Rapid cooling is necessary if core temperature must be rapidly although body temperature typically decreases more than one degree C during the first hours of general anesthesia.
Methods use to reduce brain temprature during animal experiments include packing ice,fanning,partial immersion in cold water,nasopharyngeal cooling and cardiopulnary bypass(CPB).
Rapid reduction of core temprature for mild or moderate hypothermia can be facilitated by administration of refrigerated intravenous fluid,circulating water mattresses ,forced aircooling and extracorporal means.
Intravenous administration of refrigerated (1-6) degrees C 5% albumen 5cc/kg during a period of 3 to 5 minutes,after surface cooling to 34 degress C reduces core temperature by approximately 0,6 degrees C.
To achieve maximal effectiveness,refrigerated fluids must be administered rapidly (100 ml/min) to avoid heat gains in standard intravenous tubing.
In controlling body core temperature, forced air cooling has the advantage of cooling a large skin area but seems to be even more effective when used in combination with a circulating water mattress.
In a series of eight patients with severe trauma brain injury(TBI) a heat exchanger was connected by a pressure controlled roller pump to a percutaneously introduce double lumen canule in femoral vein.
Cooling was initiated at a cooling speed of 3,5 degrees C/hour and hypothermia was maintained with 0,1 degree C at 32 degrees C brain temprature for 48 hours.
Using this technique a brain temprature of 32 degrees C,was achieved within 113 minutes(moreless 81 min) after
cooling began.
Although this invasive technique and platelet count decreased during treatment no clinical bleeding complications or problems resulted from extracorporal circulation.
SURFACE COOLING BY USE OF TUB TECHNIQUE :
After induction and intubation,when vital signs of anesthetized patient are stable,the patient is immersed in cold water keeping the chin and hands above the water level.
When the vital signs again stabilize ice cubes are added to the water. Cooling usually taken 30-6o minutes.The patient is kept in the ice water until body temperature has drops to about 40% of the desired value,since after the patient removed from the tub and transfered to the operating table,the temperature continues to drop(drift),frequently by 3 to 4 degrees C.
If the desired temprature is 28 degrees C the patient is removed from the ice water when his temprature is 32 degrees C.
As the body temperature drops, the concentration of anesthetics is gradually reduced until their administration stopped at body temperature at 28 to 30 degrees C. From this point on, no further administration of anesthetics is necessary. But controlled hyperventilation with oxygen continues for the duration of surgery.The pupils are ordinary dilated the during hypothermic period.
The patient is removed from the ice water.Then he is thoroughly dried with bath towels and placed on water filled mattresses for rewarming.
If the patient move spontaneously while being rewarmed,a mixture of 50% N2O and 50% O2 is usually sufficient to keep him quiet.
During rewarming, reflexes and spontaneous movement reappear at about 31 degrees C and conciousness return at 32 to 34 degrees C.
Extubation is carried out when respiration are adequate.
The tympanic or rectal temperature is monitored for at least 24 hr postoperatively to ensure that temprature is maintained at about 36,5 degrees C.
Also the patient is observed carefully during this period for any signs of shock or reactive hyperthermia.
LOCAL REFRIGERATION ANALGESIA :
This method can be useful for handling severely traumatilimbs that require amputation.
1.The tissue are chilled for about 1 hour then a torniquet is
applied.
The torniquet must be tight enough so the tissues distal
to the torniquet are blanched rather than congested.
2 The limb is placed in cracked ice and cooled for 2-3 hr,
depending on whether it is an arm or a leg.
The limb is placed so that the melting ice will drain away
from the bed.
3.The patient is placed on the operating table and the limb
is dried. The analgetic effects will last for 1 hr.
4.When the larger vessels have been tied the torniquet is
released and adequate hemostasis is obtained.
5.Ulceration of tissues will not occur if actual freezing and
excessive pressure are avoided.
6.Disadvantages.The labor and time involved in preparing
the patient for surgery.
INDICATIONS OF HYPOTHERMIA :
a.Aneurysmectomi
b.Carotid artery surgery in the neck.
c.Surgery of vascular tumors such as meningioma.
d.Management of severe brain injury especially brainstem
injury with high fever,coma,tachycardia,tachypnoe and
rigidity,acute cerebral vascular accident and ruptured
intracranial aneyrysm.
COMPLICATIONS :
Prolonged hypothermia can cause mask infections and coagulopathies and sign of cerebral compression.
Deep hypothermia causes myocardial depression,hypoten-
sion,arrythmia and cardiac arrest and potentially causes multifocal ischemia from reduced microcirculation and post operative shivering.
SUMMARY :
For patients at risk for ongoing brain injury,patient temperature should be monitored and aware that brain temprature might be greater.
The increase of systemic or brain temperature should be treated promptly and vigorously, because it worsen neurologic outcome after brain injury.
In TBI patient selection should focus on younger patients with GCS of 5 to 8,and ICPs between 20 and 40 mmHg because beneficial effect with hypothermia may be achieved end less effective for GCS of 3-4.
Considering of adverse effect of hypothermia patients who need extracranial surgery should be kept normothermic.
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Neuro Anethesia with Neuro surgical and Neurosciences
Perspective,Mc Graw Hill USA,1997,pp 604-6.
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