When
you or I go to the doctor, one of the first things that the nurse does,
after taking our temperature, is measure our blood pressure.
When you take your cat to the veterinarian, however, blood pressure
is not routinely measured.
It is ironic that historically, the first attempts to numerically
quantify arterial blood pressure were conducted in animals and yet, blood
pressure monitoring has yet to become a routine veterinary procedure.
There
are several reasons why measurement of blood pressure in animals is
uncommonly performed.
Direct measurement of arterial blood pressure is an invasive
procedure, involving placement of a catheter directly into an artery, a
technically difficult procedure that can be painful, and is not without
complications (large bruises, potential for infection).
Indirect (noninvasive) methods are more practical, however, the
indirect methods require a limb containing a large artery that can be
compressed by an occluding device, like a blood-pressure cuff.
This can be difficult in small animals, especially those weighing
less than 10 pounds, as the anatomy of the limb is such that the large
arteries tend to be located higher up on the limb, where securing a cuff
is difficult.
Fortunately, the last decade has seen refinements in blood pressure
measuring devices so that indirect blood pressure measurement is no longer
the cumbersome and frustrating undertaking it once was.
There
are several different techniques for indirect measurement of blood
pressure.
The two most common techniques are the Doppler technique and the
oscillometric technique.
The Doppler technique involves placing a small ultrasound probe
over one of the large arteries in the limbs or the tail.
An amplifier connected to the probe produces audible sounds for
every pulse beat.
A blood pressure cuff is placed proximal to the probe, and is
inflated until the vessel is occluded and the pulse sounds can no longer
be heard.
The cuff is then slowly deflated.
The pressure at which the pulse sounds consistently return is the
systolic arterial blood pressure.
In cats, Doppler readings have been found to underestimate the true
systolic arterial blood pressure by about 17 mm Hg, and this should be
taken into account when measurements are performed.
The
oscillometric technique utilizes a blood pressure cuff to detect pressure
oscillations as the diameter of the artery changes.
Oscillations occur when the artery pulsates.
Systolic arterial blood pressure is determined when the amplitude
of the oscillations suddenly increase.
With the technology now available, precise identification of the
change in amplitude can be measured.
A report in the September 1st, 2002 issue of the Journal
of the AVMA concluded that the oscillometric method of blood pressure
measurement is easily accomplished and is fairly accurate, although it
slightly underestimates blood pressure, especially as blood pressure
increases.
Blood pressure
measurement is important in cats because systemic hypertension (high
blood pressure) has become an increasingly recognized clinical entity in
cats. Chronic renal failure is the most common condition associated
with systemic hypertension in the cat; approximately 20% of cats with
renal failure are hypertensive. Although there are differing opinions
amongst veterinarians as to the numerical definition of hypertension,
most veterinarians would agree that systolic arterial blood pressure
above 170 mm Hg would fit the definition of hypertension. The
Veterinary Blood Pressure Society defines the average systolic blood
pressure for cats as being 124 mm Hg, adding that numbers above 150
should raise some concern.
Left
untreated, systemic hypertension may cause damage to a variety of tissues,
the most common organs being the eyes, the heart, and the kidneys.
Damage to the eyes from hypertension is well documented.
As a veterinarian specializing in cats, I’ve seen my share of
cats presenting with sudden blindness due to partially or completely
detached retinas as a result of hypertension.
The heart can also be damaged if hypertension remains untreated.
The heart has to work extra hard in order to pump blood against a
high pressure gradient.
As the heart pumps harder, the heart muscle becomes thicker and
less compliant.
Eventually, the heart can fail, and congestive heart failure can
develop.
The kidneys are also susceptible to damage from high blood
pressure, and cats with chronic renal failure and uncontrolled
hypertension experience an accelerated progression of their kidney
disease.
It is important to
recognize and treat hypertension early. Cardiac changes secondary to
hypertension are common, however, they are thought to be reversible if
the high blood pressure is brought under control. Vision loss from
hypertension rarely returns, however, so early recognition and treatment
is imperative. Kidney failure is a progressive disease, however, there
are indeed things that clients and veterinarians can do to slow the
progression of kidney failure, and controlling hypertension is an
important part of this treatment.
There have been many proposed methods of treating feline hypertension.
These include administering various classes of drugs, such as diuretics,
beta-blockers, calcium channel blockers, and angiotensin-converting
enzyme (ACE) inhibitors. Diuretics work by increasing the filtering
activity of the kidneys which decreases the blood volume, allowing for a
reduction in blood pressure. Examples of common diuretics would be
furosemide and spironolactone. Beta-blockers decrease blood pressure by
slowing the heart rate, which reduces cardiac output and therefore the
blood pressure. Propranolol was the most commonly prescribed
beta-blocker in cats, however, atenolol is favored because it is more
selective for the beta receptors located in the heart, and as an added
bonus, it is effective at once daily dosing, as opposed to the three
times daily for propranolol. (Cat owners are well aware that it is
nearly impossible to consistently medicate a cat three times a day).
Calcium channel blockers inhibit the movement of calcium ions into
cardiac muscle as well as the smooth muscle of the blood vessels. This
causes the blood vessels to dilate, lowering the blood pressure.
Angiotensin-converting enzyme inhibitors hamper the production of ACE,
an enzyme that leads to the production of angiotensin, a hormone that
causes constriction of the blood vessels. By inhibiting this
constricting hormone, the blood vessels dilate instead, decreasing blood
pressure. Angiotensin also contributes to the production of aldosterone,
a hormone that increases the blood pressure. ACE inhibitors diminish
the production of aldosterone, further alleviating the high blood
pressure.
A few years ago,
studies were published showing ACE inhibitors in combination with
beta-blockers were effective in lowering blood pressure, however, other
researchers have reported poor success with these drugs.
In the late 1990’s, researchers began to investigate the
use of amlodipine (Norvasc), a calcium-channel blocker, for the
treatment of hypertension in cats. This drug worked very well, and
amlodipine is now considered to be the drug of choice for controlling
hypertension in cats. I have had excellent success with this drug. In
one memorable case, I was able to partially restore the vision in a cat
with acute retinal detachments in both eyes due to hypertension after
administering amlodipine.
It is imperative
that cats be closely monitored once therapy has begun. Once therapy has
normalized the blood pressure, it should be rechecked a minimum of every
three months, and sooner if the owner notices any problems that might
indicate a recurrence of the hypertension.
As
you can see, advances in the recognition, diagnosis, and treatment of
hypertension are now affording veterinarians the ability to prevent
blindness, avoid heart failure, and slow the progression of kidney failure
in our feline patients.
As technology continues to improve, it can be expected that blood
pressure measurement will become as routine in animals as it is in humans.
