Non-Invasive Vascular Research Methods
Endothelial Function Testing
Endothelial pulse amplitude testing (EndoPAT) is a noninvasive method for measuring endothelial function in the peripheral arteries by using the principle of reactive hyperemia. After a temporary occlusion of a blood vessel, the vessel walls dilate and blood flow increases to compensate for the lack of blood flow during the occlusion (this is called reactive hyperemia). If a vessel does not reach an appropriate degree of dilation after a period of occlusion, this is termed endothelial dysfunction. Endothelial function is one of the earliest detectable cardiovascular changes associated with disease, and can be reversed.
EndoPAT is an FDA-approved device that uses inflatable probes on the fingertips to record pulsatile blood volume changes before and after a 5-minute occlusion of the brachial artery. It calculates a reactive hyperemia index (RHI) as a measure of endothelial function, which is a validated predictor of cardiovascular outcomes in adults.
Arterial Applanation Tonometry
Arterial applanation tonometry is a non-invasive assessment that uses a pressure sensor the size of a pencil to record pulse pressure at peripheral arterial sites. Tonometry can be used to measure augmentation index and pulse wave velocity, two measures of arterial stiffness.
Pulse Wave Velocity (PWV): As blood vessel walls stiffen, less resistance is applied to the flow of blood through the arteries, leading to an increase in the velocity of the pulse wave (PWV). By calculating the time difference between the R-wave on an electrocardogram and the arrival of the pulse wave at two distant arterial sites (carotid/brachial and femoral), the PWV can be calculated.
Augmentation Index (AIx): As the blood pulse moves through the arterial tree, a reflected wave is generated that travels back to the heart. Stiff blood vessels increase the speed of this return, leading to the premature arrival of the pulse wave and causing an augmentation of the blood pressure pulse at the heart. Augmentation index is the percent of pulse pressure augmentation due to this reflected wave.
Carotid Artery Ultrasound
Carotid artery ultrasound can allow investigators to examine structural properties of the artery. Intima-media thickness (IMT) is the width of the two inner layers of an artery, the tunica intima and tunica media. The IMT is associated with atherosclerosis in the blood vessel, and carotid IMT is a significant predictor of stroke and myocardial infarction in adults. Children with hypertension, obesity, and familial hypercholesterolemia demonstrate higher IMT values than healthy children.
Additionally, cross-sectional measurements of the diameter of the artery can be taken in systole and diastole and used to calculate the stiffness index and pressure-strain elastic modulus of the arterial wall.