Non-invasive imaging of the heart continues to evolve and improve. Cardiovascular ultrasound (echocardiography) continues to play an important role in the diagnosis and assessment of responses to therapy of many cardiac conditions. Even as the clinical applications of newer techniques such as cardiac computed tomography (CT) and cardiac magnetic resonance imaging (CMRI) increase, the role of echocardiography remains strong since it has unique capabilities. One of the strengths of echocardiography is that it can be brought to the patient’s bedside easily. In fact, features of ultrasound machines such as 3D, 2D and trans-oesophageal echo, spectral and colour Doppler imaging, strain rate imaging and even intra-cardiac imaging can be performed on devices that are increasingly portable. Thus, it is easier to provide echocardiography around the hospital, office or other environments.
Devices exist that enable many cardiovascular procedures to be performed that once required open-heart surgery, but now catheters can be used in a much less invasive manner. Percutaneous coronary interventions such as balloon angioplasty and coronary artery stent placement are the most common examples of this evolution in cardiovascular care, but many other treatment opportunities are now present in the catheterisation laboratory. Many of these new treatments require imaging of the heart in a manner different from fluoroscopy so that the position of the devices can be optimised. By virtue of its ease of use, safety, lack of radiation, low cost and portability, echocardiography has come to play a key role in selecting patients for and guidance of these procedures. This article will highlight echocardiography and its role in some of these new catheter-based treatments. Table 1 lists some of these representative procedures.
Percutaneous Mitral Valvuloplasty
One of the earliest interventions in the catheterisation lab utilising echocardiography was the treatment of mitral stenosis with balloon dilation or percutaneous mitral valvuloplasty (PMV). PMV has now replaced surgery as the initial therapy for this disorder. Trans-thoracic echocardiography is used to assess the mitral valve for degrees of thickening, calcification, regurgitation and other characteristics to determine patient suitability for the procedure. Typically, trans-oesophageal echo is performed to exclude thrombus in the left atrium as this is a contraindication to the procedure. Trans-thoracic echocardiography is often performed in the catheterisation laboratory during the procedure to ensure that as the valve is dilated it is not dilated so excessively that mitral regurgitation deteriorates to an intolerable degree (see Figure 1).
Percutaneous Closure of Atrial Septal Defect and Patent Foramen Ovale
Atrial septal defects are ‘holes in the heart’ that result in a communication between the left and right atria. While the majority are recognised in childhood, many go undetected until later in life. The benefit of surgical closure of these defects has been demonstrated and recently the safety and efficacy of percutaneous device closure of a subset of these defects in comparison with surgery has been demonstrated. During the procedure, trans-oesophageal echocardiography first identifies the location, number and size of the defects to confirm that percutaneous device closure is appropriate. Imaging in realtime with echocardiography guides the placement of the device into the correct location and confirms that the defect is closed and the device is secure prior to releasing it from the delivery catheter (see Figure 2). After device delivery, this echocardiographic assessment also includes imaging of the heart valves and veins leading into the atria to ensure that no obstruction has been caused by the device.
The patent foramen ovale is found in all newborns as it allows circulation of blood in the foetal heart, but it closes soon after birth. In up to 30% of the adult population a potential communication persists and under certain conditions will allow passage of blood between the atria, thus providing a direct path from the venous circulation to the rest of the body, bypassing the lung circulation. The indications for closure of patent foramen ovale are less clear than for atrial septal defect, but there are some patients who have suffered neurological or other systemic emboli for whom closure may be the recommended treatment. In these cases, the role of trans-oesophageal echo is similar to the closure of an atrial septal defect, i.e. assessment of location, number of defects, shape of the defect and guidance of the procedure. Echocardiographic imaging is particularly important in guiding the closure of patent foramen ovale when an aneurysm of the inter-atrial septum is also present.
Left Atrial Appendage Exclusion
Most embolic strokes that are associated with atrial fibrillation are due to blood clots that form in the non-contracting left atrium. Most of these clots form in the left atrial appendage, which is a blind pouch in the anterolateral portion of the left atrium. These thrombi can be prevented with anticoagulation therapy. Unfortunately, complications can arise from anticoagulant therapy. Currently, devices are under clinical trial that can be delivered into the left atrial appendage by catheter; once expanded, these devices block the mouth of the appendage.
This prevents communication of the appendage with the remainder of the left atrium. For patients with chronic atrial fibrillation who are unable to tolerate anticoagulants or who have suffered serious complications with these drugs, the left atrial appendage exclusion device may provide an alternative method to prevent embolisation of clots from this portion of the left atrium. For placement of these devices, images of the appendage are first obtained with trans-oesophageal echo to measure its dimensions and select the correctly sized device. Subsequently, trans-oesophageal imaging and colour Doppler are used during device deployment to make sure it is positioned properly and that it seals the appendage.
Alcohol Septal Reduction for Hypertrophic Obstructive Cardiomyopathy
Surgical therapy for the obstructive form of hypertrophic cardiomyopathy involves the removal of myocardial tissue from the upper ventricular septum. This results in a widening of the left ventricular outflow tract and a reduction in dynamic obstruction and gradient. In the mid-1990s, the first reports of a non-surgical reduction of the ventricular septal myocardium appeared. This technique utilises injection of small volumes of alcohol selectively into branches of the coronary arteries that supply the upper ventricular septum. This results in an acute stunning of this small region of the ventricular myocardium and an immediate reduction in outflow tract gradient. Chronically, this portion of the septal myocardium becomes reduced in thickness and the reduction in the dynamic outflow tract gradient persists due to remodelling or enlargement of the left ventricular outflow tract.
Echocardiography plays a vital role in this procedure. First, trans-thoracic echo is the test of choice for diagnosis of hypertrophic obstructive cardiomyopathy. To be eligible for alcohol septal reduction, the septum must measure at least 16mm on the echocardiogram and the resting gradient should be greater than 30mmHg (see Figure 3A). If the gradient at rest does not meet the criteria, an infusion of dobutamine is used to determine whether a significant gradient can be provoked. The trans-thoracic echo–Doppler is also used during the catheterisation procedure to ensure that the correct coronary artery branch is selected for the alcohol infusion. When the catheter is positioned in a branch of the septal perforating artery thought to supply the upper septum, echocardiographic contrast is injected; this delineates the territory perfused by the vessel (see Figure 3B). If the contrasts fill the portion of the septum that contacts the anterior leaflet of the mitral valve during systole, alcohol infusion follows.
However, if this vessel does not perfuse the proper portion of the septum or if contrast is also noted to perfuse the right ventricular free wall or the left ventricular inferior wall, the catheter is repositioned in another branch. During alcohol infusion, monitoring of the left ventricular outflow tract gradient is performed with continuous-wave Doppler to ensure a sufficient physiological effect has occurred (see Figure 3C).
Percutaneous Valve Repair and Replacement
Mitral Valve
Mitral regurgitation arises from a variety of aetiologies. Devices are under development that can be delivered percutaneously to reduce mitral regurgitation. One type of device clips the mid-portions of the mitral leaflets when myxomatous mitral valve prolapse pathology is present. Another type of device has been applied to ischaemic mitral regurgitation and reshapes the mitral annulus, thus altering the geometric relationships of the valve, annulus and subvalvular apparatus. In all of these types of interventions, echocardiography is used to guide device placement and assess the effects on mitral regurgitation in realtime.
Aortic Valve
As our population’s life expectancy increases, the prevalence of aortic stenosis is expected to rise. In contrast to balloon dilation of mitral stenosis, balloon aortic valvuloplasty has not provided adequate long-term success. Thus, it is typically reserved as a short-term solution for select populations such as those with critical aortic stenosis who are too unstable to immediately undergo aortic valve replacement, as a form of palliative therapy for a patient with other serious co-morbid conditions or prior to urgent non-cardiac surgery. Percutaneous replacement of the aortic valve offers the prospect of a sustained improvement in aortic valve area for aortic stenosis patients who are not suitable candidates for surgical valve replacement. Prosthetic valves mounted on expandable stents have been developed (see Figure 4). These valves are being tested in patients with severe aortic stenosis who are not candidates for surgery.
Again, echocardiography plays an important role in selecting suitable candidates based on size of the annulus, aortic valve area and leaflet morphology. Trans-oesophageal echo is then used during the procedure to assist in positioning of the valve and to ensure that there is minimal or no aortic regurgitation after deployment. Significant aortic regurgitation is a sign that the valve was undersized or inadequately expanded.
The Future
Efforts are now under way to regenerate portions of diseased hearts through delivery of stem cells, genes and small molecules. Current investigations involve patients with ventricular dysfunction from extensive myocardial infarction and other cardiomyopathies. While catheters can perform the delivery, imaging with, for example, trans-thoracic, trans-oesophageal or intra-cardiac echocardiography is critical to identify the diseased portions of the heart and to direct the delivery catheters to specific regions. In summary, echocardiography is critical for the success of many new percutaneous interventions as it enables proper selection of patients, guidance of the procedure and late follow-up. Due to new and evolving uses of non-invasive cardiac imaging that are critical to patient care, the future for echocardiography remains bright.