A variety of other techniques, many of which employ imaging modalities, can be used to measure or estimate cardiac output. All of them use time dependent images of the heart to estimate the difference between end-dias-tolic and end-systolic volumes. This difference gives stroke volume and, with heart rate, allows calculation of cardiac output.
Radionuclide Techniques. In radionuclide techniques, a radioactive substance (usually technetium-99) can be made to circulate throughout the vascular system by attaching (tagging) it to red blood cells or albumin. The radiation (gamma rays) emitted by the large pool(s) of blood in the cardiac chambers is measured using a specially designed gamma camera. The emitted radiation is proportional to the amount of technetium bound to the blood (easily determined by sampling the tagged blood) and the volume of blood in the heart. Using computerized analysis, the amount of radiation emitted by the left (or right) ventricle during various portions of the cardiac cycle can be determined (Fig. 14.12A and B). The amount
Imaging techniques for measuring cardiac output. A and B, Radionuclide angiograms. The white arrows in A show the boot-shaped left ventricle during cardiac diastole when it is maximally filled with radionuclide-labeled blood. In B, much of the apex appears to be missing (white arrows) because cardiac systole has caused the blood to be ejected as the in-traventricular volume decreases. C and D, Two-dimensional echocardiograms. In this cross-sectional view, the left ventricle appears as a ring. White arrows indicate wall thickness. In diastole (C), the ventricle is large and the wall is thinned, during systole (D), the wall thickens and the ventricular size decreases. E and F, Ultrafast (cine) computed tomography. The ventricular size and wall thickness can be assessed during diastole and systole, and the change in ventricular size can be used to calculate cardiac output.
of blood ejected with each heartbeat (stroke volume) is determined by comparing the amount of radiation measured at the end of systole with that at the end of diastole, multiplying this number by the heart rate yields cardiac output.
Echocardiography. Echocardiography (ultrasound car-diography) provides two-dimensional, real-time images of the heart. In addition, the velocity of blood flow can be determined by measuring the Doppler shift (change in sound frequency) that occurs when the ultrasound wave is re flected off moving blood. Echocardiography can, therefore, be used to measure changes in ventricular chamber size (Fig. 14.12C and D), aortic diameter, and aortic blood flow velocity occurring throughout the cardiac cycle. With this information, cardiac output may be estimated in one of two ways. First, the change in ventricular volume occurring with each beat (stroke volume) can be determined and multiplied by the heart rate. Second, the average aortic blood flow velocity can be measured (just above or below the aortic valve) and multiplied by the measured aortic cross-sectional area to give aortic blood flow (which is nearly identical to cardiac output).
Computed Tomography. Ultrafast (cine) computed tomography and magnetic resonance imaging (MRI) provide cross-sectional views of the heart during different phases of the cardiac cycle (Fig. 14.12E and F). Stroke volume (and cardiac output) can be calculated using the same principles described for radionuclide techniques or echocardiogra-phy. When ventricular volume changes are estimated from cross-sectional data, assumptions are made about ventricular geometry. Although these assumptions can lead to errors in calculating cardiac output, these methods have proven to be highly useful.
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