Introduction

The maintenance of adequate cerebral blood flow and perfusion may require cerebral revascularization in the face of occlusive disease or as a result of treatment for complex intracranial vascular lesions. With refined criteria and improvements in patient selection, cerebral revascularization has seen resurgence over the last decade. While the most commonly used bypass graft has been the superficial temporal artery (STA), some patients require a higher demand for blood flow, and therefore need a graft that can capacitate these increased needs. Flow requirements can be determined by balloon test occlusion (BTO). Sacrifice of a large parent artery, such as the internal carotid or vertebral artery, may necessitate a high-flow bypass graft, especially if there is minimal collateral reserve demonstrated by BTO. Ultimately, careful patient selection is the most important predictor of successful revascularization strategies. The most commonly utilized high-flow bypass grafts are the radial artery (RA) and the saphenous vein (SV). In this chapter, we discuss the indications and uses of the SV for high-flow augmentation in cerebral revascularization.

Indications for high-flow revascularization

Intracranial and extracranial occlusive vascular disease, can lead to progressive compromise of cerebral blood flow that may necessitate augmentation of cerebral blood flow. Trauma may result in loss or compromise of a major arterial supply to the brain and hence a need for revascularization. Further, as a greater number of complex aneurysm recurrences are seen after endovascular treatment, revascularization strategies have increased in importance. Treating these complex vascular lesions may require complex clip reconstruction techniques requiring extended temporary artery occlusion times or parent artery sacrifice. Lesions located on the M1 and P1 segments may require prophylactic bypass due to the frequent presence of perforating vessels and low tolerance of temporary occlusion in these territories. Further, many vascular lesions involve extensive atherosclerotic changes in the neck, very broad based, or giant in size, thus potentially necessitating branch or parent artery sacrifice. Finally, previously endovascularly treated vascular lesions may require complex vascular reconstructions at retreatment, that is, removal of coil mass, potentially demanding, long parent artery occlusion times. The choice of the anastomosis site is selected based on the location of the lesion, intended territory of revascularization, and properties of the donor and recipient vessels. BTO can facilitate decision making in most cases (Table 12–1). After temporary occlusion of a parent artery, the global cerebral hemodynamics are examined, with particular attention to the vascular reserve provided by the leptomeningial and circle of Willis vessels and subsequently the flow dynamics including venous washout are carefully studied.

Table 12–1 Balloon Test Occlusion Determination of Amount of Required Cerebral Blood Flow.

While angiographic study of the collateral circulation around a particular lesion is helpful, it does not provide a clinical assessment of the patient’s potential cerebral blood flow requirements. In order to determine clinically significant changes regarding the need for reserve flow, the BTO includes clinical examinations, electroencephalogram (EEG), hypotensive challenges with clinical examinations, and single-photon emission computed tomography (SPECT) imaging after performance of the BTO and removal of the balloon. Clinical exams are performed at baseline and every 5 minutes after balloon inflation. If the patient fails the BTO and develops deficits at normotension (120 to 140 systolic), then we feel the patient should undergo a high-flow bypass.

Saphenous vein considerations

The saphenous vein graft has been used for decades for bypass procedures with relatively great success. Lougheed is credited for performing the first intracranial bypass using a saphenous vein graft in 1971.¹ Despite an overall decrease in the use of intracranial SV bypasses due to an increased use of endovascular techniques for complex intracranial vascular disease and an increased experience and usage of radial artery grafts, the SV graft still provides a highly flexible and adaptable bypass graft for a variety of uses, including a primary conduit from the carotid artery or superficial temporal artery or as an interposition graft. The saphenous vein has a measured flow rate between 70 and 200 ml/min and an average diameter of ∼5 mm.^2,3 The advantages of the SV graft are the ease of harvest, autologous source, length of graft able to be obtained, absence of atherosclerotic changes throughout the length of the vessel, absence of vasospasm, and large caliber of graft able to be harvested. Several disadvantages of SV grafts exist and are most commonly frequent caliber mismatch between donor and recipient vessels, which can often lead to intraluminal turbulent flow and eventual thrombosis, the presence of valves that can be sites of thrombus formation, greater possibility of reperfusion hemorrhage, and the potential for kinking at the site of the recipient due to the thick surrounding tissue and vessel wall. Saphenous vein bypass grafts require two separate anastomosis sites and separate incisions for graft harvest, which increases the potential for complications. Although long-term patency rates have not been directly compared between RA and SV grafts, the average patency rate at 5 years postimplantation has been reported to be ∼90% for RA grafts and ∼80% for SV grafts. In the coronary literature, RAs have been consistently reported to have higher patency rates compared to SVs.^2–4

Patients undergoing revascularization procedures are usually placed on aspirin therapy before or immediately after the revascularization procedure. In cases of hypercholesterolemia, a statin can be administered pre- and post-operatively, which has been suggested to positively affect long-term graft patency.^2,3,5 All patients receive preoperative antibiotics within 1 hour prior to incision. The utilization of a neuroanesthesia team has several advantages during all phases of the procedure. Throughout the procedure—adequate cerebral perfusion is maintained adequate pharmacological brain protection, as well as optimal brain relaxation—which reduces the necessity of brain retraction. Postoperatively, controlled emergence, that is, avoidance of hypo- and hyper-tension, and rapid emergence from anesthesia are important so that an adequate neurological exam can be completed without compromising the bypass graft. Intraoperative neurophysiological monitoring is also performed, which includes EEG and somatosensory evoked potentials (SSEPs). Other neuromonitoring, such as brainstem-evoked potentials or cranial nerve monitoring, may be employed depending on the site of surgery and necessity of cranial nerve manipulation. Intraoperative graft patency monitoring can be assessed with ICG video-angiography, micro-Doppler ultrasound, or invasive intraoperative angiography via the femoral arterial rout or via direct cervical arterial puncture.

The patient is positioned with regard to the side of the lesion and the anastomosis site. For anterior circulation lesions, the patient is placed supine with the head turned to the opposite side of the lesion. The vascular grafts are often harvested from the opposite side of the lesion. For posterior circulation lesions, the patient is often placed in the lateral position.