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Non-invasive
systolic arterial blood pressure measurement in small animal.
The
doppler ultrasound technique offers a simple, reproducible and
inexpensive method of measuring SYSTOLIC ARTERIAL BLOOD PRESSURE in
small animals. It involves using a Parks 811 series Ultrasound
Doppler Flow Detector to detect the blood flow in a peripheral
vessel. This is characterised by an audible swishing sound that is
not present when the blood flow is occluded by inflating a cuff with
a sphygmo-manometer. The systolic arterial pressure is taken as the
pressure at which the first sound returns when slowly deflating the
cuff. Measurements are taken either at the forefoot or base of the
tail and can be made in both concious and anaesthetised animals.
Applications:
Hypotension
- Modern anaesthetic agents can lower blood pressure with a risk of
hypotension and subsequent post operative complications. Systolic
Arterial Pressure Monitoring using the Doppler technique can help to
identify when patients are at risk of hypoension and allow the
clinician to take remedial action. Reported minimum values of
Systolic Arterial Pressure necessary to ensure adequate vital organ
perfusion are 80 to 100mmHg.
Hypertension
- With the advent of drug therapy, indirect monitoring of Systemic
Arterial Pressure using the Doppler Technique is an aid in the
Diagnosis and control of Primary and Secondary Hypertension. Cats
with chronic renal insufficiency should be routinely monitored for
hypertension during consultations, the only practical way to do this
with a conscious patient is by the Doppler technique. Reported values
regarded as Hypertensive are where the Systolic Arterial Pressure is
greater than 170mmHG. |
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How
Doppler Ultrasound Works:
The
principle of operation is the Doppler effect. Ultrasound energy with
a repetition of 8.2 Mhz is focused into a moving blood stream. The
wavelength of the ultrasound energy is so small that a portion of the
transmitted energy is reflected back from the individual red cells.
When there is no blood movement the reflected frequency is the same
as the transmitted frequency and there is no sound. With the blood
movement restored the reflected frequency is slightly different to
that transmitted, so a sound is heard. The pitch of the sound depends
on the frequency difference between the transmitted and received
energy. this frequency difference is proportional to the velocity of
the blood. the sound heard with the Parks 811-B Doppler flow detector
is a swishing noise, the pitch varying with the blood velocity.
What
you need:
The
flow detector of choice is the Parks 811-B Series Doppler, this is
supplied with an 8.2Mhz infant flat probe. You will also need some
ultraphonic gel, a sphygmomanometer and a range of cuffs. Optional
stereo headphones are recommended when taking determinations with
conscious patients. |
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What
size cuff should you use?
To
ensure accuracy of measurement it is necessary to have the correct
width of cuff. A cuff width of approx. 40% of the circumference of
the limb or tail at the point of occlusion is recommended for dogs,
whereas it is reported that a width of 30% of the circumference may
be more applicable in cats. If the cuff width is too wide then the
reading may be on the low side, too narrow, then the reading will be
too high.
Method:
1)
Shave an area over the radial artery between the stopper pad and the
paw of sufficient size to accomodate the probe head. If using the
tail calculate cuff width required as in the next paragraph and shave
an area on the underside distal to where the cuff will be placed. As
an alternative to shaving, try dousing the area for the probe with
plenty alcohol, so that the hair is parted and flattened.
2)
Measure the circumference of the limb above the stopper pad and
select a cuff that is approx. 30% for cats / 40% for dogs of that
measurement. Place the cuff around the limb without inflation so as
not to obstruct the blood flow. If using the tail place the cuff as
close to the base as possible.
3)
Check that the volume of the doppler is turned off.
4)
Load some ultrasound gel on the concave surface of the Doppler probe
head and place the probe head over the shaved area. Note that the
probe crystals should be aligned across the lumen of the blood vessel.
5)
Switch on the instrument and slowly increase the volume to about 20%
of its full range. If the characteristic swishing sound of the blood
flow is not heard it will be necessary to slide the probe head around
a little until a good signal is found.
6)
Once you have a good signal you can tape the probe head in place.
7)
Pump up the pressure in the cuff, using the sphygmomanometer, until
it reaches approx. 20mmHg above the point at which the sound ceases.
8)
Very gradually release the pressure in the cuff. Keep a close watch
on the dial and note the reading when the first blood flow sound is
heard. This is the systolic blood pressure. It is recommended that
you repeat this approx. 5 times and take an average of the readings.
9)
Pressures above 170mmHg are generally considered hypertensive.
10)
Pressures below 80mmHg are generally considered hypotensive.
The
Anxious Cat:
Conscious
animals, particularly cats can be stressed by the procedure of
taking the pressure measurement, leading to falsely high initial
measurements. It may be advisable that measurements be repeated in
such cases after 10 - 20 minutes following the first attempt. The use
of closed ear headphones is recommended so the animal is not
disturbed by the signal from the Doppler and this will also cut out
background noise when trying to find a signal.
A
classic example of this is explained below:
5
continuous serial measurements had been taking in an elderly,
conscious cat, an average measurement of 130mmHg was determined.
Between the 5th and 6th measurements the telephone rang and Systolic
Arterial Blood pressure increased to 175mmHg but then returned the
previous value on subsequent readings.
References:
Doppler
Assessment of Blood Flow and Pressure in Surgical and Critical Care
Patients. Dennis T Crowe, Jr. & David E Spring
Kirks
Current Veterinary Therapy XII Small Animal Practice , P.113-117
Arterial
Blood Pressure Monitoring in Aneasthetised Animals, Ann E Wagner
DVM, MS & David Brodbelt MA, VetMB
JAVMA,
Vol 210, No.9 May 1, 1997
Systemic
Hypertension in Cats, Diagnosis & Clinical Management. Rosemary Henik
European
Society of Feline Medicine News, 1997
Recognition
& Management of Hypertension. Sarah M.A. Caney BVSc CertSAM MRCVS
Feline
Update, November / December 1998 |
