The cause of the increased vascular stiffness with age in humans is not known. Differences in the progression of vascular stiffness with aging may be an important determinant of the development of hypertension, especially systolic hypertension, with the subsequent substantial risk of vascular disease.
Pulse Wave Velocity
To measure pulse-wave velocity (PWV) in the aorta, 2 continuous wave hand-held Doppler transducers are used; one to deflect the pulse wave as it reaches the carotid artery and one to detect the pulse wave as it reaches the femoral artery. The time required for the pulse wave to travel from one probe to the other, combined with the distance between the two probes, allows the calculation of pulse wave velocity. Three runs of data are used so the results can be averaged, reducing the measurement variability. The system Complior SP is an automated PWV method.
Arterial Wave Form Analysis
A number of techniques for evaluating vascular stiffness rely on analysis of the pressure waveform. The particular technique that our laboratory uses is called the DynaPulse system and is marketed by a San Diego company called PulseMetric. The system is particularly easy to use and is marketed as an automated blood pressure system. A standard automated blood pressure cuff is attached to the participant’s arm and is inflated. Using the Automated Dynapulse system (PulseMetric, Inc.), a pattern recognition algorithm identifies characteristics which correspond to systolic, diastolic and mean arterial pressure changes in the cuff pulsation signal. The cuff is inflated three times and the results averaged.
A second system called the VP1000 (Colin, Japan/WaveNexus,TX) is used for vascular stiffness assessment and obtains other useful information about the vasculature. Ankle-Brachial indices and central and peripheral vascular stiffness measures are obtained using blood pressure cuffs and applanation tonometry. Cuffs are applied to both arms and ankles for blood pressure readings and additional blood flow data. Applanation tonometry is used to obtain central vascular stiffness measures. This is used for scoring the waveforms. Two runs of data are used so the results can be averaged, reducing the measurement variability.
Pressure-Strain Elastic Modulus
Methods using ultrasound imaging to evaluate vascular stiffness have the common goal of measuring vessel diameter at end diastole and peak systole so that the relative change in lumen diameter in response to a given pressure can be calculated. Our protocol uses the distal 1 cm of the CCA to measure change in lumen diameter over three cardiac cycles using B-mode imaging. Vascular stiffness is measured by the pressure strain modulus which reflects the pressure required to increase the lumen diameter by 1%, with higher values representing stiffer vessels.