P1.40 Determination of Pressure Independent Arterial Stiffness by Correcting Pulse Wave Velocity for Pressure-Area Relationship

Abstract

Background

Intrinsic (pressure independent) arterial stiffness is becoming increasingly important as treatment target in hypertensive patients but is still difficult to assess non-invasively. We measured pulse wave velocity (PWV) and the pressure-area (p-A) relation to determine both apparent and intrinsic stiffness.

Method

Non-invasive PWV (nPWV) was measured by multiple-M-mode ultrasound in a phantom. The incisura of the diameter waveforms was used as time-reference point for calculating nPWV. A catheter was placed in the phantom to measure the pressure waveform simultaneously. Additionally, in hypertensive patients carrying a baroreceptor stimulator, finger-pressure and nPWV at the common carotid artery were measured simultanously. For both phantom and subject studies, intrinsic PWV (PWV_int) was derived employing the Bramwell-Hill equation with the incremental distensibility, dA/(A*dp), based on either a linear or exponential p-A relation.

Results

In the phantom setup, nPWV (12.3±0.8 m/s) increased with increasing pressure (r = 0.67, p<0.0001). Because a linear p-A relation was observed, intrinsic PWV was calculated as ${\text{PWV}}_{\operatorname{int} }^2 = {{\text{A}}_{\operatorname{int} }}/{\text{A*nPW}}{{\text{V}}^2}$ and will be independent of pressure (r = -0.001). During baroreceptor stimulation MAP decreased from 138±22 to 109±1 mmHg and nPWV decreased from 10.5±1.5 to 6.6±1.3 m/s (p = 0.03). In these patients the observed p-A relation was exponential and PWV_int was therefore calculated using ${\text{PWV}}_{\operatorname{int} }^2 = {{\text{P}}_{\operatorname{int} }}/{\text{p*nPW}}{{\text{V}}^2}$. PWV_int did not decrease upon stimulation (p = 0.23).

Conclusion

Commonly apparent PWV is measured. Intrinsic stiffness can be calculated using nPWV and assuming a linear or exponential p-A relation. An acute reduction of blood pressure by baroreceptor stimulation lowers apparent but not intrinsic stiffness in hypertensives.