|
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. |
