Core central temperature is defined as the temperature
blood perfusion vital major organ system.
Nasopharyngeal and esophageal temperature are more representative of core temperature.
Core temperature varies not only interindividually and diurnally but also between different parts of the body.
In unanesthetized persons thermoregulatory response is
not triggered by temperature range of 0,2 degree C.
Physiologic termoregulatory response normally triggered by hypothermia.Therefore if normothermia is not actively maintained general anesthesia may lead to unintentional low body temperature.
Perioperative hypothermia commonly results from anesthe-
tic induced inhibition of the thermoregulatory as well as a
cold ambient environment in the operating room and the
heat loss owing to surgical exposure of tissue.
Anesthetized persons often behave like poikilothermia until the core temperature approaches a new setpoint for thermoregulation.
Patients at greatest risk for perioperative hypothermia include elderly patients burn patients, neonates and patient with spinalcord injuries.Hypothermia occurs when heat loss exceeds heat production.
Clinical dose of general anesthesia decrease the thresold for response to hypothermia from approximately 37 degrees C (normal) to 33 degrees to 35 deg C).
Anesthetized patients where core temperature exceeds these values are usually poikilothermic and do not actively respond to thermal pertubations.
Hypothermia with shivering is a common post operative finding will increase oxygen consumption by as much as 400%.will raise cerebral metabolic rate for O2(CMRO2)
and possibly intra cranial pressure(ICP) and myocardial ischemia in patient with coronary artery disease.
Even mild hypothermia can prolonged emergences from general anesthesia. But deliberate mild/moderate hypothermia may be used intra operatively to protect the brain from ischemic injury.In a recent poll taken from members of The American Societry of Neurosurgical Anesthesia and Critical Care 40% clinician practiced induced hypothermia in patients undergoingcerebral aneurysma surgery.
Physiologic consideration:(3,4,5)
1.Effect on metabolism:
a.When blood is cold the oxygen dissociation curve shifts
to the left.
b.The rate of oxygen consumption falls as body tempera-
ture falls (but tissue oxygen need does not fall in a
direct ratio).
c.Essential oxydatives enzymes are not inactivated by
hypothermia.
2. Effect on respiration :
As the body temperature falls the solubility of CO2 in
the blood increases.
3. Effect on the heart :
The heart rate,coronary blood flow and oxygen uptake
of the heart are decreased by 50% when the body temp-
rature is 25 degrees C.
ECG features :
The QRS complex lengthen,the PR interval prolonged
and there is elevatio of the early part of the ST
segment(the precursor of venrtricle fibrillation).
The atrium frequently begin to develops flutter or
fibrillation at temperature below 30 degree C and the
ventricel usually fibrilate below 28 degrees C.
4.The general circulation changes :
a. Cerebral,renal and splancnic areas blood flow decrease
as body temperature decrease.
b.The mean arterial pressure(MAP) is decreased by 5% for
each degrees centrigrade below 37 degrees C.
5.The Central Nervous System (CNS):
a. CMRO2 decreases to 50% of normal with hypothermia
to 30 degrees C and 25% of normal at 25 degrees C,
15% at 20 degrees C and 10% at 15 degrees C .
Period of circulatory arrest tolerated at normother-
mia in only 4 to 5 minutes but it doubles for every 8
degrees temperature reduction.
Period of tolerated circulatory arrest at 38 degrees C
for 4-5 min, at 30 degrees C for 8-10 min, at 25 deg-
ress C for 16-20 min, at 20 degrees C for 32-40 min,
and at 10 degrees C for 64-8o min.
b.Cerebral blood flow(CBF) decreases by 7% for each
degrees centrigrade below 37 degress C.
c. The Cerebrospinal fluid (CSF) pressure and CSF
production rate decrease.
d. Cortical function is decreased so there is retrograde
amnesia for events happening under hypothermia.
6.Effect on renal function:
a.Hypothermia protects the kidney from adverse effects
when the blood supply is occluded during surgery.
b.The Glomerular Filtration Rate(GFR) and Renal Blood
Flow(RBF) are reduced 35% at 25 degress C.
7.Effect on liver function:
A.Changes in clotting mechanism:
a.the bleeding time is increased
b.the platelet count falls and the postthrombine time
to shortened.
c.clot retraction is poor.
Hypothermia induced coagulopathy is caused by multiple
factors:
1.Hypothermia reduced platelets count probably from
splenic squestration.
2.It causes a reversible pletelets dysfunction by decreas-
ing adhesiveness .
3.It slow down the enzyme mediated steps in the
coagulation cascade.
4.It decreases the metabolism of heparin.
The dilutional effects of priming solutions with
cardiopulmonary by pass(CPB)on factors I,II,V,VII and
XII also contribution to difficulty with homeostasis.
Hypothermia also causes an increase in viscocity
leading to sludging of erythrocytes.
B. Morphine is slow to be conjugated and detoxified in
the liver.
C.The metabolism of barbiturates is impaired.
D.It is possible that nor epinephrine and tubocurarine
could remain undestroyed in areas of vascular stasis and
on rewarming, be released into general circulation.
8.Effect on Acid base balanced:
a.The buffering capacity of the blood is reduced during
hypothermia.
b.Alveolar ventilation and kidney's ability to regulate acid
base disturbances are decreased.
c.The heart is more sensitive to low blood pH which can
increase both its irritability and its tendency to ventri-
cular fibrillation(VF) therefore the patient should be
hyperventilated to raise the blood pH.
d.The oxygen Hb curve is shifted to the left therefore the
Hb has more affinity for oxygen.
e.On rewarming there is some metabolic acidosis (mostly
lactic acidemia).
General consideration :
1.The use of dextrose solution during hypothermia carries
risk because dextrose is metabolished very slowly during
hypothermia and therefore accumulates in the ECF and by
its osmotic effects draws water from cells.
The extra water delutes the ECF and thus the serum
electrolyte.
But during rewarming dextrose solutions should be admi-
nistered because the dextrose level will then to fall.
2.The serum potassium tends to be low when pH is high
(alkalosis).
The extracellular potassium rises whenever there is
tissue anoxia.
9.Effect on anesthetic drugs:
1.The effect of thiopental are potentiated and prolonged
during hypothermia so its use should be restricted be-
cause thiopental has direct depressant on the myocardi-
um.
2.Inhalation agents:
Hypothermia potentiates the effect the anesthetics
drugs and delays their excretion .Therefore as hypo-
thermia progresses smaller amounts of anesthetics
agents are required.
Halothane is potent vasodilator and N2O is the least
toxic of the inhalation anesthetics.
3.Muscle relaxant :
Duration of action of muscle relaxant is likely to be
increased by mild hypothermia.
For example the duration of action of vecuronium is
twofold when body temperature is reduced from 36,8
degrees C to 34,4 degrees C.
Muscle stiffness may be present at 33 degrees C it may
remain until rewarming.
Caution should be remembered during rewarming the
action of non depolirizing relaxant may become
manifest.
4.Other drugs:
The action of atropine is gradually decreased as cooling
progresses. Epinephrine retains its activity in the cooled
patient at least to 25 degrees C.
Norepinephrine is less active in the hypothermic patient
and remain partially inert until rewarming when it can
causes a marked degree of hypertension.
Neostigmine maintains its action during hypothermia.
Preoperative administration of digitalis can augment
the decreased heart rate during hypothermia.
Mechanism of hypothermia protection:(3,4,6)
Mechanical cooling was first desacribed 1943.
Hypothermia is non pharmacologic method of reducing CMRO2 causes a significant in cerebral oxygen consumption and has been demonstrated to protect the brain during anoxic condition.
Hypothermia can be protection not only by virtue of its
effect on CMRO2 but cellular and biochemical efects
better explained how hypothermia protect.
Mild hypothermia (34 to 36) degrees C effectively blocks glutamate which massively increased during an ischemic insult are believed to iniate an excitotoxic cascade ulti-
mately resulting in cell death.
Energy faillure is associated with a large influx of calcium
in vitro studies have shown that mild hypothermia reduces calcium influx.
Probably hypothermia cause decrease opportunity for intracellular calcium to accumulate to concentrations sufficient to exert toxic effects.
Hypothermia diminishes membrane bound PKC activity in selectively vulnerable regions of post ischemic brain.
PKC is enzyme involved in regulating neuronal excitability and neurotransmitter release is activated in response to an increase in cytosolic calcium.
Hypothermia also can reduce accumulation of lipid peroxi-
dation products and the consumption of free radical sca-
vengers in ischemic brain. Globus et al have demonstrated that free radical production persist for at least several hours after reperfusion.
The quantity of free radical generated is reduced to almost normal values by moderate hypoyhermia (32-34) degrees C.
Hypothermia can also suppress Nitric Oxyde Synthase activity this is beneficial if Nitric Oxyde or free radical mechanism are germane of pathogenesis of neuronal death.
Possible mechanism for the neuroprotective effect of hypothermia:
1.Reduction of rate energy use for electrophysiology cortical activity.
2.Reduction of extracellular concentration of excitatory amino acids.
3.Attenuating free radical production.
4.Suppressing the post traumatic inflamatory response.
5.Maintanance of high energy phosphate.
6.Prevention of Protein kinace C down regulation
7.Recovery of post ischemic protein synthesis.
The most recent trial concluded that treatment with moderate hypothermia for 24 hours initiated soon after head injury significantly improve outcome at three and sixmonths in those with a Glasgow Coma Scale(GCS) of 5-7.
Mild hypothermia is not associated with cardiovascular and metabolic dearrangement commonly observed at lower temperature.
To be continued
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