Pulse Wave Velocity Testing in the Baltimore Longitudinal Study of Aging

Summary

Pulse wave velocity (PWV) measurement assesses arterial stiffness by a tonometry-based system that measures the speed with which arterial pressure wave travels along the arterial tree, typically approximating the time that it takes this wave to travel from the descending aorta (using the carotid artery as a surrogate) to the femoral artery. Carotid-femoral PWV increases two- to threefold across the adult lifespan.

Abstract

Carotid-femoral pulse wave velocity is considered the gold standard for measurements of central arterial stiffness obtained through noninvasive methods¹. Subjects are placed in the supine position and allowed to rest quietly for at least 10 min prior to the start of the exam. The proper cuff size is selected and a blood pressure is obtained using an oscillometric device. Once a resting blood pressure has been obtained, pressure waveforms are acquired from the right femoral and right common carotid arteries. The system then automatically calculates the pulse transit time between these two sites (using the carotid artery as a surrogate for the descending aorta). Body surface measurements are used to determine the distance traveled by the pulse wave between the two sampling sites. This distance is then divided by the pulse transit time resulting in the pulse wave velocity. The measurements are performed in triplicate and the average is used for analysis.

Introduction

Pulse Wave Velocity (PWV) is regarded as the gold standard in the assessment of arterial stiffness². While not used clinically in the United States, it has been validated and widely used in clinical settings throughout Europe³. Changes in PWV have been correlated with cardiovascular risk factors such as hypertension, obesity, impaired glucose tolerance, and dyslipidemia. Higher aortic stiffness has been associated with increased risk for a first cardiovascular event⁴, muscle mass decline⁵, and slower gait speed in subjects with peripheral arterial disease⁶. Assessment of PWV is accomplished by analyzing the speed with which a pulse of blood travels from the carotid artery to the femoral artery⁷. It is a relatively simple method with highly reproducible results. Advances in technology continue to simplify the procedure, reducing operator dependency and decreasing the amount of time required for the exam. The system used in our laboratory is produced by SphygmoCorand utilizes an EKG and a high-fidelity tonometer to acquire waveforms from the carotid and femoral pulses. The software then analyzes these variables to determine the velocity by 1) estimating the time that the pulse wave takes to travel between the carotid and femoral sites by referencing the timing of the pressure wave arrival at each arterial site relative to the electrocardiographic waveforms 2) dividing the measured distance between the two sites by the estimated time. The closer together the sampling sites, the less accurate the PWV, limiting the use of this method to larger vessels such as the femoral and carotid arteries. In addition to determining PWV, the system can calculate the pressure in the ascending aorta by applying a transfer function to the waveform produced by the radial pulse⁸ which is the "central pressure" and the pressure that most directly impacts the heart at time of ejection. This information can aid in the assessment of the arterial system's effect on the left ventricle in a reliable and noninvasive manner⁹. Alternative techniques used for the examination of vascular stiffness, such as carotid distensibility via echo tracking, require a great deal of technical expertise and are more time consuming than the PWV. Echo tracking also generally utilizes video-image analysis and therefore the accuracy of these measurements are limited³. Other techniques for examining vascular changes include assessment of carotid-intima medial thickness (IMT) which provides results more closely related to atherosclerotic progression although the cross-sectional and transverse images obtained can provide a crude approximation of carotid elastance. This method, however, is plagued by some of the same constraints as the distensibility studies described above. As researchers continue to look at the role arterial stiffness plays in various disease states, PWV measurement will likely continue to provide a quantitative and reproducible surrogate endpoint.

Protocol

1. Procedure

1. Open the software via the shortcut on your Windows desktop by double-clicking on the icon.
2. Click Patient to activate the Patient Screen.
3. To enter a new patient into the database, click the New button in the patient search area, and then click on the Yes button to confirm you would like to enter a new patient.
4. Enter patient details such as last name, first name, date of birth, and gender. Click on the Save button to advance to the next step.
5. Click the PWA or PWV button to perform measurement. If only one of these modes is available, there is no need to click on the button, simply proceed to the study screen.

2. Central Blood Pressure Measurement Using Pulse Wave Analysis (PWA)

1. Click on the Study button to enter study parameters. Enter the brachial pressure taken from a calibrated sphygmomanometer and any other details you wish to include.
2. Click on the Capture Data button to proceed with a measurement. Attach the three ECG electrodes and leads to the patient in a modified Lead II configuration or using the patient's limbs as indicated on the cables.
3. Palpate the patient's radial artery. This is located on the lateral of the wrist, immediately above the wrist joint near the base of the thumb. Gently press the radial artery against the underlying bone with the middle and index fingers.
4. Place the tonometer over the strongest pulse point. Gently press the tonometer down until you see a consistent pressure waveform displayed on the data capture screen.
5. If Auto Capture is enabled (see Note below), the system will automatically save the measurement. To manually save the measurement for analysis, press the spacebar on your keyboard (or step on the footswitch, if used).
6. If a repeat measurement is required with the same study parameters, click on the Repeat button to return to the Capture Data screen and repeat the measurement as outlined in steps 2.4-2.5 above.
7. To perform a measurement on a new patient, click on the Patient button and return to step 1.3 above.

3. Arterial Stiffness Measurement Using Pulse Wave Velocity (PWV)

1. Click on the Study button to enter study parameters.
2. Enter brachial pressure taken from a calibrated sphygmomanometer
   Enter the distance measured for the distal (supra-sternal notch to femoral pulse) and proximal (supra-sternal notch to carotid pulse) sites. Medication, Notes, and Operator fields may be entered, if desired
3. Click on the Capture button to proceed with the measurement.
4. Ensure the ECG signal is of acceptable quality.
5. Palpate the carotid artery pulse. This is located between the larynx and the sternocleidomastoid muscle in the neck. It can be felt by pushing the muscle to the side and pressing the artery against the larynx.
6. Place the tonometer on the carotid artery and gently press the tonometer down until you receive a consistent pressure waveform displayed on the data capture screen.
7. When you have a minimum of 13 sec of consistent waveforms, click the OK button or (press on the footswitch, if used).
8. Palpate the femoral artery pulse. This is located over the ventral thigh halfway between the pubic symphysis and anterior superior iliac spine.
9. Place the tonometer on the femoral artery and gently press the tonometer down until you receive a consistent pressure waveform displayed on the capture screen. When you have a minimum of 13 sec of consistent waveforms, click the OK button (or press the on the footswitch, if used).
10. The report(s) can now be reviewed. These measurements are typically conducted in triplicate. If the user wishes to measure the PWV between carotid and radial arteries, these steps just shown would be repeated except that instead of measuring at the femoral artery, the user would place the tonometer at the radial artery. To perform a measurement on a new patient, click on the Study button and repeat this procedure or use the Search feature to search for an existing patient.

4. Shut Down

1. The software automatically saves all reports. To close the software, click on System from the main menu, and then click on Exit. Place the tonometer in the module tray for storage.

Results

Table 1. PWV results are produced in meters per second. In healthy subjects, these speeds can range from an average 6.2 m/sec (in subjects under 30) to 10.9 m/sec (in subjects...

Discussion

The measurement of pulse wave velocity (PWV) is a noninvasive method for analyzing central arterial stiffness in a relatively simple and reproducible manner³. To assess PWV, the carotid (as a surrogate for the descending aorta) and femoral pulse waveforms are acquired using a high-fidelity tonometer; the time delay in pulse wave arrival at the between the two sites is estimated referencing to a concurrently recorded EKG waves by custom designed computer software that conforms to the propagative model of the ar...

Materials

Name	Company	Catalog Number	Comments
SphygmoCor System	AtCor	SPT-304, Module EM3