A photoplethysmograph is an optical detector that indicates the volume of blood in or passing through an area of tissue. By placing the photoplethysmograph at or near the site of a human artery the pulse waveform can be detected and measured.
The arterial pulse waveform results from the ejection of blood from the left ventricle and moves with a velocity much greater than the forward movement of the blood itself.
The 1st part of waveform (systolic component) is the result of pressure transmission along a direct path from the heart to the finger.
The 2nd part (diastolic component) is formed by pressure transmitted from the LV along the aorta to the lower part of the body where it is reflected back along the aorta to the finger.
As the heart beats, pressure and flow pulse waves travel away from the heart and are reflected back toward the heart from various locations in the arterial system.
The height of the diastolic component of the PW = the amount of pressure wave reflection. This relates mainly to the tone of the small arteries. The timing of the diastolic component relative to the systolic component depends on how fast the wave passed through the aorta and large arteries. The stiffer the arteries, the quicker the blood and waveform passes through. When the aorta receives the rushing pulse of blood from the heart, it also receives pressure to its own walls. This pressure travels along the aorta’s walls in wave after wave until it reaches the walls of the smaller branching arteries that take blood to the rest of the body.
There, the speed of these pressure waves – known as pulse wave velocity – slows, and some are sent back through the aorta walls, becoming what are known as wave reflections. Now add 50 years to this picture: the arteries, including the aorta, grow stiffer and dilate, their walls becoming thicker and their diameter larger. As a result, the arteries no longer expand and contract as much as they used to. Along the walls of the stiffer aorta, the pressure waves move more rapidly, and as a result the reflection waves occur sooner than they did before.
The timing of the reflection wave is how arterial stiffness is measured.