Intravascular Ultrasound

Anatomy and Physiology

Intravascular ultrasound of a coronary artery allows for identification and delineation of the lumen and the 3 layers of the arterial wall: the inner tunica intima, the muscular tunica media, and the outer tunica adventitia.  The smooth muscle cells comprising the tunica media does not reflect ultrasound waves and appears dark in the gray-scale cross-sectional image, permitting easy identification.  Intravascular imaging with IVUS characterizes and quantifies plaque. IVUS assessment can distinguish between calcified plaque, lipid, and neointimal proliferation. IVUS allows for the identification of calcified plaque that may be undetectable from angiography alone.[2]  For evaluation of stent failure with stent thrombosis or in-stent restenosis, IVUS plays a critical role to identify the mechanism of stent failure, guiding appropriate therapy. 

Indications

IVUS has indications for a wide range of uses in patients undergoing cardiac catheterization and percutaneous coronary interventions (PCI).

The following are the recommendations made regarding the use of IVUS for PCI, according to guidelines from the American College of Cardiology (ACC).[3]

CLASS IIa:

1. IVUS is a reasonable option to assess angiographically indeterminant left main CAD (Level of Evidence: B)
2. IVUS and coronary angiography are within reason 4 to 6 weeks and 1-year post cardiac transplantation to rule out donor CAD, detect rapidly progressive cardiac allograft vasculopathy, and provide prognostic information  (Level of Evidence: B)
3. IVUS is a reasonable option to determine the mechanism of stent restenosis (Level of Evidence: C)

CLASS IIb:

1. IVUS may be reasonable in assessing non–left main coronary arteries possessing angiographically intermediate coronary stenoses (i.e., 50% to 70% diameter stenosis)  (Level of Evidence: B)
2. IVUS may be considered for the guidance of coronary stent implantation, especially in cases of left main coronary artery (LMCA) stenting (Level of Evidence: B)
3. IVUS may be reasonable for the determination of the mechanism of stent thrombosis   (Level of Evidence: C)

CLASS III:

1. IVUS for routine lesion assessment is not a recommendation if revascularization with PCI or CABG is not being contemplated (Level of Evidence: C)

European Guidelines have been updated more recently, and the 2018 guidelines from the European Society of Cardiology (ESC) recommends IVUS to assess the severity and optimize the treatment of unprotected left main coronary lesions (Class IIa, level of evidence B).[4] Additionally, the ESC recommends that IVUS should be considered to detect stent related mechanical problems leading to restenosis (Class IIa, level of evidence C) and IVUS should be considered in selected patients to optimize stent implantation (Class IIa, level of evidence B).

Contraindications

While no true absolute contraindications for IVUS exist, extreme vessel tortuosity and angulation that would preclude the ability to deliver an IVUS catheter to the area of interest is a relative contraindication. As with any instrumentation of the coronary arteries, IVUS use is accompanied by systemic anticoagulation during the procedure.  Consequently, an indirect IVUS contraindication would be a patient who is not a suitable candidate for systemic anticoagulation or a patient who is not a candidate for angiography and cardiac catheterization in general.

Equipment

IVUS obtains images from a piezoelectric transducer that produces sound waves. IVUS catheters divide into two main categories based on the mechanism of transducer design.  IVUS catheters currently in use are either solid-state phased array incorporating multiple stationary piezoelectric transducers or consist of a rotational design based on a single piezoelectric transducer.[5] Solid-state phased array IVUS catheters have the advantage of a broad depth of field and are often the preferred imaging modality in larger vessels such as for peripheral interventions. Rotational IVUS offers the advantage of an automated pullback allowing for measurements in determining lesion length and stent length selection.

Multimodality IVUS catheters, combining IVUS and an additional intravascular technology are now commercially available with combined IVUS and near-infrared spectroscopy (NIRS) and under investigation for future use with IVUS and optical coherence tomography (OCT). 

Personnel

IVUS catheters transmit an image to a dedicated console for image analysis.  Ancillary catheterization laboratory staff can prepare the IVUS catheter.  An interventional cardiologist or vascular surgeon manipulates the catheter inside of the body. Online assessment can be performed to characterize plaque morphology and assess minimal lumen area (MLA), determine plaque burden, and identify segments of normal artery. 

Preparation

The preparation of IVUS imaging catheters varies slightly with each manufacturer but is mostly a straightforward process that consists of flushing the catheter system with saline and connecting the catheter to a console.  Systemic anticoagulation and intracoronary nitroglycerin should be administered before the insertion of IVUS to minimize and prevent the risk of thrombus and vasospasm.

Technique or Treatment

Performance of IVUS assessment of a vessel can be with either a manual or automatic pullback. Automated pullback allows for length measurements by producing a longitudinal profile, which is useful for the determination of the appropriate stent length that should be selected. Stent diameter selection the identification of and then measuring the proximal and distal references. Following stent implantation, IVUS can assess the minimal stent area (MSA), a prognostic indicator closely associated with the likelihood of future events related to the stent.[6]

Complications

Complications due to IVUS use are uncommon but may include dissection, perforation, arrhythmia, thrombosis, and vasospasm as can occur during any instrumentation of a coronary vessel.[7] Particular care is necessary for highly tortuous vessels.

Clinical Significance

For assessment of left main coronary arteries, IVUS can be used to determine lesion significance and guide revascularization. The use of IVUS often leads to a change in the PCI strategy.[8] Clinical data from multicenter randomized trials and meta-analyses have consistently demonstrated that the use of IVUS can lead to a reduction in adverse clinical events.[9][10][11][12] IVUS assessment following PCI can determine predictors for restenosis based on post-intervention cross-sectional area.[13]  Despite several decades of data supporting the use and benefits of IVUS, utilization remains low in the United States.[14]

Enhancing Healthcare Team Outcomes

IVUS is an important adjunctive tool in the cardiac catheterization laboratory that can be useful to determine plaque morphology and guide percutaneous coronary interventions.  Image interpretation is an important skill with IVUS use to optimize clinical outcomes. The use of IVUS has been associated with improved clinical outcomes in both an all-comers population and those with long lesions undergoing stent implantation (28 mm stent implanted).[9][10][Level I]

To provide the best outcome, interprofessional team involvement is necessary. The nurse needs to be well aware of the procedure and complications and assist the interventional cardiologist or vascular surgeon as they manipulate the catheter inside of the body. Prior to the procedure, the nurse should assist with pre-operative screen for allergies or injuries that may hinder the performance of the procedure. If there is a concern, the should report back to the clinician. Often the nurse will need to assist with position and preparation of the patient. The nurse should also monitor vital signs and the ECG during the procedure and should alert the surgeon if there are changes in vitals signs or an untoward change in the cardiac monitor. Always quickly reporting to the surgeon if there is a concern. Post-procedure the nurse must monitor for signs of bleeding or infection. Again, quickly informing the surgeon if there is a concern. The best outcomes will be achieved if an interprofessional approach is taken in regards to the performance of this procedure. [Level V] [15]