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The endovascular approach has opened new options in the treatment of vascular and nonvascular intracranial and spinal diseases. Interventional neuroradiologic (INR) procedures may seem technically straightforward, yet they carry a significant morbidity. Approximately 0.2% to 1% of the patients develop transient or permanent neurologic signs and symptoms after diagnostic cerebral angiography. When compared with diagnostic angiography, therapeutic interventions are associated with significantly more risks of neurologic complications. The primary goals of anesthesia for INR procedures are to control the level of sedation in a manner that permits prompt neurologic examination, to render the patient immobile, and to manipulate cerebral hemodynamics.
Many INR procedures such as diagnostic angiography, carotid angioplasty and stenting (CAS), and embolization of cerebral arteriovenous malformations (AVMs) can be undertaken with intravenous sedation. General anesthesia is required, however, for a growing number of INR procedures including intracranial angioplasty and embolization of aneurysms and some high-flow AVMs, diagnostic procedures in children and uncooperative adults, and prolonged procedures such as those on the spinal cord. Often the choice of anesthetic technique is a collaborative decision by the radiologist and the anesthesiologist on the basis of their assessment of each individual patient.

I. Neurovascular access and methods

Vascular access. INR procedures typically involve the insertion of catheters into the arterial circulation of the head or the neck, usually through the transfemoral route. As illustrated in Figure-1, transfemoral arterial access is accomplished by the placement of a large introducer sheath into the femoral artery, usually 5 to 7.5 Fr in size. A 5 to 7.5 Fr coaxial catheter is positioned through the introducer sheath into either the carotid or vertebral artery by fluoroscopic control. Finally, a 1.5 to 2.8 Fr superselective microcatheter is introduced into the cerebral circulation to deliver drugs, embolic agents, or balloons to distal regions of the brain. The transfemoral placement site is usually infiltrated with a local anesthetic, which can cause femoral nerve block and a temporary weakness of the quadriceps muscle. Transfemoral venous access can also be used to reach the dural sinuses and, in some cases, the arterial side of an AVM. Direct percutaneous puncture is used to access superficial lesions of the head and neck, such as tumors and arteriovenous and venous malformations.

Figure-1. Representation of a typical arrangement of a transfemoral coaxial catheter system showing the femoral introducer sheath, the coaxial catheter, and the microcatheter.


Imaging technology. Radiologic imaging techniques needed for INR procedures include high-resolution fluoroscopy and high-speed digital subtraction angiography (DSA) with road-mapping functions. DSA enables the visualization of only those vessels that are opacified by contrast injection. The road-mapping function enables the radiologist to observe the advance of the catheter against the background map of the patient's cerebral vessels "in real time." DSA involves subtraction of the images obtained before and after the injection of the radiocontrast drug. Any displacement of the cerebral vessels because of the movement of the head profoundly degrades the DSA images. Hence, it is critical that the patient remain immobile during the procedure.
Materials for embolization and infusion. Factors that affect the choice of the embolic agent include the nature of the disease, the purpose of embolization, the size and penetration of emboli and vessels, and the permanency of occlusion. The ideal choice and the combination of agents remain controversial. Embolic agents include balloons, coils, polyvinyl alcohol (PVA) particles, gelatinous embolization spheres, and glue. As used for embolization, N-butyl cyanoacrylate (NBCA) glue is available as a liquid monomer that rapidly polymerizes in contact with ionic solutions such as blood and saline.

II. Anesthetic considerations

Briefly, the primary functions of the anesthesiologist in the intervention suite are (a) to provide to the patient the level of sedation that permits prompt neurologic assessment when needed, (b) to render the patient physiologically stable and immobile, (c) to manipulate systemic blood pressure optimally as dictated by the needs of the procedure, and (d) to provide emergent management of catastrophic complications.

III. Conduct of anesthesia for INR procedures

Preoperative assessment. A careful assessment of the airway must be made. A history of snoring may suggest that partial airway obstruction might occur with sedation. Snoring results in movement artifacts that may degrade the quality of images during cerebral angiography. Patients who have a history of adverse reaction to radiocontrast drugs require pretreatment with steroids and antihistaminic drugs. The population of patients who have occlusive cerebrovascular disease might also require adequate treatment of hypertension, heart failure, or angina. Preoperative communication should exist with the INR team to develop a clear strategy for sedation and hemodynamic interventions that might be needed during the procedure.
Preoperative investigations. The routine guidelines for indicated laboratory investigations before surgery are applicable to INR procedures. Of particular interest is the baseline coagulation screen because anticoagulation is required for the procedure.
Premedication. Anxiolytics may be administered depending on the condition of the patient. Minimal premedication is required for INR procedures. Either oral nimodipine or transdermal nitroglycerin is sometimes used to decrease intraoperative vasospasm.
Room preparation. The INR suite should have the same anesthesia equipment as is available in a standard operating room. Suction, gas evacuation, oxygen, and nitrous oxide (N2O) should be available from the wall outlets. Ideally, the anesthesia machine should have the capacity to provide carbon dioxide (CO2) for deliberate hypercapnia. An extended anesthetic breathing system is necessary to reach the remotely located patient's airway. Rapid access to all critical equipment should be possible at all times during the procedure. Induction and emergency drugs must be prepared and ready for immediate use.
Patient positioning. Because INR procedures may last for several hours, it is essential that the patient be made as comfortable as possible before the start of sedation.
Intravenous access. During INR procedures, patients are often moved cephalad toward the image intensifier and away from the anesthesiologist to check the position of the catheters. This limits access to venipuncture sites and injection ports during the procedure. Therefore, adequate vascular access and a sufficient length of intravenous tubing should be in place before the start of the procedure. In adults, two intravenous cannulae are usually inserted for this purpose; one cannula is at least 18 gauge in size. The anesthetic and vasoactive drugs should be in line before the patient is draped.
Monitoring
Arterial pressure. Because of the need to manipulate systemic hemodynamics and the emergent need, at times, for hemodynamic interventions, it is usually desirable to obtain direct measurement of systemic arterial pressure during INR procedures. This is most conveniently achieved by transducing the side arm of the femoral introducer sheath. If a relatively large coaxial catheter passes through the introducer, however, the arterial pressure trace is "damped." Despite damping, the mean pressure is still reliable in this situation. To avoid excessive damping of the femoral arterial trace, the introducer sheath should be at least 0.5 Fr larger than the coaxial catheter. Radial artery cannulation may be desirable when systemic arterial pressure needs to be monitored before inserting the femoral introducer sheath. Radial arterial line may be required during the induction of general anesthesia for the coil occlusion of an intracranial aneurysm or when monitoring blood pressure in the postoperative period is indicated.
During a typical intracranial INR procedure, two other pressures may be measured in real time in addition to the systemic arterial pressure: either the internal carotid or vertebral artery pressure through the coaxial catheter or the distal cerebral arterial pressure through either the microcatheter or a balloon-tipped catheter. The coaxial catheter pressure is monitored to detect either thrombus formation or vasospasm at the catheter tip as evidenced by a damped arterial trace. A high volume of heparinized flush solution is infused continuously through the coaxial tip to discourage thrombus formation; hence, the pressure reading characteristically increases by 10 to 20 mm Hg when recorded through the coaxial catheter. The setup for measuring arterial pressures is shown in Figure-2. The pressure transducers and access stopcocks for zeroing and withdrawal of blood are mounted, depending on the institutional preferences, either on the sterile field or toward the anesthesiologist. Measurements of the distal cerebral arterial pressure made through the microcatheter are useful during embolization of AVMs. When a balloon-tipped catheter is used for internal carotid artery (ICA) occlusion, pressure measurements at the tip of the catheter provide the stump pressure.

 

Figure-2. Schematic representation of pressure monitoring and the continuous flush systems.

Other systemic monitoring. Other monitors include five-lead electrocardiogram, preferably with ST segment trending and respiratory trace, automated blood pressure, end-tidal CO2, and peripheral temperature probe. Pulse oximeter probes are placed on each of the great toes and are useful for qualitatively comparing distal pulses in the lower limbs. Loss of the oximeter pulse trace on the side of the femoral introducer sheath might provide an early warning of the occurrence of thromboembolism, vasospasm, or mechanical obstruction.
Central nervous system (CNS) monitoring. During many procedures, neurologic examination provides adequate monitoring of the integrity of the CNS. Adjuncts especially useful during general anesthesia or planned proximal occlusions include electroencephalogram, somatosensory and motor evoked potentials, transcranial Doppler (TCD) ultrasound, and 133Xe cerebral blood flow (CBF) monitoring.
Urinary output. Most patients undergoing INR procedures require catheterization of the bladder to assist in fluid management and to increase their level of comfort. Diuresis may also occur during the procedure from the increase in intravascular volume attendant upon continuous flushing of the intravascular lines and from the osmotic load conferred by the injection of either mannitol or the radiocontrast drug. The timing and volume of the injected radiocontrast need to be monitored, especially during prolonged procedures.
Laboratory tests. A baseline arterial blood gas at the time of initial arterial puncture is useful to assess the gradient between the arterial oxygen tension (Pao2) and the hemoglobin saturation (Sao2) and between the arterial CO2 tension (Paco2) and the end-tidal CO2 (ETco2). The activated clotting time (ACT) is used to monitor coagulation. The patients receive large volumes of fluid and radiocontrast and can diurese considerably, so that determination of a baseline hematocrit is helpful as well.
Dynamic sedation. The primary goals guiding the choice of anesthetic for conscious sedation are anxiolysis, the alleviation of pain and discomfort, and the assurance of patient immobility. At the same time, the anesthesiologist must provide for a rapid decrease in the level of sedation when neurologic testing is required. The procedures are not generally painful with the exception of sclerotherapy and chemotherapy. However, an element of pain is associated with distention of and traction on the vessels; the injection of contrast into the carotid artery is frequently described as "burning." Discomfort might also be caused by prolonged periods of immobilization, bladder catheterization, and, to a lesser extent, the femoral puncture site. The patient might find the procedure psychologically stressful because of the potential risk of stroke during the procedure. However, immobilization of the patient, whether by conscious effort or deep sedation, is essential. Movement not only degrades the quality of the images but can also result in vascular injury.
Anesthetic drugs are selected to achieve controlled sedation, adequate analgesia and desired immobility. The primary approach to conscious sedation is to establish a base of neuroleptic anesthesia by the titration of fentanyl, 2 to 4 mcg/kg; droperidol, 2.5 to 5 mg; and midazolam, 3 to 5 mg, after intravenous access and monitoring have been established and oxygen administration has begun. In men, a small bolus of propofol is useful just as the urinary catheter is passed. The bolus dose of propofol also helps the anesthesiologist to assess the patency of the airway under deep sedation and determine whether a nasopharyngeal airway is required. The insertion of the nasopharyngeal airway after anticoagulation can result in troublesome bleeding and is best avoided.
When the patient is in final position, draping is begun and an infusion of propofol is started at a very low dose of 10 to 20 mcg/kg/minute and then increased slowly to render the patient immobile yet breathing spontaneously. The recent availability of the alpha2 agonist, dexmedetomidine, offers an alternative to propofol and has the advantage of improved maintenance of the airway during sedation. Dexmedetomidine, administered as a loading dose of 0.5 to 1 mcg/kg over 20 minutes followed by an infusion of 0.01 to 1 mcg/kg/hour also permits neurologic examination during awake craniotomy. Some evidence, however, indicates that the recovery of cognitive functions might be delayed in patients after receiving dexmedetomidine during INR procedures. Further, patients who have received this drug tend to have lower blood pressures in the recovery period. This may not be desirable in patients who are critically dependent on the maintenance of adequate perfusion pressure to the collateral circulation. Therefore, the choice of anesthetics is on the basis of the experience of the anesthesiologist, the needs of the patient, and the requirements of the procedure.
General anesthesia with endotracheal intubation. General anesthesia with endotracheal intubation in the INR suite is similar to that in the operating room for both adult and pediatric patients. The primary reason for employing general anesthesia is to reduce motion artifact and improve the quality of the images. This is especially pertinent to the INR treatment of spinal pathology during which extensive multilevel angiography is sometimes performed. In view of the fact that chest excursion during positive-pressure ventilation can interfere with road mapping, radiologists frequently request periods of apnea during DSA for certain procedures; this can best be accomplished with endotracheal anesthesia. A theoretic' argument can be made in the context of INR for eschewing the use of N2O because of the possibility of expanding the volume of air emboli introduced into the cerebral circulation.
Anticoagulation. Careful management of coagulation is required to prevent thromboembolic complications from the presence of catheters, which act as foreign bodies, and from endothelial injury associated with the passage of microcatheters. After insertion of the femoral introducer sheath, a baseline ACT is obtained. Heparin, 2,000 to 5,000 U/70 kg, is given intravenously and another ACT is measured approximately 3 to 5 minutes later. The target ACT depends on the clinical needs and could be two to three times the baseline value. Additional heparin may be required throughout the procedure to maintain adequate anticoagulation. On occasions, at the completion of the INR procedure, heparin's anticoagulant effect is reversed with protamine, and the femoral artery catheter is removed in the angiography suite. The proliferation of percutaneous closure devices has improved hemostasis at the arteriotomy site, particularly in patients receiving thrombolytic and antiplatelet drugs.
Deliberate hypotension. Two primary indications for deliberate hypotension are to decrease blood flow through an arteriovenous fistula during the injection of glue and to test the cerebrovascular reserve of the patient undergoing carotid occlusion. In most instances, the level of sedation is decreased to permit neurologic examination during the period of deliberate hypotension. The induction of hypotension in awake or minimally sedated patients can be fairly challenging because large doses of hypotensive drugs may be required to reduce the blood pressure in these patients. Adrenergic-blocking drugs that do not directly affect CBF might be preferable to drugs that are potential cerebral vasodilators. Typically, large doses of esmolol as a 1 mg/kg bolus followed by an infusion of approximately 0.5 mg/kg/minute are required in these patients. Supplemental labetalol might also be needed during the infusion of esmolol. Drugs such as sodium nitroprusside and nitroglycerin may also be used. Because hypotension may cause nausea and vomiting, supplemental doses of antiemetic drugs such as droperidol, 1.25 mg; ondansetron, 4 mg; or dolasetron, 12.5 mg, may be given before decreasing the blood pressure.
Flow arrest. Transient flow arrest has been used successfully for treating high-flow cerebral AVMs in relatively healthy (American Society of Anesthesiologists [ASA] status I and II) patients with the embolization of NBCA glue. When flow arrest is planned, the patient is prepared for a general anesthetic. In addition to the usual preparation, a central venous catheter is inserted to inject drugs. Intravenous adenosine as a 10 to 90 mg bolus has been used for this purpose. Either external pacing pads or a transvenous pacing line is inserted for treating any persistent arrhythmias. The procedure is generally conducted in two parts. In the first part, the target feeding artery is identified and the safety of embolizing the vessel is assessed by performing a superselective Wada test under minimum alveolar concentration sedation. In the second part with the catheter positioned in the desired location, general anesthesia is induced. Conducting a dose-response study to determine the optimal dose of adenosine is recommended. The dose of intravenous adenosine is geared to produce 5 to 15 seconds of asystole and ranges from 10 to 90 mg. Small amounts of either esmolol or nitroprusside might be necessary to treat rebound hypertension or tachycardia.
Deliberate hypertension. During the occlusion of a cerebral artery, planned or inadvertent, systemic blood pressure might need to be increased to augment CBF through collateral vessels. The extent to which the blood pressure needs to be increased depends on the condition of the patient and the nature of the disease. During deliberate hypertension, the systemic blood pressure typically is raised either to 30% to 40% above the patient's baseline or until the ischemic symptoms resolve. The electrocardiogram and ST segments should be inspected for myocardial ischemia. Phenylephrine is the first-line drug for deliberate hypertension. Dopamine might be useful in patients who have a low heart rate.
Deliberate hypercapnia. Deliberate hypercapnia to a Paco2 of 50 to 60 mm Hg may be induced during the treatment of venous malformations of the head and neck. The rationale for employing hypercapnia is to increase cerebral venous outflow relative to extracranial venous drainage and to create a pressure gradient that would divert sclerosing agents away from the intracranial veins. This is usually achieved by decreasing minute ventilation. Alternatively, CO2 may be added to the inspired gases.
Radiation safety. The INR suite has three sources of radiation: direct radiation (from the x-ray tub), leakage (through the collimator's protective shielding), and scattered (or reflected from the patient and the area surrounding the body part being imaged). It is important to realize that the amount of exposure drops proportionally to the square of the distance from the source of radiation (inverse square law). It should also be realized that DSA delivers considerably more radiation than fluoroscopy. Everyone working in the INR suite must wear a lead apron, a thyroid shield, and a radiation exposure badge. Although they are heavy, using leaded glasses is a consideration for anyone who must be near the source of the radiation.

IV. Management of procedural catastrophes

Complications arising from cerebrovascular instrumentation can be rapid and dramatic and require a multidisciplinary approach to management. A catastrophe plan such as that shown in Table-1 should be clearly defined by the anesthesia team for every INR procedure. Drugs and equipment required to secure the airway must be available without any delay. Protamine should be available for immediate injection if the decision is made to reverse the heparinization. There must be effective communication between the INR team and the anesthesiologist. The appropriate neurologic and neurosurgical consultants should be contacted as soon as possible. The anesthesia team has the primary responsibility to secure the airway and ensure adequate ventilation. While securing the airway, the anesthesiologist must communicate with the INR team to determine whether the problem is occlusive or hemorrhagic.

Table-1. Management of neurologic catastrophesa

Initial resuscitation: Communicate with radiologists. Call for assistance. Secure the airway and hyperventilate with 100% O2. Determine if problem is hemorrhagic or occlusive.
Hemorrhagic: Immediate heparin reversal (1 mg protamine for each 100 U heparin given) and low-normal pressure.
Occlusive: Deliberate hypertension, titrated to neurologic examination, angiography, or physiologic imaging studies (e.g., TCD, CBF).
Further resuscitation: Head-up 15 in neutral position. Titrate ventilation to a PaCO2 of 26 to 28 mm Hg. Give 0.5 g/kg mannitol, rapid intravenous infusion. Anticonvulsants: phenytoin (give slowly, 50 mg/min) and phenobarbital. Titrate thiopental infusion to electroencephalogram burst suppression. Allow body temperature to fall as quickly as possible to 33C to 34C. Consider dexamethasone 10 mg.b
a These are only general recommendations, and drug doses must be adapted to specific clinical situations and in accordance with a patient's preexisting medical condition. In some cases of asymptomatic or minor vessel puncture or occlusion, less aggressive management might be appropriate.
bSteroids are of dubious value in the treatment of focal cerebral ischemia but might have a place in reducing mass effect from a hemorrhage, if clinically appropriate.
TCD, transcranial Doppler; CBF, cerebral blood flow.

Occlusive catastrophes. In the case of vascular occlusion, the primary strategy is to increase distal cerebral perfusion either by augmentation of the blood pressure or by thrombolysis. Both therapies may be combined.
Bleeding catastrophes may be heralded by headache, nausea, vomiting, and vascular pain related to the area of the vascular perforation. The radiologist might see extravasation of the contrast only seconds before the patient becomes symptomatic. In the case of the puncture of a vessel, reversal of the heparin before withdrawing either the offending wire or the catheter back into the lumen of the vessel keeps the perforation partially occluded until hemostatic function has been restored. Immediate reversal of heparin is indicated as soon as an intracranial hemorrhage has been diagnosed. Protamine, 1 mg for every 100 U of heparin given, is administered without undue regard for systemic blood pressure. An ACT may be obtained later to adjust the final dose. When active bleeding occurs, the blood pressure must be kept as low as possible. Once the bleeding has been controlled, the target blood pressure should be discussed with the INR team. If vascular occlusion has been used to control the hemorrhage, the INR team may request deliberate hypertension. Allergic reactions to protamine are rare but can occur.

V. Postprocedural considerations

Patients are usually observed in the intensive care unit for the first 24 hours after intracranial and spinal procedures. The groin is monitored for bleeding from the femoral puncture site. In general, INR procedures have their own inherent potential complications and require frequent neurologic, metabolic, and hemodynamic monitoring. For example, after embolization of an AVM, the tissue edema might be minimal but still sufficient to cause deterioration in the patient's neurologic status during the course of the first evening after the procedure.

VI. Specific procedures

Superselective anesthesia and functional examination (SAFE) or superselective Wada is routinely performed before therapeutic embolization to minimize the risk of occluding a nutritive vessel to eloquent regions of either the brain or the spinal cord. This could happen if the microcatheter tip is proximal to the origin of the nutritive vessel. Not all interventionalists, however, recognize the need for SAFE before embolization. The level of sedation is decreased before testing by discontinuing the infusion of propofol. In rare instances, it might be necessary to use naloxone or flumazenil to antagonize other intravenous drugs. The INR team performs a baseline focused neurologic examination under residual light sedation.
Sodium amobarbital, 30 mg/mL, or lidocaine, 30 mg/mL, mixed with contrast is injected through the superselective catheter to obtain an angiogram with the drug/contrast mixture. Sodium amobarbital is used to investigate the gray matter. Lidocaine can be used to evaluate the integrity of the white matter tracts, especially in the spinal cord. The injection of lidocaine into cortical areas, particularly those close to the motor strip, can precipitate seizures, however. Such seizure activity can cause transient neurologic deficits. Postictal paralysis may also confuse the interpretation of the test. For this reason, the barbiturate is usually given first, followed by lidocaine. If the amobarbital test is negative, the amobarbital can protect against seizures but will not interfere with the assessment of lidocaine's effect on white matter tracts.
Superselective angiography and therapeutic embolization of AVMs. Typically, patients who present for embolization have large, complex, parenchymatous AVMs, which are composed of several discrete fistulae with multiple feeding arteries. The goal of the therapeutic embolization is to obliterate as many of the fistulae as possible. The procedure can last up to 4 to 5 hours, depending on the complexity of the lesion. Various embolic materials have been used to obliterate AVM fistulae including polyvinyl chloride particles. More durable results have been achieved with NBCA glue.
In rare cases, INR treatment is aimed at total obliteration. More commonly, however, embolization is employed as an adjunct in preparation for either surgery or radiotherapy and can be beneficial in several ways. First, embolization may facilitate operation by obliterating deep feeding arteries that are difficult to approach surgically and thereby reduce the surgical risk. Second, staging the obliteration of the arteriovenous shunts also theoretically allows the surrounding brain to accommodate to the alterations in hemodynamics and may prevent normal perfusion-pressure breakthrough. Third, the obliteration of high-flow feeders can benefit patients who have either progressive neurologic deficits or intractable seizures. Neurologic improvement after high-flow AVM embolization has been attributed to the decease in cerebral steal and a decrease in the mass of the lesion. Finally, approximately 10% of patients with AVM harbor intracranial aneurysms. Such aneurysms appear to increase the risk of spontaneous hemorrhage from AVMs. The obliteration of intranidal aneurysms during the initial embolization may decrease the rate of recurrent hemorrhage during the course of treatment.
Once the catheter has been positioned for potential glue injection, SAFE is performed. If SAFE is positive (i.e., if focal neurologic deficits develop), either the catheter is repositioned or embolization of that pedicle is aborted. If the test is negative, either glue or another embolic material may be injected. Controlled deposition of the glue is necessary to decrease complications from pulmonary embolism or obstruction of the AVM's venous drainage. The achievement of flow arrest through the fistula is necessary during injection of the glue to facilitate polymerization and solidification of NBCA glue. Techniques for flow arrest include deliberate hypotension, balloon occlusion of the proximal vessel, and circulatory pause with either adenosine or controlled ventricular fibrillation. In most instances, deliberate hypotension suffices for flow arrest, which can be achieved when the mean systemic blood pressure is reduced to approximately 50 mm Hg. Flow arrest is usually not needed for embolization with PVA particles.
The measurement of the immediate postembolization pressure has been suggested as a way to follow the course of hemodynamic changes and predict postprocedure complications. A large increase in the pressure of the feeding artery after embolization may be associated with intracranial hemorrhage. Because the AVM's feeding arteries supply normal vascular territories to a variable degree, the abrupt restoration of normal perfusion pressure to a chronically hypotensive vascular bed might overwhelm its autoregulatory capacity and result in either hemorrhage or swelling, the phenomenon known as normal perfusion-pressure breakthrough. For this reason, the target range for maintenance of posttreatment blood pressure is at or slightly below the normal blood pressure of the patient.
Embolization of spinal cord lesions. Embolization can be used to treat intramedullary spinal AVMs, dural fistulae, and tumors invading the spinal canal. For cases performed under general anesthesia with endotracheal intubation, an intraoperative "wake-up test" may be requested. The wake-up test must be explained to the patient the night before and on the day of surgery. A N2O/narcotic anesthetic technique with concurrent administration of propofol may be employed for the procedure. Neuromuscular blockade, if required, should be readily reversible for the wake-up test. For selected lesions, somatosensory and motor evoked potentials may be helpful in both anesthetized and sedated patients. When motor evoked potentials are monitored, the degree of neuromuscular blockade should be titrated to the monitoring needs.
Carotid test occlusion and therapeutic carotid occlusion. Test occlusion of the carotid artery is undertaken either before the anticipated sacrifice of the vessel or when temporary carotid occlusion may be required during surgery. During the test occlusion, a catheter with a distal balloon and a lumen is placed in the ICA. A baseline neurologic examination is performed. Flow velocity is measured over the middle cerebral artery by TCD ultrasound, if available, and the CBF can be measured by the intracarotid 133Xe injection technique. Baseline femoral and carotid artery pressures are also noted. The balloon is then inflated and the pressure in the carotid artery distal to the balloon is recorded. The inflation of the balloon can cause headache and at times an increase in the systemic blood pressure. Aggressive treatment of the hypertension is probably not warranted because it may decrease collateral perfusion pressure. The anesthesiologist should be prepared to treat bradycardia with atropine. The neurologic examination is repeated a few minutes after occlusion, and TCD and 133Xe CBF measurements are taken again. After 133Xe washout data have been obtained, a radioactive tracer for single-photon emission computerized tomography (SPECT) studies may be injected. This provides a snapshot measurement of regional CBF during ICA occlusion. Because SPECT tracers usually have a long half-life and bind avidly to cerebral tissues, the imaging part of a SPECT study may be undertaken in the nuclear medicine department after the patient leaves the INR suite.
To assess the patient's cerebrovascular reserve, the systemic blood pressure can be decreased if the patient has not demonstrated any neurologic impairment during the initial ICA occlusion. The neurologic examination is repeated at frequent intervals while the blood pressure is reduced. The distal ICA or stump pressure at which neurologic deterioration occurs and whether the patient starts yawning "often a sign of impending cerebral ischemia" are noted and correlated with the corresponding TCD flow velocity. Depending on the clinical condition of the patient, another 133Xe measurement is obtained. If overt neurologic symptoms develop, the balloon is immediately deflated, hypotensive drugs are discontinued, and vasopressors might be required to increase the blood pressure to normal levels, depending on the clinical situation.
Although uniform guidelines for interpreting the results of test occlusion are yet to be formulated, a new neurologic deficit, significant asymmetry on SPECT imaging, and a 25% to 30% reduction in CBF as measured after occlusion by either 133Xe CBF or TCD may be considered as relative indications for an extracranial to intracranial bypass procedure before sacrifice of the carotid artery.
Aneurysm ablation. Many intracranial aneurysms are amenable to endovascular treatment. As the techniques of interventional therapy have continued to evolve, endovascular ablation is increasingly considered to be the primary treatment modality for ruptured and unruptured intracranial aneurysms. The indications for open surgical versus endovascular therapy are currently a topic of vigorous discussion. There are two basic approaches for endovascular obliteration of intracranial aneurysms: (a) the occlusion of the proximal parent artery, such as the carotid artery, which was discussed earlier and (b) obliteration of the aneurysmal sac itself. Endovascular obliteration of the aneurysmal sac is usually accomplished through the use of detachable coils manufactured by a wide variety of vendors. Complicated aneurysms that have wide necks and large sacs may require advanced techniques involving either temporary balloon remodeling or placing an intracranial stent. These procedures may be prolonged and require general anesthesia with endotracheal intubation. The anesthesiologist should be prepared for the occurrence of aneurysmal subarachnoid hemorrhage (SAH) either spontaneously or as a result of the intravascular manipulations. Occlusive complications may also develop and require additional maneuvers to enhance CBF and initiate revascularization. Occlusion and thrombosis of the aneurysm may be ongoing immediately after intervention. Therefore, careful attention to the control of blood pressure in the postprocedure period remains critical, especially in patients who have presented with SAH.
Angioplasty. Either mechanical angioplasty or pharmacologic dilatation may be indicated for vasospasm after SAH and for atherosclerotic cerebrovascular disease.
Angioplasty for cerebral vasospasm is usually undertaken in patients who, despite maximum medical management, continue to demonstrate neurologic signs and symptoms of cerebral ischemia. These patients are often in extremis: their tracheas are frequently intubated, they are receiving vasopressor drugs, and they have either an external ventricular drain or other device in place to monitor intracranial pressure. Angiography is first undertaken to demonstrate that a significant degree of spasm in large proximal vessels (anterior, middle, and posterior cerebral arteries) exists. A balloon catheter is guided under fluoroscopy into the spastic segment and inflated to distend the constricted area mechanically. If deliberate hypertension is being used to ameliorate a focal neurologic deficit before angioplasty, the blood pressure should be reduced to the patient's normal range after angiographic demonstration of significant widening of the spastic segment.
Pharmacologic dilatation is also used for the treatment of cerebral vasospasm by direct intra-arterial injection of vasodilators under fluoroscopic guidance. Although the technique was originally described for papaverine, the selection of vasodilators has now expanded to include the more frequent use of calcium-channel-blocking drugs such as verapamil and nicardipine. During recirculation, the anesthesiologist needs to monitor for systemic side effects of these drugs, which include hypotension and bradycardia, especially with the calcium-channel-blocking drugs.
Angioplasty for atherosclerosis. At present, patients who have high-risk factors for carotid endarterectomy are considered favorable candidates for cervical carotid angioplasty and stenting (CAS, Stenting and Angioplasty with Protection in Patients at High Risk for Endarterectomy [SAPPHIRE] Trial). Studies evaluating the safety and efficacy of carotid stenting in traditional "low-risk" patients are ongoing (Carotid Revascularization Endarterectomy versus Stent Trial [CREST]), with encouraging preliminary results. These patients generally require balloon dilatation and placement of a vascular stent. In many cases, the insertion of a cerebroprotective device may be used to decrease the risk of distal thromboembolism. Angioplasty of the ICA is usually undertaken with minimal sedation. General anesthesia is required for treating segments of the intracranial arteries. Intracranial angioplasty and stenting procedures for atherosclerosis have a higher level of risk secondary to the inherently more delicate nature of intracranial vessels. Intracranial arteries have a thinner media and therefore are more prone to dissection and perforation. The selection of anesthetic technique also depends on the patient's medical condition and ability to cooperate during the procedure as well as the presence of anticipated technical difficulty in negotiating the stenosed segment. Deliberate hypertension may be required to augment collateral blood flow. The considerations for general anesthesia are similar to those for carotid endarterectomy.
Thrombolysis for acute stroke. It is possible to recanalize the occluded vessel after an acute thromboembolic stroke by either mechanical means or superselective intra-arterial delivery of thrombolytic agents, such as recombinant tissue plasminogen activator, streptokinase, or urokinase. Significant improvement in neurologic outcome has been demonstrated when either pharmacologic thrombolysis is completed within 6 hours of the onset of ischemic symptoms (Prolyse in Acute Cerebral Thromboembolism II [PROACT II]) or mechanical thrombolysis is performed within 8 hours (Mechanical Embolus Removal in Cerebral Ischemia [MERCI] Trial). Techniques for mechanical means of clot retrieval, balloon angioplasty, and laser ablation are also being developed. These methods can restore arterial flow more rapidly than intra-arterial thrombolysis, which may require up to 2 hours. In the vertebro-basilar circulation, treatment may be effective even when administered as long as 24 hours after the onset of symptoms. The main risk of intra-arterial thrombolysis is hemorrhagic conversion of the ischemic infarct, which has a high mortality when it occurs.
Anesthetic considerations in these patients include those for elderly people and for patients who have widespread arterial disease. Hypertension occurs spontaneously after acute thromboembolic stroke and, in the face of nonhemorrhagic focal neurologic deficits, should not be treated aggressively. Once clot lysis has been accomplished, the blood pressure is maintained in the patient's normal range and ideally titrated to some index of CBF to prevent hyperperfusion injury.
Treatment of other CNS vascular malformations
Dural AVMs. Dural AVMs induce venous hypertension, and, when cortical venous drainage is involved, may cause intracranial hemorrhage. Multiple intracranial and extracranial arteries may feed these dural AVMs so that multistage embolization is usually performed. SAFE are required, as in the case of intracranial AVMs. Transarterial embolization with NBCA glue is a commonly utilized technique, as is the transvenous coil occlusion of pathologic venous pouches.
Carotid cavernous fistulae. Skull-base trauma is the most common etiology of carotid cavernous fistula. Traumatic fistulae can also occur between the vertebral artery and the paravertebral veins. Such arteriovenous fistulae can lead to chronic hypotension of the surrounding normal vascular territories. The treatment may include either transarterial occlusion with detachable balloons or transvenous occlusion of the involved cavernous sinus. The obliteration of these fistulae might result in normal perfusion-pressure breakthrough. Therefore, after obliterating these lesions, the blood pressure should be maintained in the range of 10% to 20% below the normal pressure of the patient.
Vein of Galen malformations. These relatively uncommon but complicated lesions usually present in infancy and childhood. The patients may have congestive heart failure, intractable seizures, hydrocephalus, and mental retardation. Several approaches have been attempted including transarterial and transvenous methods. Concerns during general anesthesia for INR therapy are the same as for surgical treatment. In the setting of congestive heart failure, preexisting right-to-left shunts, and pulmonary hypertension, a relatively small glue embolus can be fatal.
Intra-arterial chemotherapy and embolization of tumors. Preoperative embolization as a means of decreasing blood loss during surgery can be performed for many hypervascular intracranial or spinal tumors. Paragangliomas can cause catecholamine release from the tumor during embolization, and means of treating the ensuing hypertensive crisis should be at hand. Superselective administration of chemotherapeutic agents can also be used for treating neoplasms refractory to conventional therapy.
Spinal compression fracture therapy. Vertebroplasty and kyphoplasty procedures involve the intravertebral body administration of acrylic bone cement compounds via a percutaneous, often transpedicular, approach. Many patients report significant pain relief after these procedures. The demographics of this population (elderly, frail, osteoporotic) and prone positioning require extra care during the preparation process. Most of these procedures fortunately can be performed and well tolerated with mild sedation.

VII. Conclusions

Interventional neuroradiology offers a new approach to several intracranial and spinal disorders. To some extent, the risk/benefit ratio of INR procedures remains to be elucidated when compared with traditional surgical approaches. The anesthetic management of these procedures, while similar to traditional operative approaches, is beset with hazards and requires certain accommodations.

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