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The most remarkable discoveries in the medical field in the
last century has been the discovery of anesthesia without which
any kind of surgery could not have progressed to where it is now. In
particular anesthesia for neurosurgery has made considerable progress in
the last five decades. Hyperventilation, osmotic diuretics, steroids,
newer anesthetic agents, and narcotics and more accurate monitoring of
vital signs during anesthesia and in the postoperative period, have all contributed
to the safety of the patient even through a long neurosurgical
procedures. The neurosurgeon should be aware of some of the anesthetic
techniques that can make his procedure safer and should recognize that
close cooperation with the anesthesiologist can significantly improve the
safety and smoothness of his procedure. A better understanding of the
risks and benefits of particular positions has led to safer and more
efficient neurosurgical procedures. The use of the three point fixation
headrest has been particularly important in this regard. This paper will
discuss basic physiology in anesthesia, its application to the
neurosurgical patient, and specific positions and their risks.
The basic aims of anesthesia for neurosurgery are :
Avoiding acute fluctuations in intracranial volume,
especially in patients with poor intracranial compliance, and
Allowing patients to be reasonably awake at the end of
surgery, to permit simple neurological assessment.
Put in simple terms, smooth induction, light levels of
anesthesia and smooth emergence are the key factors.
Physiology (the effect of anesthetic agents and techniques
on ICP):
The cranial cavity is a semi-closed, non-distensable cavity
containing brain and parenchymal water (80%), blood (12%) and cerebro
spinal fluid (8%). These are all relatively incompressible; a change in
the volume of any one will require acute changes in the other two to
avoid sudden shifts in pressure. Among the three main contents of the
cranial, anesthetic agents have the greatest influence on cerebral blood
volume, which is affected through blood flow, which varies from 20 to 80
ml/100 gm/min. Estimated cerebral blood volume is 100-200 ml; changes in
flow will produce blood volume changes concomitantly.
Lesions that characteristically produce changes in
intracranial pressure (I.C.P) are :
Intracranial trauma, producing diffuse contusion of the
brain.
Rapidly expanding intracranial or subdural haematoma.
Posterior fossa masses, that cause obstruction to the free
flow of CSF. These are life threatening emergencies in which close
attention must be paid to I.C.P. control. A proper understanding of the
pressure volume curve is essential for managing these and other
neurosurgical cases. With compromised ICP adjustment properties, due to
tumors/haematomas, there is a reduced intracranial compliance. With a
small volume increase there is a marked increase in I.C.P.; even a small
further increase in volume would produce a marked rise in I.C.P.
Both anesthetic techniques and agents greatly influence the
I.C.P curve by their effect on intracranial blood volume. Normally, small
increases I.C.P. are tolerated without any untoward effect, and volume
increases which have occurred over a long period of time may produce
minimal symptoms. With a rapidly expanding mass lesion which has
progressed along the pressure volume curve, a small rise in intracranial
volume may produce a dramatic rise in I.C.P. Hypercarbia, hypoxia, and
hypertension, are poorly tolerated in this situation because of cerebral
vasodilation that accompanies them.
Factors that influence cerebral blood flow and intracranial
pressure:
A) CO2 : A high PCO2 is the
most potent stimulator of cerebral blood flow (C.B.F). Normal individuals
tolerate wide fluctuations of PCO2 but in patients with poor
intracranial compliance, even a small rise in PCO2 is poorly
tolerated.
B) PO2 : C.B.F is not affected until PO2
decreases to about 50 Torr. Beyond this, cerebral vasodilation occurs.
Hypoxia and hypercarbia produce a synergistic effect to produce a marked
increase in C.B.F. and therefore cerebral blood volume.
C) Blood pressure : Both pain and anxiety can
increase blood pressure and this in turn can raise C.B.F. This is an
important point to consider during preoperative visits. Wide fluctuations
in blood pressure are poorly tolerated in such cases as intracranial
aneurysms, A.V.M. and malignant tumors. Normally, however, cerebral
autoregulation keeps flow steady despite variations in blood pressure.
Autoregulation is the ability of the brain
to maintain an adequate perfusion pressure over a wide range of systemic
blood pressures. In a normal individual the upper limit of mean pressure
with intact autoregulation is 125-140 Toor (Mean pressure), the lower
limit 40 Torr (Mean). Below 40 Torr cerebral blood flow will drop
precipitously and symptoms of ischemia may occur. In older persons and in
the hypertensives, the upper and lower limits of autoregulation may be
increased.
During induction of anesthesia, various noxious stimuli may
produce a sudden rise in blood pressure which can be deleterious to the
patient.
The most common stimuli are:1) Laryngoscopy and intubation
2) Suctioning 3) Skeletal fixation of the head
D) Venous pressure : Increase in central venous
pressure is directly transmitted to the intracranial cavity. This can be
harmful if there is:
1) Coughing and straining on the endotracheal tube
2) Flexion of the neck, producing kinking of the neck veins
3) A head position in which the head hangs too low
E) Anesthetic agents
1) Inhalation agents - All inhalation agents are cerebral
vasodilators and increase intracranial blood volume and ultimately I.C.P.
Both halothane and enflurane cause greater in C.B.F. than isoflurane.
2) Intravenous agents - All except ketamine are cerebral
vasoconstrictors Barbiturates produce a dose dependent cerebral
vasoconstriction, and reduce both cerebral blood flow and cerebral
metabolic rate. Propofol besides doing allof the above also reduced CMRO2.
Narcotics like morphine, pethidine fentanyl have all contributed to
control of ICP.
3) Muscle relaxants - succinylcholine increases I.C.P. as
does Curare. Among the muscle relaxants, panucuronium has the least action
on I.C.P. and such is the most commonly used drug, butbhas the
disadvantage of producing tachycardia . Vecuroniun bromide is more
cardiac friendly and can also be used in patients with heart problems as
it does not have much effect on the heart rate. Atracurium besylate
though short acting causes histamine release may and cause hypotension.
Patients who are on dilantin are extremely resistant to panucuronium and
will need large amounts of the drug.
Summary of physiological considerations:
Reviewing intracranial hemodynamics and the effect of
anesthetic agent and techniques, certain broad principles can be drawn
regarding anesthetic management for intracranial procedures.
These are :
1) One should use agents and techniques that will not affect
intracranial hemodynamics.
2) All anesthetic agents used for induction and maintenance
of anesthesia should produce cerebral vasoconstriction, and thus improve
intracranial compliance , especially in cases where it is impaired.
3) Wide fluctuations of blood pressure should be avoided to
prevent such complications as rupture of an aneurysm, or ischemic
infarction in the elderly.
4) Moderate hypocapnia with hyperventilation should be the
goal.
5) Selection of anesthetic agents should be governed by
their effect on
a) Cerebral blood flow,
b) Cerebral blood volume
c) Cerebral metabolic rate,
d) Intracranial
compliance.
Neuroanesthetic management:
PREOPERATIVE evaluation should include
1) Review of relevant history, medications and neurological
and systemic problems. A clear understanding of the intracranial
pathology, and problems associated with it during anesthesia and surgery
is essential for proper planning and management. For example : in
patients with vascular malformation who may need hypotensive techniques,
a full review of cardiovascular system will be necessary. The same is
true in patients with a pituitary tumor; such patients may have multiple
endocrine anomalies, and acromegalic patients often have airway problems.
2) Preoperative fluid status. This should be fully assessed
along with electrolytes.
3) Potential problems. Airway problems, I.V. sites, C.V.P.
catheter sites, and patency of radial artery are all to be noted.
Premedication is a controversial subject
with varying opinions.
For several years now, the author's preference has been to,
not to, sedate patients heavily for neurosurgical procedures. This also
helps with the old debilitated patient and patients who are comatose or
obtunded due to their intracranial pathology. Those who do not have any
associated systematic problems are may besedated heavily. This practice
has, in more ways than one, helped a great deal in anesthetic management.
The patient's reaction to this also has always been very favourable.
For a 70 kg adult, the usual premeditation used is :
1) Diazepam 5-10 P.O. previous night.
2) Propranolol 1.5-2 mg/kg in two divided doses, half the
dose at bed time, the other half along with premedication. Tab. Atenelol
l2.5mg at bedtime and on morning of surgery. Premedication is always
given 90 minutes before surgery.
The use of beta blockers is not a common practice, but for
several years now we have been using it. All patients receive them unless
there is a specific contraindication. The rationale for heavy sedation
along with propranolol are :
1) Many of these patients are over-anxious with a
hyperactive sympathetic system. With heavy sedation, patients are often
relaxed, and many come to the operating room fast asleep.
2) Since no potent narcotics are used, in premedication,
preoperative blood gases are within normal limits.
3) Use of potent vasodilators is substantially reduced.
There is hemodynamic stability, especially during induced
hypotension.
There is a lesser incidence of reflex tachycardia.
6) There is a significant reduction in the incidence of
postoperative rebound hypertension.
MONITORING for a major craniotomy:
This should include:
1) E.C.G. lead II or CM5
2) Intra-arterial blood pressure
3) Pulse oximeter for monitoring O2 saturation
4) Train of four blockade monitor for titrating muscle
relaxants
5) Esophageal stethoscope for monitoring ventilation and
heart sounds
6) Endtidal CO2 monitor
7) Oral temperature
8) Urine output.
For those who are to be operated on in the sitting position
with the head significantly higher than the heart,in addition should
have:
1) Right arterial catheter
2) Precordial doppler for monitoring air emboli
Transesophageal doppler, in cases suspected to have a patent
foramen ovale.
INDUCTION of anesthesia:
1) Sudden fluctuations in blood pressure should be avoided.
Among all the induction agents, sodium pentothal has the most profound
effect on blood pressure due to direct myocardial depressant action and
vasodilator effect. Hence, care should be exercised when using this drug,
especially in the elderly, and in hypertensive or hypotensive patients.
Propofol also has this property but a very slow injection over comes this
problem.
2) Airway : Proper airway management is essential to avoid
the twin insults of hypoxia and hypercarbia. An obstructed airway may
also lead to a rise in intrathoracic pressure. This may produce an
elevated venous pressure, increase in intracranial blood volume and
elevated I.C.P.
Marked sympathetic response may occur at various stages of
induction and intubation. Judicious use of potent nacrotics and
barbiturates, use of long acting muscle relaxants, and hyperventilation,
may all help to contain wide fluctuations in blood pressure. Use of a
potent inhalation agent during induction of anesthesia is not ideal.
Laryngoscopy, intubation and skeletal fixation are the most powerful
stimuli of sympathetic response, producing dramatic changes in blood
pressure. Various methods are used to contain this response. Use of
additional Sodium Pentothal or Propofol before intubation, and I.V.
xylocaine 1.5 mg/kg before intubation, are commonly used.
POSITIONING:
Positioning of the patient is an important part of a
neurosurgical procedure. Both the anesthetist and neurosurgeon must have
full understanding of the implications and possible hazards of each
position so that complications can be avoided. The basic aims of
positioning are to keep the surgical field uppermost and to aid venous
drainage, avoiding a full brain and excessive bleeding.
Physiological effects of positioning:
(A) Cardiovascular
In normal individuals the protective reflexes in the great
vessels help to keep cardiovascular homeostasis during abrupt changes in
position. In anesthetized patients, however, these protective reflexes
are depressed. Most anesthetic agents are myocardial depressants, and
produce vasodilatation. Use of muscle relaxants abolishes the muscle pump
to aid venous return. This is further reduced by controlled ventilation.
Thus, the overall effect is diminished cardiac output. This is more
pronounced in hypovolemic and elderly patients.
(B) Respiratory
Studies by Froese and Bryan have clearly
demonstrated the variation in diaphragmatic excursion that occurs with
patients breathing normally, breathing spontaneously while intubated, or
being ventilated. Changes in pulmonary blood volume, lung volume, and
restricted movement of respiratory muscle, can produce ventilation/perfusion
mismatch may occur in different position, and over prolonged period of
times may have an adverse effect. Functional residual capacity decreases
in supine position, especially in the aged and obese, and during
anesthesia. Airway closure and air trapping occurs when F.R.C. falls
below closing volume, thus setting the stage for V/Q mismatch. All of
these are exaggerated in smokers and in those with pulmonary pathology.
In the obese, both cardiovascular and respiratory changes
are exaggerated. Pooling of blood in the dependent part of the body can
significantly reduce venous will increase for adequate ventilation. Over
a period of time pulmonary collapse can due to pressure on the epidural
veins during spinal surgery. In the very obese, special modifications
such as the Tarlov Seat or Cloward Frame, may be necessary to make
surgery.
Basic principles to be observed during
positioning include :
1)Gradual and deliberate maneuvers to prevent acute
cardiovascular changes. Infusion of 500-1000 ml of crystalloids
2)Proper support to the upper chest and pelvis to prevent
respiratory impediments which produce hypoventilation.
3) Padding of all vulnerable areas to prevent thermal
injuries, pressure necrosis, nerve injuries and injury to the eye.
4) The position should be changed gradually to avoid sudden
changes in blood pressure. At special risk are elderly and hypertensive
patients. For patients who can tolerate infusion of crystalloids,
preloading with 500-1000 ml of Lactated Ringers Lactated Ringers) should
be ready to treat hypotension.
5) All pressure points should be protected properly to avoid
nerve injuries.
6) Proper positioning of the neck to prevent venous
obstruction is important.
7) Eyes should be properly protected.
8) Elevation of the head and lower limbs may aid in venous
drainage.
Various methods are used to prevent venous stasis and deep
vein thrombosis. These include elevation of the lower limb, wrapping the
leg with elastic bandages and compression boots.
In general, for tumor and trauma surgery the head should be
positioned so that the intracranial lesion is uppermost. In vascular
procedures, especially intracranial aneurysms, the head may be angled
even inferiority on the neck to allow adequate access to the base of the
brain
Common positions in Neurosurgical procedures:
1) Supine, with its various modifications
2) Prone position
3)Lateral position
4)Semi-sitting position.
5)Knee-Chest position.
1) Supine position
This is the most common position used in our institution.
Use of the Mayfield head rest allows the cranium to be kept in different
positions, thus enabling the surgical field to be elevated. This position
is also used for anterior cervical approaches. It is ideal for cranial
lesions in the anterior and middle fossa. This position is also more
physiological than the sitting or prone position, and in the elderly
cardiovascular stability can be maintained.
Some surgeons elevate the head by flexing the table and
elevating the back; in order to reduce venous pressure. Approach to the
parasagittal region, as in a parasagittal meningioma, may have to be done
with the head in an elevated position, in which case precautions for air
emboli have to be taken.
The anesthetist is often on the side opposite to the surgical
field and the arterial line and other I.V. line are to be placed on this
side. Special attachments to the operating table are available that keep
the drapes off the patient's face so the anesthetist has a clear view of
the airway. Both pulse oximeter and endtidal CO2 monitor are
valuable in detecting airway disconnections.
2) Prone position
This position is used for midline lesions of the posterior
fossa and all lesions of the spine and spinal cord that are approached
posteriorly. For most prone positions, the Mayfield head rest is used to
keep the cervical and thoracic spine in line. The anesthesiologist is
usually at the foot end of the patient away from the surgical field;
rarely he may be at one side. In this position, every aspect of
anesthetic management has to be planned so that access to arterial line
and I.V. sit are not hampered. Special precautions that have to be taken
during prone position are :
A) Positioning should be done slowly to avoid sudden
haemodynamic changes, especially in the sick and elderly. It is better to
infuse 500-1000 ml of crystalloid before the patient is turned prone in
order to avoid this. Vasopressors may be needed to prevent a sudden drop
in blood pressure.
B) Airway patency is very important. Special care should be taken
to see that the endotracheal tube is secured properly and kinking does
not occur. Use of light weight hoses and a flexible endotracheal tube
will help to prevent this complication.
C) Care of the eye is very important when the head is kept
on the operating table. The forehead and chin should be supported
properly so that there is no pressure on the lower eye. This is
especially important in the elderly.
D) Acute flexion of the neck should be avoided to prevent
constriction of neck veins.
E) Pressure on the abdomen should be minimum so that oozing
from the wound can be minimised in laminectomy cases. This is very
important in the obese, in whom both hypoventilation and excess bleeding
can occur if there is pressure on the abdomen. This is common when special
laminectomy frames are used. Proper positioning on rolls or the use of
the kneeling position as in a Tarlov Seat can help in this situation.
F) Placement of the upper limb should be planned properly to
enable access to the I.V. sites and arterial line, as well as to avoid
peripheral nerve injury.
3) Lateral position
True lateral position is not commonly used for craniotomies,
except in a modified version for posterior parietal and occipital
craniotomy. Special precautions to be taken are :
A) Stability of the patient is not easy in this position.
Proper restraining of the hips and shoulder will be needed.
B) Padding and support should be arranged so as not to
hinder ventilation.
C) Proper positioning of the head and upper limb must be
made to avoid nerve injuries and injury to eyes.
D) Ventilation/perfusion mismatch can occur in the lateral
position, and over a prolonged period atelectasis of the dependent lung
can occur.
4. Semi-sitting position
No other position has generated so much discussion as the
semi-sitting position. We rarely use this position now except in special
cases because of its inherent complications. Many centres do use this
position and have advocated it for posterior fossa and cervical
laminectomy procedures.
Physiological changes in the sitting position include :
A) Reduced venous return due to pooling of blood in the
dependent parts. Controlled ventilation further reduces venous return.
B) Varying degrees of sub-atmospheric pressure in the neck
veins and dural sinuses. The higher the surgical field from the heart,
the greater the difference in pressure.
C) Cerebral perfusion pressure decreases 2 Torr for every
vertical inch elevation above the heart. This becomes critical in the
elderly and hypertensive. In this position the transducer must be kept at
the level of the external auditory meatus.
D) The effect on ventilation is minimal, which is one of the
advantages claimed for this position. Use of the Mayfield head rest keeps
the head stable. The anesthetist is on either side of the patient, or at
the foot end. This gives him a clear view of the airway, and access to
the arterial line and the I.V sites.
There are a number of special monitoring techniques used for
surgery in the sitting position. Nitrous oxide is not used, instead
oxygen with an inhalation agent and narcotics is employed. Controlled
ventilation and 5 to 8 cm Positive end expiratory pressure (PEEP) is used
to keep the venous pressure elevated. Studies in animals have shown that
0.2 to 8 cm PEEP did not affect the right arterial pressure ot the
inter-arterial pressure gradient. Both an endtidal CO2 monitor
and a precordial doppler are used to detect venous air emboli. Thus far
we have not used the transesophageal doppler to detect arterial air
emboli from a patent foramen ovale. A pulse oximeter is routinely
employed, as well as a right arterial catheter for aspirating air; the
position of the catheter is checked by chest X-ray before surgery begins.
5. Knee-Chest position.
The abdomen is suspended, epidural veins are not engorged,
inter-vertebral spaces are well exposed, operating conditions are very
good.
Complications of positioning:
1) Cardiovascular : (a) Hypotension is the most
common cardiovascular complication. This can be acute and sudden, and in
the elderly may precipitate a cerebral or myocardial infarct. (b) venous
and arterial air emboli. Incidence of air emboli vary from 15 to 40% in
cases done in the sitting position. In our institution, reviewing post.
fossa surgery for acoustic neuroma we found that 13.5% of 319 cases had
venous air emboli. We did not have any cases of known paradoxical air
emboli, even though recent studies showed the incidence of patent foramen
ovale to be between 20 and 30%.
2) Respiratory :Ventilation/perfusion abnormalities
can occur in lateral and prone position, especially in the obese.
3) Peripheral nerve injuries : this is one of the
most common complications of positioning. Parks found injury to the
brachial plexus to be the most common, followed by injury to the peroneal
nerve. Factors relevant to peroneal nerve injury include the fact that
longer and more superficially placed nerves are prone to injury, that
stretching over bony prominences and prolonged pressure can produce
ischaemic changes in the nerve, and that an abnormal course of the nerve
close to blood vessels can result in injury during injection of drugs or
extravasation.
Brachial plexus injury can occur in prone, lateral, and
supine positions if the upper limb is not positioned properly. Similarly,
peroneal nerve injury can occur in supine and sitting positions; the leg
should be carefully checked to be certain there is no pressure over the
fibular head.
4) Other complications:
1) Venous stasis can lead to deep vein thrombosis,
especially during prolonged surgery.
2) Injury of male genitals can occur in the prone position.
Similarly, pressure on the breast can cause pressure injury.
3) If the neck is not properly placed during lateral and
prone position, injury to cervical roots can occur.
4) Soft tissue swelling around the face and eyelids can
occur. Prolonged prone position can produce swelling of the tongue and
lips. In rare instances, ulceration of the lateral margins of the tongue
can result, especially if the tongue is caught between the teeth.
5) Injury to the eye is one of the more serious
complications of positioning, especially of the prone position when the
patient is placed on rolls or on the frame and the head is placed on a
headrest. Hypotension and pressure on the eye can result in retinal
artery thrombosis, and this can result in blindness. There are a few
reports of this in literature.
INTRAOPERATIVE
Anesthetic management:
The basic anesthetic management for a routine craniotomy is
as follows :
1) There should be a full complement of monitoring during
induction, after preinduction blood gases, electrolytes and other
hematological values have been checked. Monitoring should include :
a) Pulse oximeter
b) E.C.G.
c) Continuous intra-arterial blood pressure monitoring.
d) Train of four blockade monitor.
Most patients should have two I.V. lines, one for volume
infusion and another for drugs, along with an arterial line. Cases that
will need a C.V.P. line should have one.
1) Preoxygenation for 3-5 min before including patients is
common practice.
2) A loading dose of fentanyl 250-500 micrograms is given
followed by a sleep dose of sodium pentothal 3-4 mg/kg/, or propofol
1-2mg/kg till eye lash reflex is abolished. Once the airway can be
maintained, pancuronium 0.1 mg/kg is given. The patient is ventilated
with 100% O2. An additional dose of pentothal 100-200 mg will
help in preventing hypertensive responses during intubation. Laryngoscopy
and intubation is attempted only when patient is totally relaxed. Use of
I.V. xylocaine 1.5 mg/kg or top up of inducing agent is employed to
prevent the hypertensive response.
After intubation, the tube position is checked by
auscultation and the tube is securely fixed. Eyes are protected with eye
patches. An NG tube oral temperature probe and esophageal stethoscope are
inserted where indicated. The patient is put on the ventilator with 33% O2
and 66% N2() and hyperventilated; the patient is then
positioned for surgery. All pressure points are protected and acute
flexion of the neck is avoided. Just before skeletal traction, an
additional dose of pentothal/propofol is given to prevent a hypertensive
response. To be extra careful especially in patients with aneurysms, to
prevent them from rupturing at the time of applying the the three point
head rest I use Inj. Esmolol, an ultra short acting beta-blocker,1mg/kg,
few seconds before the actual application. The action lasts only for a
few minutes.
Anesthesia is maintained with intermittent doses of
narcotics and muscle relaxants. Fentanyl is used for pain relief at
regular intervals. For muscle relaxation, after the initial dose of
pancuronium, I switch over to vecuronium bromide 2mg every 45minutes or
depending on the response from the peripheral nerve stimulator, this I
have found gives very good recovery at the end of the procedure. The
initial one to two hours should give a reasonable idea about drug requirements.
With a heavy premeditation, narcotic requirement is often reduced.
Patients who are on Dilantin are quite resistant to pancuronium;
neuromuscular monitoring is therefore essential.
Control of brain volume:
Those of us who have witnessed neurosurgery before the use
of hyperventilation, osmotic agents and loop diuretics know what a
tremendous difference these have made in operating conditions. Some of
the commonly used methods to minimize brain volume are :
1) Hyperventilation : Often the PCO2 is
kept in the low 30's Torr. This produces a reduction in cerebral blood
flow, reduces venous return and causes cerebral vasoconstriction.
2) C.S.F Drainage : a) Via a ventricular catheter -
not a routine procedure.
b) Spinal catheter - the most common; it has the potential
for nerve root injury and postoperative C.S.F leak.
c) Direct drainage through the cisterns; often the most
satisfactory approach.
3) Steroids : Preoperative, intraoperative and
postoperative corticosteriods have helped a great deal in the management
of intracranial mass lesions.
4) Diuretics : a) Loop diuretics - as soon as the
Foley's Catheter is inserted, 20 mg of fursemide is given I.V. The use of
a loop diuretic before osmotic diuretics is started prevents fluid
overload, especially in those with poor cardiac reserve. The combination
of osmotic and loop diuretic often results in copious urine, and often
1500 ml - 2000 ml is put out before the dura is exposed.
Osmotic diuretics - 20% mannitol is the one most often used.
I normally start mannitol as soon as patient has been
positioned. For an average adult, we infuse 20% mannitol, at dose of 1
gm/kg..
5) Positioning of patient is another way one can improve
venous drainage, with head elevation aiding in venous return.
Fluid management:
Unless indicated, fluid restriction is the rule, in the
initial phase except where the patient has to be put in sitting position.
Many patients have a 15-20% fluid volume deficit. Fluid replacement is
titrated to urine output plus the maintenance volume of 2ml/kg/bw, in
adults, from the time of fasting is replaced. In paediatric cases I
follow the formula of Segar & Halliday i.e 4ml/kg/hr upto 10kg body
weight, 40ml plus 2ml/kg/bw upto 20kgs and there after 60ml plus 1ml per
kg/body weight/hr.
Blood or blood products are used quite freely, in fact over
correction is what we follow in our unit as we have found that there is
inexplicable drop in both Hb & Hct in the first post-op day. (May be
peculiar to this institution).
For a normal 70 kg adult, crystalloid is replaced at 150-200
ml per hr. Urine output is very high in the initial stages, but later
tapers off to about 20-30 ml per hour. Ringers Lactate solution is
commonly used. Unless the patient is a diabetic when I use 0.9% Normal
saline solution , then later depending on the hourly CBG results I modify
the fluids. Glucose containing fluid e.g. 5%Dextrose especially is not
used as it tends to cause/increase cerebral edema.
Management of common intraoperative problems:
A) Cardiovascular
1) Hypotension : Hypotension may have many causes : the most
common are :
a) Drugs : Some antibiotics like vancomycine (which
is often used) can, if given at a rapid rate, cause acute hypotension.
b) Anesthetic agents : Many of the inhalation agents
can cause drop in blood pressure, especially in the hypovolemic patients.
c) Fluid loss or acute blood loss.
d) Hypoxia and Hypercarbia - Not often the prime suspect
with the present day monitoring.
e) Surgical stimulation with vagal response.
2) Cardiac arrhythmias : Cardiac arrhythmias may be
caused by surgical stimulation of the brain stem area, the orbit
(occulocardiac reflex) or the region of the hypothalamus. The most common
arrhythmias are nodal rhythm. Premature ventricular contraction. Sinus
arrhythmias and rarely bradycardias, including sinus arrest are
encountered regularly during middle and posterior fossa surgery.
B) Air embolism
Veins in the occipital muscles and other areas of the back
of the neck do not readily collapse after being cut but are held open,
allowing air to be sucked in. Mastoid emissary vein gained notoriety for
air entry, as are the major sinuses. Venous air embolism is common in
sitting position. Air emboli occur when a gradient of 3 cm or more exists
between the area of surgery and right heart. The incidence of air emboli
during major neurosurgical procedures varies from 15 to 40%. Most of the
incidents get corrected by themselves i.e. on giving 100% Oxygen &
cutting off of Nitrous Oxide and by lowering the head and by aspirating
the central venous line. There was one death directly attributable to air
emboli and one case revived from a severe air embolism which sustained a
cardiac arrest in the last five years.
The most common symptoms and signs of air emboli are:
1) Drop in the endtidal CO2
2) Tachycardia, cardiac arrhythmias, and cardiovascular
instability leading to hypotension.
3) Drop in SpO2
4) Hissing sound in the wound
"5) Squelching gum-boot" murmur
With the availability of the end-tidal CO2
monitor, detection of air emboli should not pose any problems. A drop in
more than four is significant.
C) Paradoxical air emboli
The presence of a potent foramen ovale varies between 20 and
30% according to the Mayo Clinic. Detection of paradoxical air emboli
needs more sophisticated monitoring devices like transesophageal doppler.
Paradoxical air emboli is a major neurosurgical complication and
treatment is far from satisfactory. Many centres are trying to change
from sitting to more supine position for posterior fossa surgery. In our
institution, very sitting to more supine position for posterior fossa
surgery.
The presence of a patent foramen ovale varies between 20 and
30% according to the Mayo Clinic. Detection of paradoxical air emboli
needs more sophisticated monitoring devices like transesophageal doppler.
Paradoxical air emboli is a major neurosurgical complication and
treatment is far from satisfactory. Many centers are trying to change
from sitting to more supine position for posterior fossa surgery. In our
institution, very few cases are done in the sitting position.
D) Respiratory
Since most patients are mechanically ventilated, respiratory
complications primarily due to anesthesia are uncommon.
Endotracheal tube displacement, resulting in partial
collapse of a lobe, is one of the common ones. Partial microatelectasis
due to positioning often results in low PO2 and V/Q mismatch.
Use of PEEP may be needed to overcome this problem. Postoperative
respiratory depression may be due to variety of causes, and one has to
look into the etiology in a systematic ray.
TERMINATION of anesthesia:
Perhaps no other part of anesthesia is more important and
often neglected than the termination of anesthesia. A well planned
procedure is often rewarded by a fully awake patient appropriately
responding to all verbal commands and neurological examination. Planning
for this should start right from the preoperative visit.
I have found the following guidelines helpful :
1) I do not reverse the muscle relaxant until the head
dressing is applied and we have total control of the airway.
2) All suctioning is done before the patient gets the
reversal agent. Endotracheal suctioning is rarely done unless there is a
clear indication for it.
3) Atropine 1.2 mg and Neostigmine 3-4 mg I.V. are given.
The reversal of muscle relaxant is monitored by a twitch monitor.
Use of I.V. xylocaine 1.5 mg/kg 15-20 min before extubation
prevents coughing and bucking. I also use a small dose of propofol at the
end of the procedure.
4) Postoperative nausea may be a problem, especially in
posterior fossa surgery. I normally give an anti-emetic like Ondenstron
8mg or Perinorm10mg I.V. with premedication itself and repeat it about an
hour before the end of the procedure. This makes the emergence from
anesthesia smooth and also helps in suppressing nausea.
In cases where the endotracheal tube is left in place
overnight or for postoperative ventilatory support, instilling 2-3 cc of
4% xylocaine into the tube to suppress the cranial reflexes has been very
helpful in preventing bucking on the endotracheal tube. I have used
Propofol bolus dose 0.5-1mg/kg, followed by infusion of 6-12ml/hr of 1%
propofol, according to me it has been very satisfactory.
Patients are transferred to the I.C.U. with a full
complement of monitoring. They are provided with O2 through a
face mask and the head is kept in an elevated position.
CONCLUSION:
Perhaps no other single aspect of neurosurgery is so
important for the final outcome, as the clear understanding between the
neurosurgeon and anesthesiologist on every aspect of the procedure.
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