature
profile for a patient, manipulating their
physiological response. 15 It can be divided into
three phases:
1 Induction – an intentional change from
current temperature to a lower one
2 Maintenance – maintaining the set
temperature for a desired time
3 Rewarming – changing to a new
normothermic temperature value by increasing
temperature at a set rate.
Research exists on the optimal time to
instigate TTM, its length of duration and
rewarming time, with some studies even
performing pre-hospital MTH, unfortunately
14
HHE 2019 | hospitalhealthcare.com
without benefit to long-term survival or
disability. 16
All early studies seemed to conclude that TTM
should be started as early as is feasibly possible;
however, a randomised, controlled trial did not
prove any benefit in terms of speed to achieve
MTH in comparison to TTM and avoidance of
hyperthermia. 17
There are currently three main techniques
available for cooling post-OHCA. These are
summarised below, and the availability of each
varies depending on location and on financial
circumstances:
1 Conventional cooling methods
2 Surface cooling systems
3 Intravascular cooling systems.
Conventional cooling methods
These methods include crushed ice, ice bags and
the infusion of cold fluid. These have the
advantage of being cheap, mostly accessible, and
can be used in combination with other methods.
These methods however are somewhat crude, and
while an evidence base for adequately lowering
temperature is available, 18 they can reduce
temperature to undesired levels and be difficult
to use in maintenance of desired temperature
range.
Surface cooling methods
These devices, probably the most commonly
available, work by circulating cold fluid or air
through either blankets or pads wrapped or
applied to the patient. A number of devices exist:
blankets such as the Curewrap with CritCool by
MTRE, and Kool-Kit with Blanketrol III by
Cincinnati Sub-Zero are popular. The InnerCool
STX by Philips and Arctic Sun by Medivance are
examples of pad devices, the latter currently
being used in our hospital.
The advantages of these devices are that they
are easy to apply and rapidly initiate treatment,
while also offering auto-feedback mechanisms to
adjust water or air temperature accordingly in
keeping with patient assessment from skin and
core temperature sensors.
Unfortunately shivering is most common with
these devices, 19 which might necessitate muscle
relaxation. There is also the possibility of skin
irritation and burns, although these are rare. 19
Intravascular cooling methods
These methods involve the cannulation of a
central vein which comes with its own risks and
complications, namely infection, bleeding and
thrombosis. At time of writing, the Thermoguard
XP (Zoll) and InnerCool RTx (Philips) are the major
devices in use. The Thermoguard has the added
benefit of having a triple lumen infusion catheter
combined with its cooling system avoiding the
need for multiple lines. The greatest benefit
ascertained with these devices is precise
temperature control in maintenance and
rewarming phases and less shivering. 20
Comparison of cooling methods
Conventional methods aside, a recent
retrospective study looked at surface cooling
methods versus intravascular cooling devices. 21
The analysis identified there was no change in
outcome in either mortality or neurological