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Enalapril
Maleate
(Enacard, Vasotec)
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Brand Name: Enacard or Vasotec
Available in 1 mg, 2.5 mg, 5 mg, 10 mg and 20 mg tablets
Background
Enalapril is an Angiotensin Converting
Enzyme Inhibitor, also called an “ACE inhibitor” or simply an “ACEI.” In
order to understand what this means, it is important to have some understanding
of the body’s “renin-angiotensin system,” an important hormonal mechanism
used in times of low blood pressure.
The kidney is a uniquely well-perfused
organ, receiving directly approximately 25% of the blood pumped by the heart.
Given this fact, it is not surprising that the kidney would possess special
areas for sensing blood pressure changes. In the event of a drop in blood
pressure, as might occur with a significant bleed or in heart failure, the
kidney’s sensors perceive this drop and release a hormone called renin.
The healthy liver normally produces a substance called angiotensinogen,
which innocuously floats around in the blood in case of a blood pressure emergency.
Should angiotensinogen meet up with renin, an activation reaction occurs
that leads to the conversion of angiotensinogen to angiotensin I. Angiotensin
I in the blood circulates, eventually reaching the lungs where an enzyme
called “angiotensin converting enzyme” converts angiotensin I into angiotensin
II.
Angiotensin II acts as the superhero in this time of need. It is probably
the most powerful known constrictor of blood vessels. Angiotensin II constricts
outer blood vessels (those in the periphery - the limbs, skin etc.), closing
them off to circulation and thus centralizing the blood flow. This essentially
creates a smaller, less elaborate course for the circulation of blood so
that normal blood pressure can be maintained with a smaller than normal blood
volume. This re-routing of the circulation functions to preserve blood flow
to the most important organs: the brain, heart and kidney.
As you might guess, an angiotensin converting enzyme inhibitor curtails
the conversion of angiotensin I to angiotensin II. If angiotensin II is such
a good thing, why might we want to stop making it? It turns out, the renin-angiotensin
system evolved to maintain blood pressure in the event of blood loss such
as might occur in an attack from an enemy or predator. The system turns
out not to be such a good thing when blood pressure drops more chronically,
as in heart disease.
When blood pressure drops from heart failure, there is no blood loss; the
amount of blood is the same as always. The problem in heart failure
is that the heart is not pumping enough blood forward. When the renin-angiotensin
system activates, it ends up confining the same amount of blood to a smaller
circulatory route, essentially forcing the already diseased heart to pump
blood faster to keep up. This obviously weakens the heart further and exacerbates
heart failure.
Enalapril effectively acts as a dilator of blood vessels. This effect opens
up circulation peripherally. (If one thinks of the circulation as a roadway
system, this is analogous to achieving less highway congestion by opening
more side streets.) Blood pressure drops to normal and the heart has less
work.
How This Medication Is Used
Enalapril is used in the treatment of high blood pressure, congestive heart
failure, and renal (kidney) protein-loss.
The effect of enalapril in the kidney bears a special discussion. The kidney
consists of millions of tiny filtration units called glomeruli (which are
part of larger excretory units called nephrons).
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This depiction of a nephron shows a
glomerulus and blood vessels.
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A tiny blood vessel enters each glomerulus, carrying blood to be filtered,
and another tiny blood vessel carries blood out after it has been filtered.
ACE inhibitors dilate the exiting vessel without dilating the entering vessel.
(Think of a sink full of water with the faucet on and the drain open. As
long as water coming in matches water draining, the sink stays full. The
ACE inhibitor opens the drain further. The sink doesn't empty all the way
but there is less water maintained in the sink then there was before.) This
effect amounts to less blood pressure inside the glomerulus (the sink), which
means less filtration. When the glomerulus is leaking protein, less filtration
means less urinary protein loss, which is good. If the kidney is failing
or if the patient is very dehydrated, less filtration means less kidney function,
which is bad. This makes ACE inhibitor use a bit tricky in heart failure
patients where we do not want to treat the heart disease at the expense of
the kidneys. For patients without heart failure, where one is treating high
blood pressure or renal protein loss, the drop in kidney function that comes
with ACE inhibitor use is not significant.
In heart failure patients, when enalapril is commonly given in conjunction
with a diuretic (like furosemide), kidney parameters (BUN and creatinine)
should be measured prior to enalapril use, again 3 to 7 days after enalapril
therapy has started, and periodically thereafter.
Kidney function should also be rechecked after any dose change in the heart
failure patient.
Enalapril may be given with or without food.
Despite FDA approval as a once a day medication, usually enalapril must
be given twice a day.
Side Effects
Nausea, appetite loss, and/or diarrhea are sometimes observed with this
medication. In some patients, these effects are severe enough to preclude
the use of enalapril.
In some patients, blood pressure can drop too low as the peripheral blood
vessels are dilated. This manifests as listlessness and lethargy. Often
the dose of enalapril can be modified should this side effect occur.
Enalapril may lead to elevations in potassium blood levels.
Interactions with Other Drugs
Enalapril is commonly used in combination with diuretics, especially
furosemide. In this situation, monitoring kidney parameters is especially
important as both these medications serve to decrease blood supply to the
kidney, as they support the heart. Should a heart failure crisis occur while
a patient is on these two medications, it will become necessary to rely
on the diuretic to resolve the crisis. High doses of diuretic are typically
needed. This can easily lead to kidney failure though there is no alternative
when the heart is failing.
Blood potassium levels can become dangerously high when enalapril is used
with other medications that elevate blood potassium level. Such drugs might
include: potassium supplements (Polycitra, or Urocit-K) or spironolactone
(a potassium sparing diuretic.)
Enalapril is less effective in the presence of aspirin or other NSAIDs.
Concerns and Cautions
Enalapril is inactive when it is consumed and must be activated into
“enalaprilat” (its active form) by the liver. If the patient’s liver is
in failure and cannot reliably perform this conversion, an ACE inhibitor
that does not require conversion can be used.
Enalapril probably should not be used in patients with impaired kidney
function.
This medication should not be used in pregnancy or lactation.
Copyright 2007 - 2008 by the Veterinary Information
Network, Inc. All rights reserved.
This work was originally published by Veterinary
Information Network, Inc. (VIN) and is republished with VIN's permission.
Date Published: 1/1/2001 Date Reviewed/Revised: 06/04/2007
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Enalapril
Cautions and suggested clinical observations
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