EXERCISE AND CONTRAST ECHOCARDIOGRAPHY
This type of stress test generally involves walking on a treadmill after a baseline echocardiogram is obtained (see Plate 3-15). During treadmill exercise the heart rate increases, and if myocardial ischemia occurs, wall motion abnormalities can be detected, which return to baseline at rest or after nitroglycerin. This is highly suggestive of a high-grade stenosis of a coronary artery; identification of the specific artery depends on the distribution of the wall motion abnormality. For example, if the ventricular septum contracts normally at rest but with exercise barely moves, this suggests disease in the anterior descending coronary artery. In contrast, a similar situation with the inferior wall suggests disease of the vessel supplying the posterior descending coronary artery.
If the
patient reaches 85% to 90% of predicted maximum heart rate with no transient
wall motion changes on the echocardiogram, high-grade coronary stenosis can be
excluded in the majority of cases (i.e., excellent negative predictive value).
Early
attempts at contrast echocardiography involved intravenous injection of
agitated saline, which did not cross the pulmonary vascular bed and thus was
seen only in the right atrium and right ventricle. Right-to-left shunts could
be detected by bubbles in the left atrium or left ventricle. Left-to-right
shunts at the atrial or ventricular level could be detected as negative images.
Other contrast agents were developed later that allowed passage through the
lung after bolus injection in a peripheral vein. These contrast agents
consisted of microspheres and were helpful in defining intracardiac structures,
demonstrating intracardiac shunts, and enhancing Doppler velocity signals
through heart valves. Probably the most important use of these agents is to
identify clearly the borders of the left ventricle, which allows an easier and
better assessment of left ventricular function than when these agents are not used
(see Plate 3-15).
