We have developed digital, Apple II microcomputer-based methods for the numerical analysis of pulsed, range-gated, ultrasonic Doppler blood velocity signals. These methods were then used to analyse Doppler data recorded every 5 ms from the ascending aorta via the suprasternal notch in normal subjects ranging in age from 3 to 62 years. Normal values for peak velocity, the integral of velocity over the time of systole, and the rate of change of velocity in early ejection are reported. It was found that, after an initial step increase, the velocity of blood flow in early ejection increased in a linear manner in more than two thirds of the individual beats analysed. The time for which the linear acceleration in the aorta was constant (circa 50 ms) was unrelated to the age or size of the subject.

A computer model of the aorta and its branches was made based on a simulation of an electrical transmission line using T elements. The model represented the aorta, with branches to the arms and legs and a branch to the head. The values for the capacitance and inductance of each T element could be specified, and a linearly increasing left ventricular pressure was used to drive the model. Transmission line equations were used to select values for the components, and an attempt was made to simulate the results of measurements of blood velocity in the aorta and peripheral arteries of normal subjects. The values obtained with the model showed a close relation to those in experimental studies. The results support the hypothesis that the arterial bed can be well represented by a "lossless" branched transmission line, with impedances matched at each branch and terminated with resistances that give a reflection coefficient of 0.5. A driving function, in which a linearly increasing left ventricular pressure provided a transient input to an aortic root of relatively low impedance, gave the best simulation of the experimental results.

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