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Diagram from The National Heart, Lung, and Blood Institute
http://www.nhlbi.nih.gov/health/dci/Diseases/pda/pda_heartworks.html
This is the first article in a series I will be writing for you on
canine cardiac diseases. This article will give you an overview of
cardiac anatomy and physiology, which will help you understand the
diseases as they are presented in forthcoming articles. Keep this
article handy for review when you read the future articles. My hope
is that this information will encourage Bichon breeders to obtain
the OFA cardiac certification prior to breeding, and will help
improve the health of our beloved Bichons.
To start with the basics, the heart is a muscle that functions as a
pump. The pump has 4 chambers that run on electricity, not gasoline.
The electrical power is derived from the pacemaker of the heart,
which sends the electricity to the heart muscle via a wiring system
(nerves, i.e. conduction system). Once stimulated with electricity,
the muscle contracts and pumps out its content of blood. The heart
muscle has a great burden in that it constantly moves, so if it
becomes damaged it can’t be rested like other muscles in your body.
Heart failure means the heart muscle is weakened from some type of
damage, rendering it unable to empty its contents efficiently.
The 4 chambers of the heart are the right and left atria and the
right and left ventricles. These chambers house muscle in their
walls, as shown in pink on the above diagram. The chambers
communicate via valves that function like trap doors to keep blood
flowing in the right direction through the heart. First,
deoxygenated blood (blue) is returned from the body via veins, after
the tissues and organs have taken the oxygen they need, to the right
atrium via the superior vena cava (collects blood from the brain and
upper body) and the inferior vena cava (collects blood from the
lower body and organs). All of this “blue” blood collects in the
right atrium until the atrium is stimulated by the electricity to
contract, which opens the tricuspid valve and dumps its contents
into the right ventricle. Once the right ventricle is filled with
blood it is stimulated by the electrical current to contract,
opening the pulmonary valve and sending the “blue” blood to the
lungs where the blood is oxygenated and becomes “red”. Once the
blood is full of oxygen, it’s returned to the left side of the heart
(the left atrium) via the pulmonary veins. Once the left atrium is
filled with blood and stimulated by the electrical current, it
contracts and empties its contents into the left ventricle through
the mitral valve. The left ventricle then fills and becomes
stimulated to contract by the electrical current, emptying its
contents into the aorta through the aortic valve. From the aorta,
via arteries, the rich, oxygenated blood (“red”) is pumped to the
brain, and to the rest of the body and organs, only to return later
to the right side of the heart as “blue” blood. The miracle of this
process is that it all occurs within less than a second!
A healthy heart is a very efficient pump. The left side of the heart
contains more muscle due to pumping blood to the arteries of the
body, which requires high pressure (your blood pressure) to move
blood through this high resistance systemic circuit. The right side
of the heart does not need to generate high pressure to pump blood
only to the lungs, which is a low resistance circuit called the
pulmonary circuit. This efficient pump can be modified by diseases that
weaken the muscles, damage the valves causing them to leak, alter
the conduction system (wiring) which is called an arrhythmia, i.e.
an abnormality in the normal rhythm of the electrocardiogram (EKG),
or by malformations of the heart and great vessels (aorta and
pulmonary artery) that are present at birth. These diseases will be
discussed in future articles.
Heart failure means that one or both sides of the heart is not able
to pump efficiently due to weakened muscles in the ventricles, which
causes blood to back up in the system. To explain it simply, I use a
toilet as an analogy, yes a toilet! Imagine a toilet is one side of
the heart. If it’s the left side, the tank is the left atrium, the
flushing knob is the mitral valve, the bowl is the left ventricle,
all the plumbing gadgets in the tank reflect the strength of the
muscle, and the sewer pipe below the toilet is the aorta. The goal
is to get a good flush, i.e. a good cardiac output that sends plenty
of fluid (blood) out the sewer pipe (to the body). If the toilet is
the right side of the heart, the tank would be the right atrium, the
knob the tricuspid valve, and the bowl the right ventricle, and the
sewer pipe the pulmonary artery.
Diseases of the heart can be demonstrated by the affect on the
toilet. A good flush could be impaired by:
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The gadgets are not working together properly (weak muscles)
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There is too much water in the tank (left atrium)—fluid overload
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The flushing knob is faulty (mitral valve)
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There is too much water in the bowl (left ventricle)—fluid overload
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The sewer pipe is occluded by buildup of excrements (atherosclerosis—build up of cholesterol and plaque)
If the toilet has trouble flushing all of its water from the tank or
out of the sewer pipe, it backs up out of the toilet bowl onto the
floor. In the left side of the heart blood would back up into the
lungs causing breathing problems. In the right side of the heart
blood would back up into the venous system causing enlarged neck
veins and liver, plus edema (swelling due to fluid retention) in the
extremities. Chronic heart failure will cause a combination of the
above symptoms since both sides of the heart are involved.
There are basically 3 categories of heart diseases in dogs:
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Congenital (present at birth and genetically transmitted) heart diseases, such as patent ductus arteriosis (PDA)
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Hereditary diseases that may appear later than birth, such as valve diseases, dilated cardiomyopathy (disese of the cardiac muscle), and sub aortic stenosis (a problem below the aortic valve)
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Acquired diseases such as coronary artery disease and chronic valve diseases, which also may be hereditary
These diseases will be discussed in future articles, including the
prevalence in Bichons. The Health Committee always encourages you to
submit information on any Bichons that have cardiac diseases for our
data collection, which is of course confidential.
References:
- Diseases of the Heart by Charles K. Friedberg
- “Matters of the Heart” by Mara Bovsun. AKC Gazette, October, 2005
- “Facts on Canine Cardiac Health” by Kevin Schargen. AKC Gazette, March, 2005
- OFA web site:
http://www.offa.org/cardiacinfo.html
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