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Doppler Measurement of Cardiac Output — A Critique

The use of suprasternal doppler techniques represents a potentially exciting advance in routine cardiovascular monitoring. Noninvasive devices using the doppler shift in ultrasound aim to provide the anaesthetist or intensive care physician with a one-time or continuous readout of cardiac output derived from a measurement of aortic blood flow velocity. However, doppler techniques for cardiac output measurement have a number of practical and theoretical problems which should be appreciated to ensure accurate measurement. Briefly, a probe incorporating a continuous wave or pulsed doppler wave transducer is placed in the suprasternal notch. From the quality and clarity of the reflected sound, the transducer is angulated to detect maximal velocity in the ascending aorta. The mean systolic velocity is derived by spectral analysis of the maximum velocity during systole. The cardiac output is the product of mean systolic blood flow velocity and aortic cross-sectional area. The latter is derived from aortic diameter may be read from a nomogram for height, weight, age and sex or measured with 2D or M-mode echocardiography. The aortic cross-sectional area is assumed to be constant throughout systole. The doppler equation is used to calculate the velocity (V) of a moving target, in this case, red blood cells, according to the formula: