Doppler flow determination.

Doppler ultrasound provides a non-invasive alternative to the established methods of blood flow measurement. The standard principle of the Doppler-echographic determination of volume flow rate is based upon the assessment of three quantities: the vascular cross-section, the angle of incidence of the ultrasound relative to the direction of the velocity vector, and the cross-sectional average velocity. In the literature reporting comparisons between Doppler and invasive cardiac output measurements, correlation coefficients ranging from 0.8 to 0.95 are given for adult patients. Error limits of less than 20% for individual measurements, however, require a skillful investigator, a high quality instrument, and an appropriate subject. From a technical point of view, the highest accuracy is achieved by directing the ultrasound beam parallel to the flow, by measuring the true cross-sectional area instead of the diameter, and in the presence of well-behaved transluminal blood velocity distributions.