Cardiovascular system is the system composed of heart and blood vessels. Pulse wave travels from the
heart, where it can be detected at the arteries, small arteries, and arterioles. Properties of the arteries are
nonlinear; therefore, pulse wave is highly dependent on the properties of artery. Both structural and
functional artery changes influence pulse wave shape as well as pulse pressure, and pulse wave velocity.
For instance, blood viscosity, elasticity of blood vessel, resistance of blood flow; temperature also has
influence to pulse shape (changing amplitude). Pulse wave analysis provides meaningful information about
heart and vascular system function, for the further treatment and cardiovascular risk prediction [16-18].
Stiffen arteries usually is the consequence of age or atherosclerosis, which is the cause to develop
myocardial infarction or stroke; therefore, arterial stiffness index is a good myocardial infarction or stroke
predictor. Moreover, derived data from pulse wave analysis can detect a patient who has coronary artery
disease or hypertension.
Age is the one key feature to change the elasticity of arteries, which is a consequence of increased pulse
wave velocity . Therefore, stiffness index has a close correlation (r=0.573, P<0.05) to pulse wave
velocity , as well as both are good cardiovascular risk indicators. An adjustment of the SI is required,
because young people with higher heart rate may have greater SI than older people with lower heart rate.
After adjustment (Figure above) correlation between age and SI improved from r=0.275, P=9.8E-019 to r=0.536,
Digital volume pulse (DVP) obtained from peripheral branch, simply measuring blood volume flow at the right
index finger using PPG, which is considered to be the reliable method to estimate SI and PWV . Time to
record DVP suggested by  was 10 seconds, where values averaged to obtain a single value.
Commonly, right hand index finger is used [17-18]; however, pulse wave can be recorded at different body
surfaces (ear, ankle, wrist, chest and, etc.). One study showed that pulse wave shape is almost the same at
the different hand index fingers .
TDVP is expressed as time delay between systolic and diastolic pressure peak waves. Artery path length is
unknown, therefore, assumed that it will be proportional to the subject height h. Then, SI can be expressed: