Both vasodilation and vasoconstriction play important roles in determining vascular tone. Vasodilation refers to the widening of blood vessels which is caused by relaxation of the smooth muscle cells that are found within the walls of blood vessels. As the blood vessels dilate and become wider, blood flow increases because vascular resistance is lessened. Vasoconstriction is the opposite process and refers to the narrowing of the arteries resulting from contraction of the muscular walls of the blood vessels. Vasoconstriction results in decreased or restricted blood flows.

There are many different competing vasoconstrictor and vasodilator influences that act on blood vessels and, at any given time, vascular tone is determined by the balance of these competing vasoconstrictor and vasodilator effectors. The measure of vascular tone of the small arteries is known as the Reflection Index or RI.

Influences that cause variations in RI can be as simple as the effect of caffeine or exercise.

The Stiffness Index (SI)

The Stiffness Index, SI, is a measure of large artery stiffness. The Stiffness Index is determined by time. There is a time delay between the pulse wave and the wave reflection. This time delay is closely related to pulse wave velocity (PWV) in the aorta and the large arteries. Pulse wave velocity is a well-established technique for obtaining a measure of arterial stiffness between two locations in the arterial tree. This is because the velocity (speed) of the pulse wave along an artery is dependent on the stiffness of the artery. The stiffer the artery, the shorter the time gap between the pulse wave and the wave reflection.

SI is calculated from the time it takes the reflected pressure wave to travel from the lower body back to the finger divided by the subject’s height. By including patient height the path length traversed by the wave reflection is taken into account. This means that SI can be calculated more accurately. The SI calculation gives a value similar to aortic pulse wave velocity.

The term Stiffness Index came to the fore following a series of studies carried out by researchers from King’s College London and the University of Wales College of Medicine (see in particular “Noninvasive Assessment of the Digital Volume Pulse Comparison with the Peripheral Pressure”, Millasseau et al. Hypertension. 2000;36:952-956). The researchers used photoplethysmography to devise a reproducible parameter that they termed ‘stiffness index’ by measuring the time delay between direct and reflected waves in the digital volume pulse. They were able to demonstrate a significant correlation between the stiffness index and carotid-femoral pulse wave velocity (PWV) which is not surprising given that SI is determined, to a large extent, by the velocity of the arterial waveform in the aorta and large arteries. The research results clearly demonstrated that the SI could be used as a valid surrogate for aortic PWV.

PWV is typically measured between the carotid and femoral arteries. The carotid artery supplies the head and neck with oxygenated blood, with the femoral artery being the large artery in the muscles of the thigh. Although PWV is considered to be the gold standard for measuring arterial stiffness, SI has been proven to be a reliable measure and monitor of arterial stiffness that uses a simple, reproducible technique.