does Bio Clip work?
Bio Clip works by measuring a person’s pulse waves which are detected via an
infrared light emitted by the Bio Clip when it is attached to a finger. Infrared
light can be used in this way because of the light absorbing characteristics of
oxyhaemoglobin (HbO2) that is found in arterial blood. The amount of light is
directly proportional to the volume of blood in the finger pulse. With the skin
being so richly perfused (full of the blood that is supplying oxygen), it is
relatively easy to detect the pulsatile component of the cardiac cycle. Bio Clip
takes a short recording lasting 10-30 seconds and from this, it calculates your
average pulse. Bio Clip then produces a typical pulse wave form and
automatically derives your Stiffness Index (SI) and your Reflection Index
does Bio Clip derive SI and RI from the pulse wave contour?
time the heart beats and the aortic valve opens, the aorta receives a rushing
pulse of blood from the heart. It also receives pressure that spreads from the
walls of the heart to the walls of the aorta. These pressure (or pulse) waves
travel from the heart down the arterial walls in advance of blood flow. When the
wave hits major branching points, such as at the renal and femoral arteries, it
is reflected back so that it reverses direction and travels back to its point of
origin, becoming what is known as ‘wave reflection’.
Pulse waves are an observable and measurable physiological phenomenon in the
arterial system during blood circulation. They produce a wave pattern.
Researchers have been able to prove that the contour of the finger pulse is very
sensitive to vascular tone of the whole cardiovascular system.
Because the digital volume pulse that Bio Clip detects via the infrared light
signals is essentially the summation of both the pulse wave and the wave
reflection, Bio Clip is able to automatically calculate both your SI and RI from
the information provided by the pulse wave pattern.
The pulse wave part of the pulse that is measured by Bio Clip is the pressure
wave that is transmitted from the left ventricle of the heart to the finger via
the most direct route. The wave reflection element is formed by the pressure
transmitted from the heart to the lower body where it is reflected back up the
aorta and on to the finger. This defines the arterial pulse shape or pulse wave
contour as captured by the Bio Clip finger attachment.
The amount of reflection in the lower body determines the relative amplitude
of the pulse wave and the wave reflection. The amount of reflection itself is
governed by the vascular tone of the small arteries. Vascular tone refers to the
degree of constriction experienced by a blood vessel relative to its maximally
Reflection Index (RI)
Both vasodilation and vasoconstriction play important roles in determining
vascular tone. Vasodilation refers to the widening
of blood vessels which is caused by relaxation of the smooth muscle cells that
are found within the walls of blood vessels. As the blood vessels dilate and
become wider, blood flow increases because vascular resistance is lessened.
Vasoconstiction is the opposite process and refers to the narrowing of the
arteries resulting from contraction of the muscular walls of the blood vessels.
Vasoconstriction results in decreased or restricted blood flows.
There are many different competing vasoconstrictor and vasodilator influences
that act on blood vessels and, at any given time, vascular tone is determined by
the balance of these competing vasoconstrictor and vasodilator effectors. The
measure of vascular tone of the small arteries is known as the Reflection Index
Influences that cause variations in RI can be as simple as the effect of
caffeine or exercise.
Stiffness Index (SI)
The Stiffness Index, SI, is a measure of large artery stiffness. The
Stiffness Index is determined by time. There is a time delay
between the pulse wave and the wave reflection. This time delay is closely
related to pulse wave velocity (PWV) in the aorta and the large arteries. Pulse
wave velocity is a well-established technique for obtaining a measure of
arterial stiffness between two locations in the arterial tree. This is because
the velocity (speed) of the pulse wave along an artery is dependent on the
stiffness of the artery. The stiffer the artery, the shorter the time gap
between the pulse wave and the wave reflection.
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. By
including patient height the path length traversed by the wave reflection is
taken into account. This means that SI can be calculated more accurately. The SI
calculation gives a value similar to aortic pulse wave velocity.
The term Stiffness Index came to the fore following a series of studies
carried out by researchers from King’s College London and the University of
Wales College of Medicine (see in particular “Noninvasive Assessment of the
Digital Volume Pulse Comparison with the Peripheral Pressure”, Millasseau et al.
Hypertension. 2000;36:952-956). The researchers used photoplethysmography to
devise a reproducible parameter that they termed ‘stiffness index' by measuring
the time delay between direct and reflected waves in the digital volume pulse.
They were able to demonstrate a significant correlation between the stiffness
index and carotid-femoral pulse wave velocity (PWV) which is not surprising
given that SI is determined, to a large extent, by the velocity of the arterial
waveform in the aorta and large arteries. The research results clearly
demonstrated that the SI could be used as a valid surrogate for aortic PWV.
PWV is typically measured between the carotid and femoral arteries. The
carotid artery supplies the head and neck with oxygenated blood, with the
femoral artery being the large artery in the muscles of the thigh. Although PWV
is considered to be the gold standard for measuring arterial stiffness, SI has
been proven to be a reliable measure and monitor of arterial stiffness that uses
a simple, reproducible technique.