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The photoelectrical Plethysmography displays the contour of the peripheral pulse waveform related with the volume changes in artery.
The pulse wave form analysis provides 2 markers (Stiffness Index and Reflection Index) related to the endothelium vascular function.
How the changes in vascular function in a fingertip, ensure that the endothelium of the entire vascular system has been checked?
The endothelium is the same throughout the body, and when damage is noted in the fingertip, it indicates that the endothelium is damaged throughout the body—that it’s a systemic dysfunction.
Pulse waveform analysis
Analysis of the contour of the peripheral pulse to assess arterial properties was first described in the nineteenth century.
With the recognition of the importance of arterial stiffness there has been a resurgence of interest in pulse wave analysis, particularly the analysis of the radial pressure pulse acquired using a tonometer.
An alternative technique utilizes a volume pulse. This may conveniently be acquired optically from a finger (digital volume pulse). lthough less widely used, this technique deserves further consideration because of its simplicity and ease of use.
As with the pressure pulse, the contour of the digital volume pulse is sensitive to changes in arterial tone induced by vasoactive drugs and is influenced by ageing and large artery stiffness.
Measurements taken directly from the digital volume pulse or from its second derivative can be used to assess these properties. The arterial pulse waveform is a contour wave generated by the heart when it contracts, and it travels along the arterial walls of the arterial tree. Generally, there are 2 main components of this wave: forward moving wave and a reflected wave.
The forward wave is generated when the heart (ventricles) contracts during systole. This wave travels down the large aorta from the heart and gets reflected at the bifurcation or the “cross-road” of the aorta into 2 iliac vessels. In a normal healthy person, the reflected wave usually returns in the diastolic phase, after the closure of the aorta valves. The returned wave give a notch and it also helps in the perfusion of the heart through the coronary vessels as it pushes the blood through the coronaries.
Therefore the velocity at which the reflected returns becomes very important : the stiffer the arteries are, the faster it returns.
This may then enter into the systolic phase and augment final blood pressure reading.
Diagram shown a typical healthy Digital Pulse Waveform.
Using this technique, Dawber et al. obtained the DVP in 1778 individuals from the Framingham cohort recruited in 1965 and 1966.
They proposed that the DVP be classified into one of the following classes :
Class 1, a distinct notch is seen on the downward slope of the pulse wave;
Class 2, no notch develops but the line of descent becomes horizontal;
Class 3, no notch develops but there is a well-defined change in the angle of the descent;
Class 4, no notch develops or no change in angle of descent occurs.
They found class 1 to be prevalent in younger individuals and class 4 present in older participants and in individuals with established coronary artery disease.
In men aged 65–74, the prevalence of myocardial infarction was approximately fourfold greater in participants with a class 4 waveform compared with a class 1 waveform.
Second derivative of the original Pulse wave
A sophisticated approach to contour analysis of the DVP has been developed by investigators in Japan. Takazawa et al., Takada et al. and Imanaga et al. have proposed using the second derivative of the DVP (d2DVP/dt2, sometimes referred to as the ‘acceleration photoplethysmograph’). This facilitates the distinction of five sequential waves called a, b, c, d and e waves . The relative heights of these waves (b/a, c/a, d/a and e/a ratios), particularly the d/a ratio, have been related to age, arterial blood pressure , large artery stiffness and effects of vasoactive drugs .
The b/a ratio has been related to ageing and carotid distensibility . Following analysis of the correlation of the b/a, c/a, d/a and e/a ratios with age, a more complex ‘ageing index’ was defined as (b–c–d–e)/a .
In a study to assess arterial distensibility in adolescents, the d/a ratio identified individuals at increased risk of developing atherosclerosis. The second-derivative approach has recently been applied to the study of the peripheral pressure pulse. Other mathematical approaches to analysis of the DVP include artificial neural networks, the extraction of periodic components using frequency analysis or nonlinear dynamical analysis. The physiological and clinical characteristics relating to the derived mathematical parameters, however, have not been clearly identified.
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Features of the Digital Pulse Waveform