Intravascular Ultrasound

The concept of using ultrasound technology to evaluate vasculature is not new for vascular surgeons. The majority of diagnostic vascular studies in the modern era are based on ultrasound technology. The introduction of ultrasound to image vasculature from the inside out takes this technology to the next level. This system was first utilized by interventional cardiologists during coronary interventions [1–4]. The IVUS catheter is inserted into a blood vessel over a guidewire. The tip of the catheter produces sound waves, which are transmitted through the blood to the blood vessel wall. The sound waves then come back to the catheter tip and create an image of the vessel wall, providing a plethora of useful information about the vasculature, including vessel diameter, characteristics of the intima, echogenicity or echolucency, length of lesions, and characteristics of lesions [5].

A typical image from an IVUS is shown in Fig. 24.1. The innermost circle represents the IVUS catheter. Moving outward in a radial fashion, much more information can be gleaned from this image. The black halo around the catheter represents blood. The radiopaque circle surrounding the blood is the intima of the blood vessel. The layers outside the intimal layer are the media and the adventitia. Penetration into deeper structures can be obtained by decreasing the frequency of sound waves, but this is accompanied by a decline in the resolution of the image. Generally, lower-frequency IVUS catheters are used for large-diameter blood vessels, and vice versa. The use of IVUS may help determine accurate sizing of stents and may help avoid or reduce the use of contrast, reduce radiation dose, and decrease operative times. In addition, IVUS can provide invaluable information regarding plaque characteristics.

Currently, two manufacturers have devices that are approved for non-coronary use in the United States: Volcano (Volcano Corp., San Diego, CA) and Boston Scientific (Boston Scientific Corp., Marlborough, MA). The Boston Scientific device is compatible with an 0.018″ system, and Volcano devices are compatible with 0.014″, 0.018″, and 0.035″ systems.

Clinically, IVUS can be used in the various clinical scenarios discussed below.

Peripheral Endovascular Interventions

Most patients with peripheral arterial disease are now treated with endovascular interventions, which are usually performed with angiographic guidance. Conventional angiography provides a two-dimensional view of the vessel of interest. IVUS has the advantage of providing a three-dimensional view of the vessel and any lesions present. It is also of signficant help in determining stent patency (Fig. 24.2). A recent Japanese study revealed superior 5-year patency rates for interventions performed under IVUS guidance when compared with those performed without IVUS (65% vs. 35%) [6]. Another study showed that use of IVUS helps determine the presence of under-expanded stents in 40% of patients [7]. It is worth noting that conventional angiography in these cases had failed to determine this crucial finding. Similarly, another study showed improved patency rates for interventions when IVUS was used to guide therapy [6]. The use of IVUS with re-entry devices has been shown to be associated with accurate re-entry into the true lumen and successful recanalization of occluded arterial segments [8, 9].

Aortic Dissection

Traditionally, type B thoracic aortic dissections have been managed conservatively with medical therapy. With advancements in the endovascular treatment of these dissections, a growing number of experts are now advocating endovascular repair of the aorta in these patients to prevent long-term sequelae such as aneurysmal degeneration. Nonetheless, the accurate placement of the guidewire in the true lumen can be quite challenging under direct fluoroscopy alone, which produces two-dimensional images. IVUS can be very useful in delineating the true lumen, false lumen, origins of visceral branches of the aorta, and the proximal and distal extent of dissections (Fig. 24.3) [10]. Further, the use of IVUS may assist in the accurate sizing of aortic endografts and their placement in the true lumen without compromising perfusion to the visceral vasculature [11].

Venous Stenting

In May-Thurner syndrome, the left common iliac vein is compressed between the right common iliac artery and the vertebrae. This pathology is treated in appropriately selected patients by placing self-expanding stents in the compressed area. It may be difficult to determine the exact area of extrinsic compression under conventional fluoroscopy alone. The use of IVUS in such cases can accurately determine the exact area of stenosis by extrinsic compression and can help in accurate stent placement (Fig. 24.4). In addition, the use of IVUS offers a complete assessment of thrombus after thrombolysis [12, 13]. It also can help with proper sizing to avoid stent migration in these compressed vessels.

To summarize, IVUS can be used in many clinical scenarios and can help with accurate visualization of intravascular pathology and in guiding successful endovascular treatment.

Embolic Protection Devices

Conceptually, embolic protection devices (EPDs) are designed to prevent distal embolization. To understand the history of the development of EPDs, it is important to review the history of carotid artery stenting (CAS). Carotid endarterectomy (CEA) is considered the gold standard for treatment of severe carotid disease. CAS as an alternative to CEA was introduced by Mathias in 1977 [14]. Since the introduction of CAS, surgeons have raised concerns regarding the fracture of atherosclerotic plaque and distal embolization, which may have disastrous consequences, primarily embolic stroke. Theron et al. [15] coined the term “cerebral protection” to describe any methods used to prevent cerebral embolism. Cerebral protection is now categorized into proximal and distal types, with respect to the location of the carotid lesion. Early distal protection techniques included the placement of a balloon in the distal internal carotid artery (ICA). While the balloon prevents emboli from traveling upstream during the carotid stenting procedure, it also prevents blood flow in the occluded vessel. To counter this problem, filter devices were developed to allow for distal perfusion while preventing embolic events. The EVA-3S trial [16] showed that CAS procedures performed in the absence of distal protection had a four-fold increase in stroke when compared with CAS procedures performed with distal embolic protection. Proximal protection by a balloon inflated in the common carotid artery (CCA) was first described by Parodi et al. [17]. Subsequently, Criado et al. [18] described the technique of flow reversal from the CCA. Because the sheath for flow reversal is placed in the CCA, it is considered a form of proximal protection. The ROADSTER trial [19] investigated the safety of this technique and showed a rate of 30-day stroke lower than the rate reported in any previous trial.

Distal Protection

Balloon Occlusion

Balloon occlusion is the earliest described method of cerebral protection. Theron et al. [15] described the use of balloon occlusion distal to the carotid lesion prior to passing a stent. This technique entails first placing a wire in the distal ICA, followed by over-the-wire placement of a guiding catheter in the CCA. A small polyethylene catheter with a non-detachable balloon is then inserted into the ICA, and the balloon is then inflated to occlude the ICA. After endovascular intervention like angioplasty, the ICA should be thoroughly aspirated with an angioplasty catheter, and then the balloon is withdrawn. Aspiration and flushing of the guiding catheter and sheath removes any particles of debris from the atherosclerotic plaque.

Filters

The next generation of EPDs comprises filters. In contrast to occlusive balloons, filters can capture embolic material while maintaining distal perfusion. Several filters are currently available, which can be distinguished from each other based on porosity and landing zones. Table 24.1 lists the filters currently approved by the US Food and Drug Administration (FDA).

Table 24.1

Filters Approved by the US Food and Drug Administration (FDA) for Distal Embolic Protection in Carotid Artery Stenting

Type of distal filters	Manufacturer	Trial	Number of patients	30-day stroke rate	Landing zone (MM)	Porosity (μM)	Year approved by FDA
Angioguard RX	Cordis Corporation, Miami Lakes, FL	SAPPHIRE [21]	167	3.6%	5.9	100	2004
Rx Accunet	Abbott Vascular, Abbott Park, IL	ARCHeR [22]	581	5.5%	15.1	150	2004
FilterWire EZ	Boston Scientific, Marlborough, MA	ASTI [23]	100	2%	13.4	110	2006
Emboshield nav6	Abbott Vascular, Santa Clara, CA	PROTECT [24]	220	1.2%	19–22.5	140	2005
SpiderFX	Ev3, Plymouth, MN	CREATE SpideRX [25]	160	3.1%	17.3	50–300	2006
FiberNet	Lumen Biomedical, Plymouth, MN	EPIC [26]	237	2.1%	15	40	2008

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