Stiffness index

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.

Adjusted stiffness index and age for the healthy people (r=0.536, p=7.279E-040)
Age is the one key feature to change the elasticity of arteries, which is a consequence of increased pulse wave velocity [16]. Therefore, stiffness index has a close correlation (r=0.573, P<0.05) to pulse wave velocity [33], 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, P=7.2E-040 [17].

Pulse wave volume characteristics at the age for the healthy subjects, measured at the right index

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 [16]. Time to record DVP suggested by [16] 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 [114].

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: SMA

2015 Heart Risk Warner