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 PWVint2=Aint/A*nPWV2 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 PWVint2=pint/p*nPWV2. 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.