Principles of neuroanesthesia: 


Dr. (Lt.Col.) M. Rammohan Naidu,  

Neuro-Anesthesiologist, Apollo Hospitals,  Chennai , India.

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 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.


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.


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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