Estimation of aortic and pulmonary blood flow in a paediatric population using Doppler ultrasound: an in-vitro study

The noninvasive measurement of blood flow using ultrasound has been attempted by numerous workers. In order to evaluate the technique of duplex scanning applied to the problem of noninvasive assessment of aortic and pulmonary blood flow in a paediatric population with ventricular septal defect, we attempted an in-vitro simulation of these flow conditions. The factors which have been considered in the design of the apparatus include the availability of a range of tube diameters, an aortic-"like" waveform, laminar flow, variable downstream peripheral resistance, a compliance factor and viscosity of the flow medium. The ultrasound probe beam characteristics, sample volume size and shape were also determined. From the results of the beam plots it was apparent that the region of sensitivity of the CW Doppler probes were inappropriate for the measurement of blood flow in neonates. In order to simulate physiological parameters in a paediatric population as closely as possible, five tubes of diameters ranging from 9-18 mm were used, in each tube a range of flow values from 0.8 L min-1 to 5 L min-1 were measured. The flow values were measured by both "bucket and stopwatch" and duplex scanner for both steady and pulsatile flow conditions. The results are presented as correlations between the direct and noninvasive pulsed Doppler assessment of flow measurement and are in the range 0.92-0.99 significant at p less than .001. A discussion of the reliability of flow measurements made using a conventional duplex scanner is given.