With Doppler echocardiography, blood flow velocity is usually measured either within or close to the heart (e.g. aortic root, transmitral). Echocardiographic measurement of cross-sectional area at this point enables calculation of cardiac output as the product of blood velocity averaged over 1 min and cross-sectional area. Non-uniform flow distribution, particularly in the presence of turbulence, may render this measurement unreliable. Further error derives from difficulty in orienting the Doppler signal to the direction of blood flow, and from beat-to-beat variation in stroke volume.
Aortic vessel area at the valve ring (or just distal to the sinuses of Valsalva) is calculated from aortic root diameters determined from two-dimensional images, assuming a circular shape. However, both the aorta and the main pulmonary trunk are more ellipsoidal than circular, and root area varies with timing in the cardiac cycle and with changes in preload, afterload, and cardiac dynamics. Planimetric measurement, enhanced by recently developed edge-detection technology, may improve the accuracy of calculated outputs although it may still prove dificult to obtain accurate short-axis views. Similar methodology has been applied to record flow across the left ventricular outflow tract, the pulmonary artery, and, in particular, the mitral valve. The area of the mitral valve is measured by planimetry or estimated from diameters obtained in two- and four-chamber views assuming an elliptical shape. Transesophageal echocardiographic imaging allows cardiac output estimation in the critically ill more readily than the transthoracic approach.
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