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Pulse Contour Analysis
Cardiovascular Disease (CVD) is the leading cause of death and serious illness and in 1948, the Framingham Heart Study embarked on an ambitious project in health research. Pulse wave shape was one of the parameters collected during the study. The tools available to the investigators at that time precluded a detailed analysis of the waveform, but visual inspection of waveform changes correlated with increased risk of developing CVD . It is only recently that research workers from around the world have revisited this exciting observation and in particular the research group at St Thomas hospital showed that the finger volume pulse derived from a digital photoplethysmographic probe is directly related to the radial and brachial artery pressure pulse.
The Digital Volume Pulse (DVP)
The digital volume pulse (DVP) is recorded by measuring the transmission of infra-red light absorbed through the finger. The amount of light is directly proportional to the volume of blood in the finger pulp.
To minimise the occurrence of poor signals from vasoconstricted and poorly perfused subjects, a unique control system maintains the light transmission at the optimum level to accurately follow blood volume changes, independant of the subjects finger size to obtain an extremely accurate and noise free signal.
How the Digital Volume Pulse (DVP) is formed?
The first part of the waveform (systolic component) is formed as a result of pressure transmission along a direct path from the aortic root to the finger. The second part (diastolic component) is formed by pressure transmitted from the ventricle along the aorta to the lower body where it is reflected back along the aorta to the finger. The upper limb provides a common channel for both the directly transmitted pressure wave and the reflected wave and, therefore, has little influence on the contour of the DVP.
Indices derived from the Digital Volume Pulse (DVP)
The height of the diastolic component of the DVP relates to the amount of pressure wave reflection. This in turn relates mainly to the tone of small arteries.
The timing of the diastolic component relative to the systolic component depends on the pulse wave velocity (PWV) of pressure waves in the aorta and large arteries. This in turn depends upon large artery stiffness.
Indices derived from the Digital Volume Pulse (DVP)
Reflection Index RI is the height of the diastolic component of the DVP expressed as a percentage of the systolic peak and is a measure of the amount of pulse wave reflection and the tone of small arteries:
The Stiffness Index SI is an estimate of pulse wave velocity in large arteries and is obtained from subject height divided by the time between the systolic and diastolic peaks of the DVP. It is a measure of large artery stiffness.
The Digital Volume Pulse (DVP) is 'the same' as the pressure pulse in the wrist
It has been known for some time that the peripheral pressure pulse contains information on arterial stiffness and vascular tone and that increased arterial stiffness correlates with increased risk of a major cardiovascular event. The specific validation of Pulse Trace was done at St Thomas' Hospital and has been published. These papers demonstrated: A simple linear relationship between the shape of the Digital Volume Pulse and that of the peripheral pressure pulse, which remains constant irrespective of the effects of hypertension or effects of vasodilatation produced by NTG and that the Stiffness Index (SI) parameter correlates with PWV the gold standard for arterial stiffness (see below).
It is stable, simple, easy to use device with a low Coefficient of Variation
In comparison to other methods to measure arterial stiffness and vascular tone Pulse Trace is the only one that is operator independent with a Coefficient of Variation equivalent or better than the other established techniques. This has been demonstrated in a number of independent studies.
The Stiffness Index (SI) parameter correlates with the 'gold standard' measurement for arterial stiffness PWVcf
The Stiffness Index (SI) is calculated from the time it takes the reflected pressure wave to travel from the lower body back to the finger divided into the subject's height. Whilst many factors influence the volume pulse contour, arterial stiffness is the dominant factor. This was clearly demonstrated in a comparison of the 'gold' standard method of measuring arterial stiffness using the PWV (carotid - femoral) with the Pulse Trace SI parameter. This paper and others show that SI is a measure of arterial stiffness. The link between arterial stiffness and the risk of a major cardiovascular event is well established and SI can be used to measure and monitor arterial stiffness in a simple, non-operator dependant, and reproducible manner.
The Reflection Index (RI) parameter measures vessel tone and can be used to assess endothelial function.
The Reflection Index (RI) is calculated as the % ratio of the height of the diastolic notch to the peak pulse height. In numerous studies it has been shown to correlate with vascular tone (large vessel diameter) and can be used as a bases for a non invasive test for endothelial function and in other studies where the disease process or drug action is known to modify vascular tone e.g. PIH.