Aortic compliance measured by non-invasive Doppler ultrasound: Description of a method and its reproducibility

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Abstract
1. Aortic compliance (expressed as percentage increase in lumen diameter for a pulse pressure of 10 mmHg) was calculated from the characteristic pulse wave velocity deduced from ''foot to foot'' transit times of the blood flow pulse over a measured length of aorta. Two continuous wave Doppler ultrasound transducers were used: one insonating the root of the left subclavian artery, the other the abdominal aorta proximal to its bifurcation. Measurements were made after the subject had been supine for 5 min to achieve stable conditions. 2. Transit times were calculated on-line in real-time from the delay between the start of the systolic upstroke in the flow velocity sonograms recorded by the two transducers. 3. Within-visit variation was minimized by measuring over 45 cardiac cycles repeated three separate times and calculating an overall mean value for aortic compliance. A full measurement took 5-10 min. 4. Reproducibility of this overall mean value of aortic compliance was assessed as 1 month in 30 subjects (aged 60 .+-. 7 years, mean .+-. SD) and at 3 months in 23 different subjects (59 .+-. 8 years). Aortic compliance values ranged from 0.1 to 2.0%/10 mm Hg (see eqn 4). 5. The mean paired difference between aortic compliance values was -0.02 between 0 and 1 month, and 0.03 between 0 and 3 months, with respective 95% confidence limits of -0.06 to 0.03 and -0.03 to 0.09 embracing zero, thus not differing significantly from baseline. 6. Aortic compliance can be both simply and reproducibly measured non-invasively; if an individual was measured on two occassions, then 95% of the time the difference between the two measurements of aortic compliance would be < 0.28 (twice the SD of the 3 month differences).