Utilizing Contrast Echocardiography in Practice

Endocardial Enhancement

Currently, the most common application of contrast echocardiography in clinical practice is to assess left ventricular (LV) function and regional wall motion when endocardial delineation is otherwise difficult. Despite advances in ultrasound imaging technology that have continuously improved image resolution and quality, the inability to adequately visualize the endocardial borders is still common. The intravenous administration of microbubbles in most subjects allows excellent discrimination between the LV blood pool and myocardium, thereby improving the ability to assess ventricular chamber dimensions and both global and regional systolic function ( Video 12.1, Video 12.2 ).

Enhanced definition of the endocardial border is achieved by opacification of the LV cavity blood pool that provides contour recognition against the darker myocardium. Because only approximately 5%–10% of the mass of the myocardium is attributable to its microvascular blood volume (MBV) the contrast signal in the myocardium is a small fraction of that within the LV cavity. An exception to this occurs when microbubble concentration is very high, well beyond saturation of the dynamic range for the blood pool, and the myocardial signal can approach that of the blood pool, making endocardial definition difficult ( Fig. 12.2A ). Very high concentration of microbubbles in the blood pool also causes attenuation of the imaging beam, thereby producing shadowing of far field structures. Accordingly, left ventricular opacification (LVO) is generally performed with either small repetitive intravenous boluses of contrast or a continuous infusion of contrast at modest rates that result in full opacification of the blood pool without attenuation when imaged using low-power ultrasound to avoid microbubble destruction. Contemporary ultrasound imaging systems incorporate contrast-specific presets based on amplitude modulation or phase-inversion because of their ability to reliably produce high contrast signal and clear definition of the endocardial borders. However, the application of these algorithms can sometimes be disadvantageous when assessing regional wall motion during exercise or dobutamine stress due to the inherent reduction in frame rate with multipulse protocols, the production of “flash” artifact from tissue motion, and visualization of hyperemic myocardial perfusion at peak stress that can make differentiation of the border between myocardium and blood pool less clear. Although LVO is very effective at defining endocardial borders, it is important to recognize that, like all forms of imaging, the quality is affected by rib attenuation and beam-altering artifacts (see Fig. 12.2B , Video 12.3 ).

Assessing Left Ventricular Wall Motion and Systolic Function

Defining the LV endocardial borders is necessary for detecting the presence of wall motion abnormalities, assessing LV dimensions, and calculating left ventricular ejection fraction (LVEF). Endocardial definition is insufficient in up to 20%–30% of patients referred for stress echocardiography. Poor echocardiographic windows are particularly a problem in subjects who are obese, suffer from chronic obstructive pulmonary disease, are on ventilators, or cannot be optimally positioned for imaging. LVO with contrast echocardiography provides an opportunity to improve endocardial border resolution. Studies using intravenously injected lipid or albumin encapsulated contrast agents have included both unselected patients and those with technically difficult conventional two-dimensional (2D) imaging. These studies have unequivocally demonstrated that the use of contrast in both populations increases the number of interpretable studies, increases the number of interpretable LV segments with regard to evaluating wall motion, decreases interobserver variability, and increases reader confidence. The ability of contrast to transform an uninterpretable echocardiographic study into a diagnostic one appears to be particularly impactful in intensive care unit patients who have technically limited acoustic windows. Interobserver variability for detecting regional wall motion abnormalities with contrast echocardiography has been shown in a multicenter study to be superior to that of cardiac magnetic resonance imaging (MRI), cine ventriculography, and noncontrast echocardiography.

The range of clinical decisions that are based on a precise measurement of LV dimensions or LVEF continues to expand and evolve. Quantitative measurements are a component of patient selection for implantable defibrillators, cardiac resynchronization therapy, left-sided valve replacement/repair and for guiding optimal drug therapy with medications used for heart failure or cardiotoxic chemotherapeutic regimens. The gravity of the aforementioned decisions, both from the patient’s perspective and based on the socioeconomic impact of the treatments, emphasizes the importance of reliable and reproducible measurements of LV volumes and LVEF. When using radionuclide or invasive left ventriculography or cardiac MRI as a gold standard, LVO with intravenous contrast administration has been shown to be more accurate and more precise than unenhanced images with regard to measuring LV volumes and LVEF even in a population not selected for difficult acoustic windows ( Fig. 12.3 ). Contrast administration has been shown to consistently improve interobserver variability with regard to measurement of LV volumes or LVEF, particularly in patients with two or more adjacent poorly visualized segments, and results in the lowest interobserver variability compared to cardiac MRI, noncontrast echocardiography, and ventriculography.

The cost effectiveness of using contrast echocardiography in selected patients has been examined in several studies. Although the cost of the contrast agent and additional time for preparing for and performing contrast echocardiography represent added costs, the added time may be minimized by imaging protocols that allow the decision to use contrast to be made early in the study, thereby eliminating the “struggle time” wasted when a sonographer tries unsuccessfully to improve unenhanced images. Another consideration in assessing the cost-effectiveness of contrast echocardiography is downstream costs of other diagnostic tests that must be used or inappropriate therapies that are used as a result of nondiagnostic echocardiograms. All studies performed to date examining cost-effectiveness have demonstrated that the reduction in downstream resource utilization makes the routine performance of contrast echocardiography in patients with technically limited windows an effective strategy. Beyond just cost-savings, the ability to better understand regional and global LV function in inpatients and outpatients with technically difficult echocardiograms has been demonstrated to have an impact on management regarding changes in therapy or subsequent procedures performed ( Fig. 12.4 ). It should be recognized that in this type of analysis the lack of change in management does not necessarily indicate that the information did not impact patient care. It has been proposed that the superior information provided by contrast when image quality is otherwise limited is the reason for the finding of a significant one-third reduction in mortality in those receiving contrast in retrospective studies performed in critically ill patients undergoing echocardiogiography.

Stress Echocardiography

Stress echocardiography has become a cornerstone in the noninvasive evaluation of patients with suspected coronary artery disease (CAD). When performed in appropriate pretest probability populations, the sensitivity of stress echocardiography for detecting obstructive epicardial disease is between 80% and 90%, while the specificity is just under 80%. Conventionally, stress echocardiography relies on the detection of regional abnormalities and contractile reserve. Accordingly, optimal performance of stress echocardiography relies on the ability to see every segment, the ability to see every segment well , and a high level of reader confidence. Contrast echocardiography for LVO during stress has been shown to increase the number of interpretable segments, to increase subjective study quality, and to increase reader confidence. LVO has a greater impact in those patients with technically difficult baseline images and when images are interpreted by less experienced readers. The impact of LVO is particularly high in segments that most commonly suffer from poor endocardial discrimination, such as the basal lateral and basal inferior regions. The ability to ensure that the true LV apex is imaged and not foreshortened is also a valuable contribution of LVO. It has been advocated by some that contrast should be used in the majority of patients referred for stress echocardiography because of difficulties in being able to predict which patients who have adequate baseline images will have a deterioration in poststress image quality due to hyperventilation or excessive cardiac translation. However, this recommendation is tempered by the finding that the impact of using contrast is greatest in those with poor or marginal image quality at baseline.