NEW IMAGING TECHNIQUES FOR ASSESSING CARDIAC FUNCTION

This article covers some of the newer imaging modalities available for assessing cardiac function and anatomy, with an emphasis on applications in patients who are being cared for in an intensive care setting. Some of these imaging modalities can be performed at the bedside, and some require patient transportation to an imaging suite. Among the newer imaging techniques discussed are MR imaging and radionuclide techniques, including bedside assessment of cardiac function using first-pass radionuclide angiography and combined perfusion and function using technetium-based myocardial perfusion tracers and gated tomographic acquisitions. Echocardiography has become an indispensable tool for evaluating cardiac function in critically ill patients. The major advantage of echocardiography is that the machine is portable and imaging can easily be performed at the bedside. The disadvantage is that good ultrasound windows can be difficult to obtain on patients who are critically ill, are on respirators, have chest bandages or tubes, are obese, or have hyperinflated lungs secondary to chronic obstructive lung disease. Overall anatomy of the cardiac chambers and valves can be assessed grossly on echocardiographic studies performed with suboptimal windows, but for accurate assessment of global left or right ventricular function, the ejection fraction numbers are considered clinically more useful. Ejection fraction can be derived from a two-dimensional echocardiographic study either by the "eye-ball" method or using software that quantitates the left ventricular volumes at end-diastole and end-systole using an area length or a Simpson's Rule method, both of which depend on the identification of the entire endocardial surface of the left ventricle in the imaging planes. In very ill patients with technically limited studies, visualization of the endocardial surface is usually incomplete. In addition, the "eye-ball" method for assigning a single number to global left ventricular function can be inaccurate in the presence of regional wall motion abnormalities, which are present in patients with coronary heart disease and in patients with cardiomyopathies or valvular heart disease.