Abstract-Segmentsof 45 human thoracic and 20 abdominal aortas, including 13 pairs, aged 30-88 yr at autopsy. were perfused with 37 C Tyrode's solution at in-sifcd length. Diameter changes due to 20 mmHp pressure steps, between 20 and I80 mmHg. were measured to I pm accuracy with balanced transducers.

We tested Portapres, an innovative portable, battery-operated device for the continuous, noninvasive, 24-hour ambulatory measurement of blood pressure in the finger. Portapres is based on Finapres, a stationary device for the measurement of finger arterial pressure. Systems were added to record signals on tape, to alternate measurements between fingers automatically each 30 minutes, and to correct for the hydrostatic height of the hand. We compared the pressure as measured by Portapres with contralateral intrabrachial pressure measured with an Oxford device. Results were obtained in eight volunteers and 16 hypertensive patients. Time lost due to artifact was about 10% for each device. In two patients a full 24-hour Oxford profile was not obtained. In the remaining 22 subjects finger systolic, diastolic, and mean pressures differed +1 (SD 9), -8 (6), and -10 (6) mmHg, respectively, from intrabrachial pressure. These diastolic and mean pressure underestimations are similar to what was found earlier for Finapres, are typical for the technique, and are systematic. Avoiding brisk hand movements resulted in fewer waveform artifacts. The hand had to be kept covered to continue recording at low outside temperatures. Sleep was not disturbed by Portapres, and arterial pressure showed a marked fall during siesta and nighttime. There were no major limitations in behavior, and no discomfort that originated from continuous monitoring was reported.

A new structural model is described for the tension-radius relationship of blood vessels, taking into account their mechanically important constituents: collagen, elastin and smooth muscle. The model has four characteristic parameters: EC, the Young's modulus of the collagen fibres; ESE, the Young's modulus of the combined smooth-muscle/elastin network; epsilon mu, the amount of strain at which the high stiffness region on the tension-radius curve is reached, and eta an indicator for the degree of collagen fibre stretching. The structural stiffness of the wall constituents is reflected by EC and ESE whereas the global stiffness of the entire blood vessel is described by epsilon mu and eta. All these elasticity parameters are pressure independent, in contrast to generally quoted values for the incremental modulus or vascular compliance which are strongly pressure dependent. Hence, an objective comparison of the mechanical properties for various types of blood vessel, based on the present model parameters, has been made possible. The model was successfully fitted to tension-radius data of 65 human aortas, age range 30-88 years, with moderate or severe atherosclerosis. The structural as well as the global stiffness changes with age, e.g. collagen stiffness shows a ninefold increase over 60 years. Global stiffness depends on atherosclerosis.

The clinical utility for autonomic research purposes of the Finapres, a device for measuring finger arterial pressure continuously and noninvasively, was assessed by estimating its accuracy, precision and within-subject variability in 48 subjects, aged 18-65 years, in comparison with intraarterial brachial blood pressure. At differences of -3.5 +/- 12, -8.0 +/- 8 and -4.4 +/- 8 mmHg from simultaneous intrabrachial systolic, mean and diastolic pressure, the Finapres meets the Association for the Advancement of Medical Instruments (AAMI) accuracy requirements for systolic and diastolic, but not for mean pressure. The precision requirements were nearly met for mean and diastolic, but not for systolic pressure. These results compare to those of others under widely varying circumstances in anaesthetized and awake subjects, and are comparable to those published for the auscultatory technique. The within-subject precision of Finapres is high; the 95% confidence intervals are 3.4-4.5 mmHg for systolic, 1.5-2.0 mmHg for mean and 1.7-2.2 mmHg for diastolic. This makes the Finapres suitable for tracking changes in blood pressure. Four case studies are provided as examples of the value of the Finapres in the clinical laboratory.

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