Measurements of
pressures in the toes are considered
useful for the evaluation and practical
applications in certain groups of patients,
such as those with diabetes mellitus. In
normal subjects toe pressures are always
found to be lower than ankle pressures and
usually lower than brachial systolic
pressures. The decrease in systolic pressure
between the ankle and the toe is a result of
the smaller diameter of the arteries of the
feet, which result in damping of the
pressure wave and greater resistance.
Measurement of toe pressures allows
assessment of the occlusive process in those
cases in which arterial calcification
interferes with the measurement of the more
proximal pressures. It is important to carry
out the measurements with the patients and
their limbs comfortably warm. Pressures
measured with cuffs will be overestimated if
the cuff is too narrow and may be
underestimated if it is too wide. Since a
certain length of a digit is needed to apply
the sensing device (PPG transducer) distal
to the cuff, the 2.5 cm cuff is usually
adequate. The lower limit of normal for the
toe pressure has been found to be 50 mmHg
and 64% of the brachial systolic pressure.
For the ankle-toe gradient the upper limit
of normal was 70 mmHg.
How to calculate the TBI
Divide the highest toe pressure by the
highest brachial pressure. The result is the
TBI. 0.65 and 0.7 and above is normal for
TBI.
TBI= Toe Systolic Pressure/Brachial Systolic
Pressure.
Interpretation (see reference below)
0.64± 0.20 limbs
normal
0.52± 0.20 claudication in limbs
0.23± 0.19 limbs with ulcers or ischemic
rest pain
A toe systolic pressure greater than 30mm/hg
may be an indicator that there is healing
potential in a foot with ulcers.
How To Perform a Toe Brachial Index (TBI)
Zierler RE, Sumner DS, "Physiologic
Assessment of Peripheral Arterial Occlusive
Disease", Chapter 6 Vascular Surgery 4th
Edition 1:85-117 WB Saunders CO., Orlando FL
1995
The Hokanson photo plethysmographs consists
of dual infrared light-emitting diodes which
transmit light into the subcutaneous tissue,
and dual receiving diodes to pick up the
light reflected from the blood near the
skin. The output of the PPG changes as the
blood volume near the skin surfaces changes
in response to pulse or other causes. The
photo sensor is placed on the toe to record
pulse changes. When the cuff at the base of
the toe is inflated to above systolic level,
blanching of the skin occurs and pulsation
disappears. During slow deflation of the
cuff the first reappearance of the pulse
waveform signifies the systolic pressure
under the digit cuff.
Digit pulses are much affected by
temperature. It is important that the
patient be comfortable and extremities be
warm.
As with limb cuffs, digit cuffs should be
20% wider than the girth of the digit. Cuff
width should equal or exceed the diameter of
the toe being tested. A DC2.5cm (2.5 x 9 cm)
cuff is recommended. Pressures are measured
with the patient in the supine position with
the foot supported so that arterial
circulation is not restricted.
Procedure:
- Attach the photo transducer to the
distal pad of the toe with clear
double-stick tape. The transducer can
also be held in place by a digit cuff
(not inflated) or a Velcro strap.
(Applying a thin layer of ultrasound gel
to the face of the transducer can
enhance coupling and light
transmission.)
- Apply a digit cuff (DC2.5) to the
base of the toe.
- Inflate the digit cuff above
systolic pressure with an Hokanson S300
aneroid sphygmomanometer until the pulse
wave form goes flat line due to the
cessation of blood flow into the toe.
(see example below)
- Slowly deflate the cuff and note the
systolic pressure on the S300 when
pulsation’s appear. The first pulsation
is considered the systolic pressure at
the level of the cuff.
- Compute the toe/brachial index by
dividing the toe pressure by the HIGHEST
brachial systolic pressure.
