The Measurement of Input Impedance and Apparent Phase Velocity in the Human Aorta

Abstract Measurement of the input impedance in the aorta in 3 patients with rheumatic mitral disease is described. The aortic flow was measured by recording the pressure gradient along the ascending aorta with a differential manometer through a double‐lumen catheter; a separate strain gauge on one side of the system enabled the arterial pressure to be recorded simultaneously. Records were made before and during the administration of i.v. nor‐adrenaline. Computation of blood flow, apparent phase velocity and impedance was performed on a digital computer. The moduli of aortic input impedance decreased with frequency over the first few harmonics both before and during nor‐adrenaline but were higher during infusion. The pattern of impedances tended to suggest that some reflection was reaching the ascending aorta; this was also suggested by the high values obtained for apparent phase velocity at low frequencies.