Partial standing wave formation above an abdominal aortic stenosis.

Reflection from an arterial discontinuity, such as a stenosis, would be expected to produce partial standing waves of pressure and flow proximal to the stenosis. This phenomenon is demonstrated by determining the amplitude changes of the harmonic components of pressure and flow waves recorded at three sites at different distances from an experimental stenosis of the abdominal aorta in dogs. To minimise the effect of reflections from other arterial sites, such as the peripheral beds, the animals were vasodilated. Impedance and propagation velocity measurements were made to estimate the contribution of peripheral reflections in the harmonic components of the aortic pressure and flow pulses. In general, the peripheral contribution appeared to be small for harmonics greater than the first. The results indicate that, for the 'closed' type of reflection at the stenosis, a pressure antinode and a flow node occur immediately proximal to the stenosis. As the distance from the stenosis to the measurement site is increased, nodes and antinodes of pressure and flow occur at frequencies which correspond to integer multiples of lambda/4. Similar fluctuations take place in the impedance modulus proximal to the stenosis, such that close to the stenosis the modulus is a maximum, at the lambda/4 distance the modulus is a minimum and at the lambda/2 distance it is again a maximum. The extent of these impedance changes with distance indicates that the attenuation of the reflected, backwardgoing waves is greater than found for forwardgoing waves and that the diagnostic assessment of a vascular obstruction by means of a proximal measurement of pressure or flow may be subject to error if the measurement is not made close to the obstruction.