Echo maintains crucial role in clinical cardiac practice

New portable handheld devices deliver quality previously seen only in less convenient consoles

With the introduction of new cardiac imaging modalities, echocardiography may have lost some of its luster, but it still remains an important diagnostic test in the cardiology practice. Echocardiography offers distinct advantages over competing imaging modalities: greater safety, portability, wider spectrum of clinical applications, and lower cost.

Ultrasound systems are designed to fit specific clinical applications and environments. Portable handheld devices now available are capable of producing 2D, color, and spectral Doppler images with a quality comparable to state-of-the art equipment available five years ago. These portable devices facilitate the performance of ultrasound studies in remote locations and are capable of transmitting data via high-speed wired or wireless networks for remote expert interpretation. These capabilities help establish the diagnosis of acute cardiovascular emergencies such as aortic dissection, cardiac tamponade, or myocardial infarction even before the patient arrives at the hospital.

Echocardiographic studies are also easy to perform in the emergency room, intensive care, and the operating room, since the equipment occupies limited physical space, imaging can be done under sterile conditions, and the modality does not involve radiation to either patient or medical personnel. Unlike other imaging modalities, echocardiography provides real-time images with ultrahigh temporal resolution. Therefore, patients may be imaged even if they have fast or irregular heart rates. With the introduction of harmonic imaging and ultrasound contrast agents, the number of patients who provide suboptimal images has fallen below 5% in most experienced centers.

The spectrum of clinical applications of echocardiography continues to expand. In patients being evaluated for hypertension, coronary artery disease, or congestive heart failure, echocardiography provides reliable assessment of left ventricular mass, volumes, ejection fraction, and diastolic function. The LV ejection fraction derived by echocardiography is the most widely used index of LV systolic function. Over the years, it has established its value as a strong prognostic index in patients with ischemic cardiomyopathy, dilated cardiomyopathy, and valvular heart disease.

Studies have shown that six-month survival after an acute myocardial infarction decreases from more than 97% for those patients with LV ejection fraction above 40% to less than 85% for those with LV ejection fraction below 30%.1 A reduced LV ejection fraction is the strongest predictor of sudden death in patients with heart failure, regardless of the etiology, and is now considered an indication for prophylactic implantation of intracardiac defibrillator devices.2

Two-D echocardiography is the most commonly used method to evaluate the LV ejection fraction, using single-plane or biplane area times length or Simpson's stacked disc methods.3 In those cases in which 2D echocardiography has limited image quality, the use of intravenous contrast may improve accuracy and reproducibility.4 The use of real-time 3D echocardiography for measurement of ventricular volumes and ejection fraction in patients with heart failure has also been shown to improve accuracy, particularly in patients with dilated hearts.5

NEW METHODS

In patients with heart failure, newer echocardiographic methods have recently been applied for the evaluation of mechanical dyssynchrony. About 30% to 50% of patients with heart failure, wide QRS intervals, and low LV ejection fraction manifest clinical improvement6 and have improved survival with cardiac resynchronization therapy.7 However, 25% to 30% experience clinical deterioration, and a similar percentage have no significant change in their clinical status.

Tissue Doppler echocardiography identifies patients who respond to resynchronization therapy more accurately than QRS width. In a recent study, a 65-msec delay measured by this method predicted with 80% accuracy patients who experienced improvement in quality of life and with 92% accuracy patients who experienced more than a 15% reduction in LV end-systolic volume.8

Stress echocardiography has gained increasing acceptance with the introduction of digital echocardiography, harmonic imaging, and the use of contrast agents, all of which have incrementally contributed to increased image quality, reproducibility, and accuracy. Stress echocardiography is used to establish diagnosis, determine prognosis, or evaluate the need for revascularization in patients with known or suspected CAD.

Exercise stress testing may be performed with treadmill, supine or prone bicycle, and even arm ergometry. Several studies have reported sensitivities ranging from 71% to 97% and specificities ranging from 64% to over 90%. Exercise echocardiographic variables have incremental independent prognostic utility over other variables such as the Duke treadmill score.9

Stress echocardiography may be performed with pharmacologic stressors such as dobutamine. Reported sensitivities and specificities of dobutamine echocardiography have been similar to those for exercise echocardiography. Echocardiographic variables obtained during pharmacologic stress have also been shown to have significant prognostic value.10 A normal dobutamine stress echocardiogram is associated with a low cardiac event rate in patients with suspected CAD and in those at clinically determined intermediate or high cardiac risk undergoing noncardiac surgery.

The presence of stress-induced regional wall motion abnormalities, particularly when detected at low heart rates, is a strong predictor of cardiac events. Dobutamine stress echocardiography allows further risk stratification, even in patients receiving peri-operative beta blockers with intermediate or high risk.11 In patients with ischemic heart disease and chronic LV dysfunction, dobutamine echocardiography is useful to identify myocardial viability.

Improvement in regional contractility at lower rates of dobutamine (5 to 10 mcg/kg/min) in segments that are akinetic or hypokinetic at rest predicts functional recovery after revascularization. The sensitivity for dobutamine echocardiography predicting recovery of function ranges between 74% and 88%, and specificity ranges between 73% and 87%. Dobutamine stress echocardiography has higher specificity but lower sensitivity and overall similar accuracy for predicting functional recovery, compared with radionuclide perfusion techniques.12

Echocardiographic contrast agents have been developed for the assessment of myocardial perfusion. These new agents consist of inert perfluorocarbon gases encapsulated in a biodegradable shell. Since the LV myocardium has a dense capillary bed, the injection of contrast microbubbles results in myocardial enhancement that is proportional to blood volume.

During vasodilator stress, in the presence of a flow-limiting stenosis, there is a reduction in capillary blood flow and myocardial blood volume in the segments supplied by the stenotic vessel. This may be detected as either a delay in myocardial enhancement following contrast injection or a relative reduction in enhancement in ischemic compared with normal segments. Recent studies have shown relatively good agreement between myocardial contrast echocardiography and SPECT for the detection of ischemia.13,14

NEW AGENTS

Echocardiography is the preferred method for evaluating valvular disease. In patients with valvular stenosis, transvalvular pressure gradients and valve area may be estimated with accuracy comparable to that obtained from direct invasive catheter measurements (see figure). Color Doppler echocardiography is also the preferred method for the evaluation of regurgitant valve lesions.

Ventricular size and function, which are important parameters to follow in patients with valvular heart disease, may be determined under resting conditions or during exercise. Transesophageal echocardiography allows closer examination of valve anatomy and function, with exquisite spatial and temporal resolution. It is the preferred method to establish the diagnosis of endocarditis of native and prosthetic heart valves.

Dr. Garcia is director of noninvasive cardiology at Mount Sinai Medical Center in New York, NY. He has received grants and research support from Philips Medical Systems.

References

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