5.6. Evaluation of elasticity and pulse wave velocity

In the previous section it was determined that pressure variation inside the artery affects the movement of the arterial wall. When the pressure increases, the movement of the arterial wall decreases. By measuring the blood pressure and radial velocity of the arterial wall it is possible to evaluate the elastic modulus and estimate the elasticity of the arterial wall, as equation (27) predicts. However, the pressure variation is small, about 20 mmHg, if only the effect of the baroreflex is considered. To achieve a larger pressure range, the blood pressure must be manipulated in some way. The easiest way of accomplishing this is to move the limb. A three-step tilt-test was used during the measurement. In the first step, the volunteer pointed his hand downwards at an angle of 45 degrees from the horizontal plane. Then he lifted his hand to the horizontal plane for a new measurement and, finally, to complete the sequence, he raised the hand upwards to an angle of 45 degrees. As each step took 10 seconds, the duration of the entire sequence was 30 seconds. Fig. 24 presents a measurement sequence from a volunteer, and the different steps are shown in Fig. 24 (a).

In the first step, with the hand held down, both MAP and PWV are at a high level and the corresponding Doppler frequency at a low level. On raising the hand in step two, the MAP value starts to decrease and the flexibility of the vessel wall increases. The wall then absorbs an increased amount of the pulse"s energy and the PWV decreases. In the last step, MAP reaches 65 mmHg, PWV decreases to 3.5 m/s and the Doppler frequency increases to 2.5 kHz.

The measurements were done on 16 healthy volunteers who didn’t have any diagnosed diseases. During the measurement sequence the ECG, blood pressure and Doppler signals were recorded for further data analysis. More information about the measurements can be found in original papers V and VI. The preliminary results are presented in paper VI and the corrected results based on more extensive measurements in paper V.

Fig. 25 presents the PWV for all volunteers as a function of MAP. A trend line is fitted to the measured data, indicating that PWV increases when the MAP value increases. For the measurement range between 60–120 mmHg, the average PWV varies between 3.9–4.8 m/s. The relative change in PWV within the MAP range of 60–120 mmHg is 19%. In this range, the standard deviation is approximately 0.5 m/s.

Fig. 26 presents the elastic modulus for all volunteers as a function of MAP. A trend line fitted to the data indicates that E increases when the MAP value increases. As the figure shows, E exhibits a large deviation. The relative change for E in the pressure range 60–120 mmHg is approximately 80%.

Because the deviation of E is large, the individual values of E were calculated in certain pressure ranges. Table 6 presents the calculated values for MAP, PWV and E for the pressures 40, 60, 80, 100, 120 and 140 mmHg, ± 5 mmHg. The standard deviations (std) for PWV and E are also presented. The last column presents the elastic modulus of a canine artery (Bergel 1961). The figures demonstrate that the mean values of E in this measurement are in same range as in the work of Bergel. For example, at 100 mmHg, we have E = 636.8 ± 314 kPa, while Bergel reports E = 690 ± 100 kPa at identical pressure. The corresponding values at 40 mmHg are 111.1 ± 96 kPa and 120 ± 20 kPa (Bergel 1961), respectively.