Indirect or
Physiologic Testing
We also use what is commonly referred to as
indirect or physiologic testing. This can be used to
evaluate the arterial as well as the venous systems. For
arterial applications, it is primarily used in arm and
leg artery testing. This modality allows us to obtain
blood pressure and blood volume information and proves to
be very valuable in determining the overall clinical
significance of artery blockages. Most "significant"
blockages that limit blood flow enough to result in
symptoms occur in the larger arteries and these can be
readily visualized with ultrasound. However, the
body is often good at compensating for blockages by
enlarging smaller vessels - in essence creating many
small bypasses. These are usually too small to be seen
directly so the physiologic testing allows us to
determine the "bottom line" - how much blood is getting
to the leg!
For venous
evaluation, it can be used to evaluate how effective the
patient can "eject" blood from the leg through the veins
as well as how fast it refills, a measure of the valvular
function of the vein.
Arterial
Testing
This testing involves using a series of
blood pressure cuffs on the limb to measure blood
pressure and blood volume at various
levels. Most "significant" blockages that limit blood
flow enough to result in symptoms occur in the larger
arteries and these can be readily visualized with the
ultrasound. However, the body is often good at
compensating for blockages by enlarging smaller vessels -
in essence creating many small bypasses. These are
usually too small to be seen directly so the physiologic
testing allows us to determine the "bottom line" - how
much blood is getting to the leg! This testing involves
using a series of blood pressure cuffs on the limb to
measure blood pressure at various levels throughout the
legs. A blood pressure is also obtained at each arm for a
comparison. In a normal artery, the pressure should be
essentially the same throughout. For example, if your arm
pressure is 120 mm/Hg, the pressure at your ankle should
essentially be the same. If the pressure at the ankle is
60 mm/Hg, there is a problem. Additionally, we will
sometimes find individuals with complete blockages that
still have normal blood flow to the foot - the body has
done a good job developing collateral channels to get
blood flow around the blockage. (Click
here to go to section
on
exercise)
Every time your heart contracts and pumps
blood into the leg, the size of the leg actually gets
slightly larger - proportional to the amount of blood
going into the leg with every heart beat. We can
measure this volume change at various levels throughout
the limb and calculate how much blood enters the limb
with every heartbeat. The resultant volume changes
are displayed graphically and are called pulse
volume waveforms. Coupled with the pressure information,
this physiologic testing allows us to determine the
overall significance of the blockages.
Fig
1
Fig
2
Fig 3
Fig 1 - Blood pressure cuffs can be placed at
numerous locations on the limbs and allows measurement of
pressure and volume at specific levels
Fig 2 - Using a Doppler to detect flow and
bllod pressure at the ankle
Fig 3 - We can even measure the blood flow
patterns and pressure at the toe!
This is a chart of the pulse
volume waveforms. One can easily see the right leg (the
graphs on the left side) has dramatically limited flow
compared to the left. The pressure information is also
listed.
This graph shows repetive ankle
pressure monitored over time following exercise. The
right leg pressure represented by the red line shows a
fairly stable pressure following exercise - a normal
finding. In contrast, the blue line depicting pressure at the left
ankle, shows a gradual decrease in pressure which suggest
this person does not have enough blood to adequately
supply the the tissues during
exercise
Venous
Testing
We also use a technique
called photoplethysmography (PPG). This uses an infrared
light that is directed into the skin and this is
reflected back dependent upon the amount of red blood
cells within the skin and subcutaneous tissues.
This reflected light is displayed as a waveform. This can
be used to tell us about artery blood flow and is
also useful in venous testing in several
ways.
Blood leaves the low pressure venous system of the
leg but the contraction of the calf muscle. This is so
important it is called the calf muscle pump. When the
calf muscle contracts and blood is ejected out of the calf,
the numbers of red blood cells decreases and allows one
to roughly estimate the effectiveness of this
pump. Additionally, we can measure some called venous
recovery time. When blood is ejected out of the calf veins
with muscle contraction, it should slowly refill as blood
travels through the capillary beds into the vein. In
patients with leaking vein valves, blood will fall back down
the legs because of these malfunctioning valves and the
refill times will be signficantly
shorter.
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