Introduction
Many cats are living longer lives,
and unfortunately, are acquiring diseases that were not seen as
commonly in the recent past. Hypertension (high blood pressure) is
one of these diseases. The increase in blood pressure affects many
organs, particularly theliver
eyes, kidneys
and heart. Hypertension hastens the progress of these diseases and
substantially predisposes your cat to blindness.
We have only recently been
measuring blood pressures in dogs and cats, so our database is not as
complete as in humanoids. We are at the beginning stages of
understanding if an elevated blood pressure is the result of a
disease, a cause of a disease, or has no bearing on a
disease.
High blood pressure can be
primary, where the cause is unknown. In most animals though, it is
secondary to some other disease.
We have a short Quicktime
video on the use of our
doppler blood pressure monitor. It will take a few minutes to
download-you need Quicktime from www.apple.com to view it.
The are two main factors that determine blood
pressure.
Cardiac output
It is the amount of blood pumped by the heart
in a specific period of time. The determinants of cardiac ouptut
are the heart rate(measure in beats per minute) and
the stroke volume (the amount of blood in ml ejected
with each beat of the heart).
Vascular resistance
This is how constricted or dilated the artery is as the blood
is flowing through it. a dilated artery has a larger diameter, so less blood
pressure needs to be generated by the heart for blood to flow through this
dilated vessel. Arteries constantly constrict and dilate, all depending on
the needs of the body overall and the specific organ they are supplying blood
to. For example, the arteries to your muscles dilate when you exercise. This
allows the muscles to receive extra nutrients and oxygen. When you are done
exercising they start constricting and blood is diverted to other areas of
the body where it might now be needed. Maybe now you are eating a meal and
the digestive system needs the added blood flow.
As it turns out, is is cardiac
output X vascular resistance that determines the blood
pressure. If you get scared, adrenaline secretion will increase the heart
rate and your blood pressure will rise due to the increased cardiac
output. If you become dehydrated, the stroke volume
might decrease due to a lack of fluid, and your blood pressure will decrease
due to a decreased cardiac output. Older pets tend to have arteries
that are not as elastic as when they were younger, the animal world version
of arteriosclerosis. These blood vessels stay constricted more than dilated.
This increases the vascular resistance, resulting in increased
blood pressure.
Normal regulation of the blood pressure
involves a complicated set of metabolic processes. Many body
systems are involved, including the nervous system, the
renal
system, the cardiovascular system, and the endocrine system. It is
a highly refined system that can make minute changes in rapid
response to changing physiologic needs.
In a nut shell, the kidneys secrete a hormone called renin.
This can be in response to a decreased blood flow to the kidneys, stimulation
of the nervous system, secretion of hormones like adrenaline (epinephrine),
or low sodium levels. Renin will activate the conversion of angoitensin I
to angiotensin II in the lungs. Angiotensin II will constrict the blood vessels
(increased vascular resistance) and stimulate the secretion
of aldosterone. Aldosterone will increase water retention by its effects on
sodium. Constriction of blood vessels (increased vascular resistance)
and increased water retention (increased stroke volume) lead
to an increased blood pressure. Pretty easy huh?
Pathophysiology
Abnormally high blood pressure causes blood vessel damage,
particularly in the eye, kidney, heart and brain. These damaged blood vessels
will bleed, cause clots, fluid buildup, and tissue death. The mechanism for
this is complex.
Hypertension also places excessive strain on the
cardiac (heart) muscle. The heart has to pump against more pressure
(vascular resistance), causing further deterioration.
as it progresses the heart enlarges and a murmur can be heard with
the stethoscope.
Symptoms
There are no specific set of
symptoms of high blood pressure. That's why its called the silent
killer in people. What might appear are the symptoms of the disease
that is causing the high blood pressure in the first place.
The primary symptom some owners
notice is a sudden onset of blindness, as evidenced by dilated pupils
and bumping into objects. Cats that were apparently fine just a day
or two earlier are now completely blind. Prior
to the onset of blindness an owner might notice other symptoms. These
might include weight loss, excess drinking and urinating, vomiting,
change in appetite (up or down) and fast heart rate.
Ollie
is exhibiting all the classic signs of blindness. Both
pupils are dilated and stay that way, even when a
light is shined on them. In addition, his left eye
shows signs of potential hemorrhage.
Cause
Feline hypertension is almost always secondary
to other problems, namely hyperthyroidism
and kidney
failure. The majority of cats with these two diseases will eventually develop
hypertension. Any cat that has been diagnosed with one or both of these diseases
should also be monitored for hypertension every 3-6 months.
Kidney Failure
Every beat of the heart sends a significant amount of blood
to the kidneys. As cats age the kidneys do not function properly, and through
complex mechanisms mentioned in the physiology, the blood pressure will elevate.
Hyperthyroidism
The increased level of thyroid hormone
(thyroxine) in the blood stream causes the heart to increase its
output of blood, leading to a racing heart and an increase in
blood pressure.
Diagnosis
As in people, hypertension is a silent disease.
You don't feel ill, and there are no obvious symptoms until it is too late.
Fortunately, we have sophisticated medical equipment that will help us make
this diagnosis.
Signalment
Found in cats and dogs. There
is no breed predilection, but tends to occur more in males and
older animals.
History
Older cats that have racing and
pounding heart rates, along with blindness, give an indication of
hypertension. Hypertension is suspected in cats that have been
diagnosed with kidney or heart disease in addition to
hyperthyroidism.
Physical
Exam
In most cases there are no
obvious physical abnormalities. Blood samples that are easily
obtained, or pulsate in the syringe when obtained, are another
indirect indication of hypertension. The pupils might be dilated,
the thryoid gland might be enlarged, the kidneys might feel
abnormal, and there could be blood in the urine (hematuria) or
nose bleed (epistaxis).
An examination of the retina by an ophthalmologist
will sometimes give an indication of hypertension. There might be areas of
hemorrhage or even detachment of the retina. Every blind cat should be seen
by an ophthalmologist to check for hypertension, along with other causes of
blindness like FeLV,
FIP,
fungal infections, and Toxoplasmosis.
Diagnostic
Tests
An important tool in the diagnosis of hypertension
is a blood pressure monitor. Cat arteries are very small, and the usual method
to detect hypertension in people is not accurate in cats. A special blood
pressure unit called a Doppler is used. Its basis in principle is the Doppler
effect (obviously). The Doppler effect is the change in frequency of a sound
wave as it comes towards you then moves away from you. The best analogy of
the Doppler effect is the sound a speeding race car makes as it comes towards
you, then passes away from you. In the case of a Doppler blood pressure unit,
it is the movement of red blood cells through the artery that is being measured.
During the frequency change that occurs during this red blood cell flow the
reflected sound wave goes from the ultrasonic to the audible range.
Taking a blood pressure reading on a cat
is more difficult than in a person. Their arteries are very small, and of
course are covered with hair. Also, cats are easily stressed, which can lead
to a false reading. We will routinely take 7 readings, discard the lowest
and highest, and average the rest. Sometimes we find a cat with consistent
readings each time, so not as many readings are needed. A cat with a systolic
pressure over 170-180 mm Hg is considered hypertensive. In dogs we consider
over 180 mm Hg to be high. Sight hounds, overweight, and older animals tend
to have higher numbers. These are not hard and fast numbers, just a guideline
for each individual case. In many cases of high readings we repeat the blood
pressure readings a week later to check for consistency in the readings.
We prefer owners stay with their cat in one
of our quiet exams rooms. After clipping the fur on the bottom of the rear
foot (we also use the front foot and tail) we set up our equipment. We take
our first pressure reading when everything is calm and all equipment is set
up. Our new doppler unit has the added advantage of having head phones so
that the sound of the beating artery is not disturbing to your cat.
This
is an overall view of our equipment. The transducer is
under our nurse's hand at the bottom, the blue
pressure-wrap is lightly around the ankle, and the
pressure gauge is on the top.
This
is the hi-tech part of the unit. It is a doppler
transducer that allows us to detect the faint arterial
blood flow to the foot
We
usually inflate the pressure to over 200 or until the
artery sound can no longer be heard. The pressure is
slowly released until the artery sound can be heard
again- this is the systolic pressure. Measurement of
the diastolic pressure is not accurate by this
method.
The
use of the head phones is a big aid in maintaining a
stress free environment. When we have obtained an
adequate number of readings we can take the headphones
off and allow you to listen to the artery sound with
the external speaker.
If
you double click on this video when it is done
downloading you can see the doppler in action. Turn up
the volume on your speakers since the whooshing sound
you are listening for is subtle.
Low blood pressure can be of
significance in animals. Usually this is encountered during
anesthesia.
Our Doppler unit allows us to monitor the blood pressure during
anesthesia and make corrections as needed. We also encounter
low blood pressure during shock, trauma, bleeding, and from
certain medications.
Treatment
Underlying treatment of the
disease that is causing the hypertension is sometimes all that is
needed to prevent hypertension. Use of K/D food, with its decreased
salt (sodium chloride) might be beneficial, but this is unproven. If
hypertension still persists after treating the primary problem then
we sometimes will use specific medication to lower the blood
pressure:
Heart
Drugs
ACE inhibitors like Enalapril or
Lotensin (prevent conversion of angiotensin to angiotensin II, thus decreasing
stroke volume andvascular resistance)
Beta-adrenergic blockers
like Propranalol or atenolol (decrease theheart
rate)
Calcium channel blockers
like Norvasc (decreasevascular
resistance)
Diuretics
Lasix or aldactone
(decrease thestroke volume)
Vasodilators
Hydralazine (decrease
vascular resistance)
Blind cats need to be placed on a
drug called Norvasc to rapidly lower the blood pressure. There is a
chance of regaining sight with the use of this drug, especially if
utilized as soon as blindness is noted. Cats that are on this drug
should have their kidney tests monitored 2 weeks after starting
Norvasc, then every 3 months.
The best treatment for hypertension and its
associated blindness is prevention. Any cat over 8 years of ago, or diagnosed
with kidney disease, diabetes mellitus or hyperthyroidism, should be monitored
periodically for hypertension. This will allow diagnosis of the problem before
it causes blindness.
Monitoring
All pets with hypertension should have their
blood pressure checked every 3 months. In addition, blood panels, thyroid tests,
urinalysis, and eye exams should be performed every 3-6 months.
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