The therapeutic idea of anastomosis of the external carotid artery, or one of its branches, with the internal carotid artery above the area of narrowing was advocated by C. Miller Fisher in 1951 as a means of cerebral revascularization.1 In the late 1960s, Yasargil2 brought this idea into reality and performed the first bypass procedure for a patient with occlusive cerebrovascular disease. Currently, there are 2 major indications for cerebral revascularization bypass surgery: (1) flow augmentation and (2) flow replacement.3,4 The most common procedure for flow augmentation is the superficial temporal artery (STA) to middle cerebral artery (MCA) bypass, also known as the extracranial-intracranial (ECIC) bypass. The treatment of patients with moyamoya disease is now the most common indication for this procedure. There is also a renewed interest in its use for stroke prevention in patients with occlusive cerebrovascular disease. Another use of bypass surgery is in patients with complex cerebral aneurysms and tumors, where treatment of the lesion requires sacrificing of a major artery, and thus a flow replacement bypass is needed to provide blood flow to the compromised area.4 Anesthetic considerations for ECIC bypass for patients with moyamoya disease have been well reviewed, but there is limited information for occlusive cerebrovascular disease and for the treatment of intracranial aneurysms and tumors. The purpose of this article is to provide a comprehensive review on both anesthetic and surgical considerations for patients undergoing cerebral revascularization procedures according to the indications of flow augmentation and flow replacement.
TYPES OF BYPASS PROCEDURES
Cerebral bypass procedures are classified according to the location of blood vessels used for anastomosis, the amount of blood flow through the bypass, and the indication for the procedure (Table 1). The actual donor and recipient blood vessels used for the bypass vary depending on the location of the brain that requires flow augmentation or replacement. These procedures are thus described as ECIC when a vessel outside of the cranium is anastomosed to an intracranial vessel. The most common ECIC procedure is the STA to MCA bypass. If both vessels are intracranial, the procedure is named an intracranial-intracranial (ICIC) bypass such as posterior inferior cerebellar artery (PICA) to PICA bypass. In a low flow bypass, the blood flow through the graft is about 10 to 30 mL/min, whereas a high flow bypass may have graft blood flow up to 100 mL/min.5
A flow augmentation procedure is considered when the blood flow to the area of concern is borderline, that is, the risk of the occurrence of cerebral ischemia and infarction is considered likely due to inadequate blood flow. Moyamoya disease and intracranial occlusive cerebrovascular disease are the typical indications of flow augmentation procedures. An ECIC bypass with the use of STA or occipital arteries as the graft is usually considered in this situation. A STA-MCA bypass will initially have a blood flow approximately 30 mL/min and has been shown to have a capacity to dilate and increase this flow over time.6 Flow augmentation procedures are also classified as direct or indirect. A direct procedure has the anastomosis placed between 2 vessels, whereas indirect refers to the placement of an intact scalp artery on the surface of the piamater/arachnoid to allow for revascularization.
A flow replacement procedure may be indicated when an artery contributing to, or branching from, the Circle of Willis will be sacrificed in the management of complex intracranial vascular pathology or tumor. These procedures usually need high flow grafts, which can deliver larger volumes of blood. They also may require a graft that is longer in length due to the longer distance between the anastomosis sites. The usual source for the bypass graft is the saphenous vein or radial artery. At times, however, an STA graft may be chosen for flow replacement if it is deemed to be adequate in caliber and estimated to meet the requirements of the receiving territory. These bypasses may be either ECIC vessels or ICIC vessels.
Occlusive Cerebrovascular Disease
Cerebral atherosclerosis, a major cause of occlusive cerebrovascular disease, can be divided into extracranial atherosclerosis (ECAS) and intracranial atherosclerosis (ICAS). There are differences in prevalence, risk factors, stroke mechanism, treatment, and functional outcome between ECAS and ICAS diseases.7,8 In contrast to ECAS, which is prevalent in Caucasians, ICAS is more common in the Asian population contributing up to 33% to 50% of strokes in this population.9,10 Typically, most of the risk factors (hypertension, hyperlipidemia, diabetes, history of smoking) are similar between the 2 types, though patients with ICAS are often younger, female, and may have a metabolic syndrome.7 Patients with ICAS are also at higher risk for poor functional outcome.11 Although carotid endarterectomy and stenting are the common surgical interventions in ECAS, patients with ICAS often present for intracranial bypass procedures.
The first ECIC procedures were performed using STA to MCA bypass for patients with occlusive cerebrovascular disease.3,4 In 1985, results of the International Cooperative Study of Extracranial Intracranial Arterial Anastomosis (ECIC bypass trial), a prospective, randomized controlled study comparing medical treatment to ECIC, showed no reduction in major and fatal strokes, ipsilateral strokes, or all strokes and death combined with ECIC.12 After this, there was a decrease in the use of ECIC for occlusive cerebrovascular disease.13 Many investigators also believed that the failure of the ECIC bypass trial was largely due to the poor selection of patients. With a better understanding of cerebral ischemia and metabolism, and advances in neurological imaging, there is a renewed interest in performing ECIC bypass procedures for patients with occlusive cerebrovascular disease.13–16 The newer imaging modalities, such as magnetic resonance imaging, xenon-computerized tomography, or positron emission tomography scanning, will help to identify a subgroup of patients most likely to benefit from cerebral revascularization.17,18 These imaging modalities can be used to demonstrate perfusion abnormalities of the cerebral perfusion, which has been shown to significantly increase the risk of stroke.17,18 Today, cerebral revascularization bypass surgery is performed in patients in whom the radiographic and metabolism studies demonstrate that they have decreased cerebral blood flow (CBF) and poor or absent cerebral vascular reserve. Using positron emission tomography, Grubb et al.19 described the stages of hemodynamic failure based on the changes in CBF, cerebral perfusion, and oxygen extraction (Table 2). In hypoperfused cerebral regions, where the microcirculation is maximally vasodilated to maintain adequate CBF and oxygen delivery, further reduction in cerebral perfusion would lead to an increase in oxygen extraction. This compensatory mechanism is used as an indicator of exhausted vascular reserve and is considered as “stage 2 hemodynamic failure. ” Thus, the indications for ECIC include patients who have perfusion abnormalities resulting in reduction or loss of cerebral vascular reserve capacity to reduce the risk of stroke.
Another study, the Carotid Occlusion Surgery Study (COSS) was a prospective, open-labeled, randomized controlled trial to compare STA-MCA bypass versus best medical therapy for reducing subsequent ipsilateral stroke in patients with complete internal carotid artery occlusion and an elevated oxygen extraction fraction in the cerebral hemisphere distal to the occlusion.20 The results showed no overall benefit with ipsilateral recurrence of stroke in the surgical group, despite a high bypass graft patency rate and demonstrated improvement in cerebral hemodynamics. The study design and methodology have been criticized for failure to select patients of at risk of stroke and the use of a semiquantitative method of measuring oxygen extraction ratio.21 At present, there is no clear evidence to reliably identify patients with atherosclerotic cerebrovascular disease who are most likely to benefit from bypass therapy. Thus, offering patients this procedure to prevent strokes is still controversial. Currently, the practice remains highly individualized, with patients being offered ECIC bypass only in the presence of severe hemodynamic impairment (as assessed by impaired cerebral vascular reserve capacity on magnetic imaging with a CO2 challenge) and the exclusion of emboli as a major contributor to their symptoms.22
Moyamoya is a progressive disorder caused by blocked arteries or spontaneous occlusions of the blood vessels of the Circle of Willis.23–25 The name in Japanese refers to a puff of cigarette smoke drifting in the air and describes the look of the tangle of tiny vessels of secondary neovascularization of the lenticulostriate and leptomeningeal circulation which are formed to compensate for the blockage (Fig. 1). Moyamoya syndrome is traditionally considered in patients who have the characteristic vasculopathy and associated conditions such as neurofibromatosis or sickle cell disease. Classically, moyamoya manifests itself bilaterally and these occlusive lesions are progressive; thus, many patients present for bilateral procedures.23–25 In pediatric patients with moyamoya, which is more prevalent in Japan, the first symptom is often a stroke or transient ischemic attack (TIA), muscular weakness, paralysis, or seizures.23 In Caucasians, moyamoya is seen most commonly in adults and usually presents with either headaches or stroke (ischemic or hemorrhagic).23 The intraoperative management of moyamoya may also be complicated by the presence of associated intracranial aneurysm.
Surgical revascularization procedures for blood flow augmentation include both direct and indirect bypass techniques.23,26 Direct bypass is where an extracranial source of blood augmentation (typically the STA) is directly anastomosed to an intracranial artery (typically a cortical MCA branch) (Fig. 2). This should result in an immediate increase in blood flow to the brain; however, the increase may be unpredictable and is dependent on the flow within the donor vessel, and the pattern of the underlying disease.27 This procedure involves identifying (usually with micro-Doppler ultrasound) the ipsilateral STA, which is then harvested free on the distal end. A small craniotomy is performed to find a cortical branch of the MCA suitable for anastomosis. Heparin is administered (50–100 units/kg) at this stage. This MCA branch is temporarily occluded with arterial clips and the STA is anastomosed (using 9-0 or 10-0 Ethilon suture) as an end to side anastomosis to it. After distal anastomosis is completed, temporary clips are removed, hemostasis is achieved, and the heparin may be totally or partially reversed with protamine (0.5–1.0 mg/kg). Occasionally, the anterior communicating artery may also be used for bypass with STA. Other ECIC bypass procedures are also performed using different arteries such as STA-to-posterior cerebral artery, STA-to-superior cerebellar artery, or occipital artery to posterior cerebral artery. The donor and recipient vessels chosen for bypass are determined in part by the area of interest and the quality of the vessels, which can be assessed by the preoperative angiography.
Indirect bypass is performed as an alternative, mainly in pediatric patients with moyamoya disease, when the small size of the cortical vessels may preclude direct anastomosis. The rationale with indirect bypass is to stimulate pial vascularization by allowing direct contact between the pia-arachnoid and an extracranial source of vascularized tissue.24,26 These procedures include encephaloduroarteriosynangiosis, which is the laying of an intact scalp artery (usually the STA) onto the surface of the pia/arachnoid to allow for revascularization, and encephalomyoarteriosynangiosis, which is the placement of a pedicle of the temporalis muscle over the brain surface28 (Fig. 2). Other techniques less commonly used include creation of burr holes and transplantation of omentum.28 Some procedures may include a combination of both a direct and an indirect bypass. Indirect bypass techniques do not generally result in immediate hemodynamic shifts within the intracranial circulation because the neovascularization relies on ingrowth of capillaries that takes a number of weeks or months to mature.
The anesthetic management of patients with moyamoya for ECIC has been well described for both pediatric and adult populations.29–37 In contrast, there is limited information on the anesthetic management of patients with occlusive cerebrovascular disease. Most of the following discussion reflects the accepted anesthetic management of patients with moyamoya, but this can also be used as a guide for patients with occlusive cerebrovascular disease for ECIC or ICIC procedures. In addition, patients with occlusive cerebrovascular disease are managed in a similar manner to patients presenting for carotid endarterectomy. Anesthetic considerations of cerebral revascularization procedures are summarized in Table 3.
The age of patients presenting for ECIC surgery can range from pediatrics to adults. Depending on the mode of presentation of their disease, these patients may have significant preexisting neurologic deficits. Hypertension may occur as a result of adaptation to cerebral vascular insufficiency (hypoperfusion). Inappropriate treatment of this compensatory hypertension may lead to cerebral ischemia and stroke. Patients with occlusive cerebrovascular disease often also have associated cardiovascular disease.38
Patients with symptomatic moyamoya disease and carotid stenosis or occlusion will usually be taking antiplatelet medications. The actual practice varies, but low-dose aspirin is often continued until the day of surgery. Though in some centers, it may be stopped 7 to 10 days before the surgery and bridged with low molecular weight heparin. In neurologically unstable patients needing reversible anticoagulation before the angiography and surgery, low molecular weight heparin (0.5 mg/kg twice a day subcutaneously) can be used. Aspirin is then restarted on the first postoperative day.37
Intraoperative monitoring during ECIC includes all routine requirements with addition of an intraarterial catheter for continuous arterial blood pressure measurement intra- and postoperatively. Cerebral ischemia is a potential perioperative risk especially during temporary occlusion of cerebral vessels. However, in a standard STA–MCA bypass, vessel occlusion is usually brief in duration and has been shown to carry a low risk of intraoperative ischemia.39 Cerebral function monitoring with electroencephalogram (EEG) and somatosensory evoked potentials (SSEP) have been used though there is limited information on the usefulness during these ECIC procedures.40 Significant slowing of the ipsilateral EEG and SSEP changes were reported in 2 of 45 (4.4%) cases.40 Monitoring techniques of global cerebral function with cerebral oximetry,41,42 and jugular bulb oxygen saturation41,43,44 have been reported; however, the usefulness of these techniques has not yet been assessed.
Indocyanine green (ICG) videoangiography has been established as a noninvasive technique to assess the patency of a bypass graft45 (Fig. 1). This allows a quick, real-time intraoperative assessment of relevant vasculature, including vessels smaller than 1 mm, and to check for graft patency and collateral blood flow reliability.45,46 The angiography requires IV injection of ICG and the use of a microscope with an integrated ICG camera with near-infrared light applied directly on the operative field.45,46 ICG is a preservative-free powder (25 mg), which is diluted with 10cc of aqueous solvent (2.5 mg/mL). The usual dose range for intraoperative videography is 5 to 25 mg. The main disadvantage of ICG angiogram is the inability to visualize deeper vasculature structures. Injection of ICG may cause erroneous desaturation readings on pulse oximeter47 and both anaphylactic and anaphylactoid reactions have been reported with the use of ICG.48,49
Appropriate hemodynamic control is critical throughout the perioperative period. Hypotension may lead to ischemia during the procedure and later with thrombosis of the bypass graft. Reduction in CBF is poorly tolerated in children, who have a higher cerebral metabolic rate, diminished autoregulatory response, and higher oxygen extraction ratio than adults.33 Hypertension may lead to bleeding especially at the site of the anastomosis during and especially after surgery. The optimal or best level of arterial blood pressure during and after surgery may be difficult to define. The general recommendation is to maintain “normotension” or to keep the blood pressure within 10% to 20% of the preoperative established baseline blood pressure for all patients.31,33,35–37 Control of blood pressure during induction and maintenance of anesthesia is with careful titration of anesthetic drugs to the level of surgical stimulation. Any episodes of hypotension (systolic less than 100 mm Hg) should be treated aggressively with vasoactive drugs such as phenylephrine or ephedrine. The goal of blood pressure management postoperatively should be discussed with the surgeon and appropriate treatment instituted if need be. Persistent hypertension can be controlled with drugs such as hydralazine, esmolol, or labetalol.
Intraoperative ventilation of the patient’s lungs should be aimed at providing normocarbia and normo-oxygenation.29,31,33–37 Hyperventilation-induced hypocapnia causes cerebral vasoconstriction and can lead to cerebral ischemia especially in areas which have poor CBF.37 Hyperventilation induced by crying or exercise has been shown to trigger a TIA in children with moyamoya.36 In a case series by Sumikawa and Nagai,29 intraoperative hypocapnia (PaCO2 30–35 mm Hg) was shown to be associated with delayed recovery of consciousness and postoperative neurological deficits when compared with patients who had general anesthesia with PaCO2 between 40 and 50 mm Hg.
Hypercapnia can also have undesirable effects. The collateral network of vessels in patients with moyamoya is in a state of maximal vasodilation. During hypercapnia, a decrease in CBF may occur by means of an “intracerebral steal” effect when other normal vessels vasodilate.50–52
Induction of anesthesia is with standard and carefully titrated induction drugs and good control of blood pressure, oxygenation, and CO2 level. The ideal anesthetic technique (inhaled or total IV) and the actual choice of drugs for maintenance of anesthesia have been debated and both techniques have been used.41,53,54 In a study assessing the perioperative course of 216 patients undergoing revascularization surgery, there were no differences in patient outcome among different anesthetic techniques (inhaled, IV, and balanced).55 The severity of disease and surgical procedures were the only determinants of neurological deterioration.
The goal of fluid management is to maintain normovolemia, though some have recommended a hypervolemic state to avoid the problems related to hypotension and decreased cerebral perfusion.33,35–37 The patient’s temperature should be maintained at normal to prevent postoperative shivering. The role of cerebral-protective techniques or drugs, such as hypothermia or administration of barbiturates or propofol at the time of temporary occlusion, is debatable and is dependent on the institution and/or surgeons’ preferences. As discussed in the surgical considerations, anticoagulation is used during the temporary occlusion of the vessels and this may increase the risk of bleeding.
Emergence and Postoperative Care
A smooth emergence with good hemodynamic control is critical to prevent hemorrhagic complications and graft thrombosis. Monitoring and care of the patient continues into the postoperative period in the postanesthetic care unit or a neurosurgical critical care unit. Neurological assessment is performed as soon as possible. Direct pressure on the side of the head where the ECIC with the STA has been used must be avoided. Blood pressure should be maintained at an established level for each patient and deviations treated. Recommendations have been suggested to maintain the systolic blood pressure <120 mm Hg in normotensive patients and <140 mm Hg in hypertensive patients for a week postoperatively.56 Also if intraoperative pial hyperemia is observed, the systolic blood pressure should be kept <120 mm Hg for 1 week.56 Intraoperative and postoperative hypercapnia have been associated with an increased incidence of perioperative TIA50 and transient neurological deficits.43 Hence, the level of PaCO2 should be actively controlled during the emergence and in the postoperative period. Oxygen supply and carrying capacity should be optimized by appropriate oxygen therapy and transfusion. Complications such as seizures need to be treated quickly. Appropriate analgesia should be given to prevent agitation and increased stress, which may affect the bypass.
COMPLICATIONS AND OUTCOME
Intraoperative complications in patients with moyamoya have been shown to be mostly cardiopulmonary and have minimal impact on patient outcome.55 Postoperative cerebral ischemia has been reported to be approximately 3.5% in adult moyamoya patients44 and 16.9% in pediatric moyamoya patients.50 The severity of the disease, preoperative TIA history, and indirect bypass procedures were the major determinants for postoperative neurological deficits in adult moyamoya patients.55 Notably, the risk of cerebral ischemia persists into the postoperative period for patients with indirect bypass procedures because neovascularization requires months to complete. Similarly, frequent preoperative TIA and intraoperative hypercapnia were associated with perioperative infarction in pediatric patients.50 In contrast, the perioperative stroke rate (30 days) for patients with occlusive cerebrovascular diseases undergoing bypass procedures was reported as 12.2% in the ECIC bypass trial12 and 15% in the COSS study.28 The mechanisms of stroke were usually related to hypoperfusion and/or the occurrence of artery-to-artery emboli.12,20
Cerebral Hyperperfusion Syndrome
Cerebral hyperperfusion syndrome has been well described in patients undergoing carotid revascularization (endarterectomy or stenting)38 but is less well known after ECIC bypass. There has been some speculation that hyperperfusion syndrome may develop after ECIC bypass in a previously underperfused cerebral vascular bed due to impaired cerebral autoregulation. The incidence of postoperative transient neurological deterioration due to cerebral hyperperfusion was reported in 17% to 38% of patients with adult-onset moyamoya disease.57–60 The incidence of cerebral hyperperfusion in patients with occlusive cerebrovascular disease was reported at 17% after bypass procedures,60 and 0.4% to 20% after carotid endarterectomy.38 Previous studies have shown that adult onset, hemorrhagic presentation, and increased preoperative cerebral blood volume were risk factors for cerebral hyperperfusion.60 In contrast to the classical presentation of cerebral hyperperfusion in patients undergoing carotid endarterectomy who commonly exhibit progressive headaches, ophthalmoplegia, and seizures, transient focal neurological deficits that mimic an ischemic attack are the most common presentation among patients with moyamoya diseases and occlusive cerebrovascular diseases.60 Because cerebral hyperperfusion is a well-known risk factor for intracranial hemorrhage with a high risk of mortality, most investigators emphasize the importance of maintaining strict control of blood pressure in the postoperative period to prevent this complication.57–60 Control of blood pressure at targets less than 120/80 mm Hg, and routine postoperative monitoring of CBF using transcranial Doppler and or dynamic imaging techniques (computerized tomography or magnetic resonance perfusion, single-photon emission computed tomography) have been shown to be effective in preventing permanent neurological sequelae secondary to cerebral hyperperfusion syndrome.60
REPLACEMENT OF BLOOD FLOW
Revascularization techniques are also used in patients with complex cerebral aneurysms and skull base tumors.4,56,61 A small subset of patients with complex cerebral aneurysms, which are not amenable to coiling or clipping, require that an artery be sacrificed for safe exclusion of the aneurysm from the circulation. In order to prevent cerebral ischemia, replacement of blood flow with a bypass is thus required. These patients are usually identified by preoperative angiographic anatomical assessment of their collateral circulation from the Circle of Willis, endovascular balloon occlusion testing, functional measurements of CBF, and perfusion imaging to assess collateral reserves. The balloon test occlusion technique was first described and used in humans by Serbinenko62 in 1974. The test occlusions are performed at normal blood pressure and during temporarily lowered blood pressure (hypotensive challenge). The presence of a neurological deficit in response to the endovascular balloon occlusion warrants a bypass procedure if the parent vessel cannot be preserved surgically.61 Other angiographic criteria such as venous circulation times are also used by many centers in deciding on the adequacy of collateral circulation.63 A hypotensive challenge test by lowering mean arterial blood pressure 20 mm Hg or 25% from baseline by pharmacological means, such as with sodium nitroprusside, may also help to determine the need for either a high flow or low flow bypass.4
Patients with anterior and middle fossa skull base tumors that encase the internal carotid artery may require sacrificing of the artery for resection of the tumor. Grading systems have been described to show the involvement of segments of the internal carotid artery within the tumor64 and are used in the consideration of the need for cerebral bypass during treatment.
Flow replacement bypass with an interposition graft is usually regarded as a technically difficult operation and is associated with higher morbidity and mortality than other types of cerebral revascularization procedures. The additional risks are due to harvesting and handling of a long graft, construction of several anastomoses, and the surgical treatment of the underlying complex aneurysm or tumor.56,61 Other surgical considerations include the question of the need for a high flow bypass and the proper choice of the vascular conduit, either long saphenous vein or radial artery.
The usual sequence of the procedure includes harvesting of the graft, dissection of the neck to expose the common carotid or external carotid, craniotomy for exposure of the lesion and for placement of the distal recipient artery, preauricular tunneling for the bypass graft, construction of the distal and proximal anastomosis, and then the definitive treatment of the aneurysm or tumor. The anastomosis is typically from the ipsilateral carotid artery system (common carotid or external carotid) with tunneling of the graft subcutaneously and anastomosing it to the intracranial artery of interest (Fig. 3). Intracranial sites of the carotid artery system may also be used for the proximal anastomosis. Depending on the location of the aneurysm, other types of bypass connections may also be used, such as PICA-to-PICA bypass for a brainstem aneurysm. The standard method of anastomosis is with heparinization of the patient, temporary occlusion of the recipient artery, and with the use of microsutures of 8-0 to 10-0 monofilament for the anastomosis. Generally, the duration of temporary occlusion is aimed to be less than 20 minutes along with controlled hemodynamics, and with or without administration of cerebral protection.65
More recently, the development of the Excimer Laser-Assisted Non-occlusive Anastomosis (ELANA) techniquea has allowed for the construction of a high flow bypass with an end-to-side anastomosis between a donor vessel and a recipient artery without the need to temporarily occlude the recipient artery.66,67 Most of the procedure follows the conventional techniques, but the laser is used to create a hole in the artery, though not the anastomosis. The advantages of ELANA are the reduction in the risk of ischemia because there is no need for temporary occlusion, less exposure of vessels is required and thus less brain retraction, heparinization may be minimized, and cerebral protection may become unnecessary.67,68
The definitive treatment of the aneurysm is dependent on the intraoperative assessment of the anatomy. Commonly used treatment modalities include either complete clipping of the neck of the aneurysm or trapping the aneurysm by occlusion of the feeding artery (proximally or distally) allowing for spontaneous thrombosis56,61 (Fig. 3). Securing the aneurysm is often performed at the same time as the bypass procedure. In difficult cases, either staged operations, or combined with postoperative endovascular coiling and/or stenting are also used.
The definitive tumor resection is dependent on the specifics of the tumor and patient. This may be performed as a second operation, often 3 to 7 days after the bypass procedure to avoid long operations and hemorrhagic complications due to the use of perioperative anticoagulants.56
The actual anesthetic management of flow replacement bypass procedures cannot be generalized because each patient will have different requirements. In particular, those with complex aneurysms present most challenges to anesthesiologists (Table 3). The patient may have also had a recent subarachnoid hemorrhage along with its associated implications. As well, there is the risk of intraoperative rupture. Patients with giant aneurysms may present with compressive symptoms. Patients with large or complex tumors may have increased intracranial pressure. Overall, the considerations usually include the surgical need for a relaxed brain for exposure, prevention of aneurysmal rupture with tight hemodynamic control, and prolonged surgery.
Perioperative Cerebral Ischemia
Cerebral ischemia may occur at many different stages during the treatment of these patients. These stages include (i) preoperative endovascular balloon occlusion testing, (ii) temporary occlusion of vessels during bypass construction, (iii) securing of the aneurysm, and (iv) postbypass graft thrombosis and graft spasm.69 Intraoperatively, during the clipping of the aneurysm, obscured perforators hidden by the calcified or thrombosed aneurysm may be inadvertently occluded. In addition, systemic hypotension can cause cerebral hypoperfusion at any stage and may result in cerebral ischemia. Patients undergoing flow replacement bypass are usually known to have inadequate collateral reserves as demonstrated by preoperative balloon testing and may not tolerate systemic hypotension if temporary occlusion times are prolonged. The treatment strategy for intraoperative cerebral ischemia during a flow replacement procedure is summarized in Table 4.
Intraoperative occlusion of the graft may occur and is usually due to either a mechanical cause such as kinking of the vessel or due to a hypercoagulable state.56 The extent and location of the occlusion needs to be determined and treated promptly. Use of traction sutures on the graft has been reported to be a successful strategy treating mechanical kinking of the bypass.56 If a thrombosis occurs, balloon catheter embolectomy may be used. Postoperative thrombosis of a graft may be treated with endovascular thrombolysis.56 Reconstruction of the bypass graft may be required if other measures fail. Management of radial artery and venous graft spasm can also be challenging;56,70 intraoperative graft spasm may be treated with topical papaverine and postoperative spasm may be treated with intraluminal angioplasty or intraarterial nitroglycerin.71
Various methods have been used to assess the patency of the bypass and to reduce the risk of graft thrombosis and vasospasm. These include the visual inspection and palpation of graft pulsation, use of a micro Doppler, intraoperative angiography, ICG, and intraoperative sonographic flowmetry.72,73 Intraoperative sonographic flowmetry uses a perivascular probe that can be placed directly on the bypass graft to measure the ultrasonic transit time of the blood with a display of the flow rate.46,73 This technique of flow measurement is highly recommended for both graft selection and to assess the adequacy of flow in the afferent, bypass, and efferent vessels before and after the bypass.73 As discussed in the previous section, video angiography with ICG has become one of the standard practices to assess the vessel patency.
Macroscopic assessment of bypass flow and graft patency does not necessarily provide information on the adequacy of overall cerebral perfusion. Hence, there is a need for continuous neurophysiological monitoring during flow replacement bypass surgery to indicate any early signs of cerebral ischemia to allow for preventative measures. Intraoperative neuromonitoring with EEG, SSEP, and motor evoked potentials (MEP) have been reported.69,74–76 Dengler et al.69 reported the use of SSEP and MEP monitoring in 31 patients with complex aneurysms. During 11 procedures, 15 significant changes were observed in MEP and were treated successfully in all but 1 patient. In all cases, blood flow monitoring had shown good perfusion of the bypass grafts. New postoperative motor deficits were present transiently in 1 patient and permanently in another. Other monitoring techniques, including near-infrared spectroscopy and jugular venous oxygen saturation, have not yet been investigated during the flow replacement procedures.
The consideration of neuroprotection is important especially when there is a plan for temporary occlusion of major cerebral vessels. At present, there is a lack of evidence on the efficacy of neuroprotection in terms of neurological outcome. Maintaining adequate blood flow with appropriate blood pressure manipulation is critical. Cerebral-protective drugs and maneuvers have been reported in the literature, though there is minimal information as to the best technique.74,75 Most commonly, these reports have indicated the use of barbiturates or propofol with EEG monitoring and mild hypothermia. Cerebral-protection strategies usually vary with the individual patient and the institutional practice. Deep hypothermia with circulatory arrest is used when prolonged ischemic time is needed.77 With the use of the ELANA technique, Muench et al.68 stated that because there is no temporary occlusion of vessels, there is less need for brain-protective strategies and induced hypertension.
Monitoring of a patient’s neurological status and hemodynamic variables continues to be critical into the postoperative period and should take place in a neurocritical care unit. Assessment of the neurological status of the patient is a good measure of adequate cerebral perfusion, and hence, early awakening after surgery is helpful. Blood pressure should be maintained at an established level for each patient according to the patient’s preoperative blood pressure and all deviations treated. It has been recommended to maintain systolic blood pressure less than 140 mm Hg for 2 to 3 days postoperatively.56 Postoperative hyperperfusion is uncommon but may occur in a patient with restricted blood flow distal to the lesion before the bypass. When a vein graft is used for the bypass, it requires time to adapt to the arterial blood pressure and the higher sheer stress. To prevent kinking or thrombosis of the graft, it is essential to maintain adequate flow across the bypass. The patient may either be heparinized or started on antiplatelet therapy immediately after surgery.56 Muench et al.68 recommend postoperative management of ELANA procedures with high-flow bypass to maintain systolic blood pressure between 140 and 160 mm Hg. All other routine postoperative measures such as prevention of nausea, vomiting, pain, hypoxia, and shivering are also important considerations to prevent agitation and increased stress which may affect the bypass. Postoperative angiography is usually performed to assess bypass patency and the residual aneurysm filling.
Favorable outcomes after cerebral revascularization for complex aneurysms have been shown to range from 50% to 93%.76,78–80 In a systematic review of untreatable anterior circulation aneurysms, Schaller78 found that neurological function and subsequent stroke due to hemodynamic insufficiency improved in 74% of patients with bypass surgery. Sia et al.81 reported a procedure-related complication rate of 14.7% based on their experience with 170 high-flow bypass procedures and concluded that high flow bypasses for aneurysms and tumors are uncommon procedures that may be best performed at high volume centers and with attention to detail. In a recent review, Kalani et al.80 showed a favorable outcome in 82% of patients. However, the authors also stated that with the new and expanding techniques of endovascular treatment, there has been a decrease in the need for revascularization, but subsets of complex cerebral aneurysms will still require it.
In summary, cerebral revascularization procedures are used for different purposes in a wide spectrum of patients. Overall, there are 2 separate groups of patients. The anesthetic management is similar in many respects for most of these patients and includes maintaining adequate cerebral perfusion to prevent the development of intraoperative cerebral ischemia especially during the temporary occlusion of cerebral vessels. Patients with complex aneurysms and tumors add further requirements to the bypass surgery because these procedures include more complex anastomosis and surgical treatment of the underlying pathology. More prospective studies are needed to determine the influence of anesthetic variables and drugs on the outcome.
Name: Jason Chui, MBChB, FANZCA, FHKCA.
Contribution: This author reviewed the literature and helped write the manuscript.
Attestation: Jason Chui approved the final manuscript.
Name: Pirjo Manninen, MD, FRCPC.
Contribution: This author helped review the literature and write the manuscript.
Attestation: Pirjo Manninen approved the final manuscript.
Name: Raphael H. Sacho, MBBCh, MD, FRCS (Eng).
Contribution: This author helped write the manuscript.
Attestation: Raphael H. Sacho approved the final manuscript.
Name: Lashmi Venkatraghavan, MD, FRCA, FRCPC.
Contribution: This author helped with idea, content, and write the manuscript.
Attestation: Lashmi Venkatraghavan approved the final manuscript.
This manuscript was handled by: Gregory J. Crosby, MD.
a Excimer Laser-Assisted Non-occlusive Anastomosis (ELANA): ELANA is an anastomosis technique that using an excimer laser/catheter system to perform end-to-side (T-shaped) cerebrovascular anastomosis without temporary occlusion of the recipient artery. It was first developed by Tulleken et al. for the treatment of untreatable giant aneurysms in 1990s.67
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