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The Circulatory System

The heart

The heart is a four chambered muscular organ that pumps the blood

around the body

The heart lies in the mediastinum within the thoracic cavity

It is conical in shape and lies at a slight angle in the thorax

The heart lies to the left on the midline with its apex near its sternum

The heart is enclosed by a double layered membrane called the

Pericardium

The inner layer of this is called the Epicardium, which is a serous

membrane and directly covers the heart wall

Within the pericardial cavity is a little serous fluid which

lubricates the heart as it beats

The wall of the heart consists of a layer of cardiac muscle –

Myocardium and a thin inner epithelial layer that lines the heart

called the Endocardium

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The heart is divided up into right and left chambers by a partition

called the Septum

Each side is then divided up into two chambers

They are:

The thin walled collecting chambers = Atrium

The thicker walled pumping chambers = Ventricles

The right side of the heart pumps blood from the heart to the lungs

and back

The left side of the heart pumps blood all around the body

Heart valves

Within the heart there are two sets of valves, the function of these

valves is to prevent backflow of blood

They are:

1. Atrioventricular valves

2. Semi lunar valves

Atrioventricular valves

These lie between the atria and ventricles

They prevent the flow of blood back into the atria when the

ventricles contract

They are attached to the papillary muscles in the heart wall by

fibres called Chordea tendinae these prevent the valves being

turned inside out by the pressure of the blood

The right atrioventricular valve lies between the right atrium and

right ventricle

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It comprises of three fibrous cusps and is also known as tricuspid

valve

The left atrioventricular valve lies between the left atrium and

left ventricle

It comprises of two cusps and is also known as the bicuspid valve

Semilunar valve

These are situated at the base of the two major vessels leaving the heart

The Pulmonary Valve lies at the base of the pulmonary artery and

prevents backflow of blood from the pulmonary artery to the right

ventricle

The Aortic Valve lies at the base of the aorta and prevents

backflow of blood from the aorta to the left ventricle

It is the semi lunar valve closing that can be heard in the second heart

sound (dubb) if the valve is faulty it doesn’t close properly causing

turbulence. This is now heard as a whoosh instead of a lubb or dub this is

known as a Heart murmur

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Circulation of blood through the heart

Deoxygenated blood is carried back to the heart in Veins

Two major veins collect all the blood and enter the right side of the

heart these are called:

Cranial vena cava

Caudal vena cava

The two atria (left and right) and the two ventricles (left and right)

contract in unison

Both vena cava empty into the Right Atrium which contracts when

it is full

This sends deoxygenated blood into the right ventricle via the

right atrioventricular valve

The right ventricle contracts, pumping blood into the pulmonary

artery via the pulmonary valve

The deoxygenated blood is carried in the pulmonary circulation to

the Lungs

The blood is oxygenated in the lungs and is then carried back to

the left side of the heart in the pulmonary veins which enter the

Left Atrium

When the left atrium is full of blood it contracts forcing

oxygenated blood into the left ventricle via the left

atrioventricular valve

The left ventricle contracts and pumps blood into the Aorta (by

the aortic valve) which then carries the oxygenated blood around

the body.

Oxygenated blood is delivered to the tissues; the deoxygenated blood is

collected up by the veins and transported back to the heart

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Control of the heart beat

The heart is made of cardiac muscle which is a specialised type of muscle

that has the ability to initiate contraction from within the muscle itself

(without nervous control

The mechanism that is responsible for controlling the rate of contraction

of heart muscle is called the conduction mechanism

Within the wall of the right atrium is an area of modified cardiac muscle

called the Sino atrial node

This nodes job is to determine the rate of the heart beat and is referred

to as the Pacemaker

If the muscles have been working hard and require more oxygen the

sinoatrial node will increase the heart rate so that the oxygen can get to

those muscles

During sleep the sinoatrial node slows the heart rate down as the muscles

are relaxed

The sinoatrial node starts a wave of contraction, which passes

over the walls of the atria

The impulse passes to another specialised area called the Bundle of

his which is situated within the septum

The impulse is then conducted to the apex of the heart where it

spreads out into the ventricles in specialised nerve cells called

Purkinje Fibres

The wave of contraction in the heart muscle (myocardium) of the

ventricles starts at the apex and spreads upwards forcing blood

into the arteries that are situated at the top of the ventricles.

This is called the cardiac cycle

The period of contraction within the heart is called Systole and is

when the blood is being pumped into the ventricles or circulation

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The period of relaxation of the heart is called Diastole and is when

the atria are filling with blood

The circulatory system is made up of a network of channels that

transport the blood from the heart to the tissues, where oxygen and

nutrients are delivered, and then transport it back again to the heart.

This network consists of arteries, capillaries and veins

Arteries

A large vessel which carries blood under pressure away from the

heart

Most arteries carry oxygenated blood except for the pulmonary

artery which carries deoxygenated blood

Arteries have thick muscular walls which enables the vessel to

dilate or constrict

Blood travels along the arteries reflecting the heartbeat which can

be felt as the pulse

As the arteries enter the tissues they branch getting smaller and

smaller these are then called Arterioles which then flow into

capillary networks

Capillaries

The capillaries form a branching network in all the tissues and link

the arteries and the veins

They are narrow with thin walled consisting of a single layer of

endothelial cells with no muscle or elastic tissue

They are permeable to gases, nutrients and waste products which

diffuse between the blood and tissues and from the tissues back

into the blood

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Veins

A vein is a large vessel which carries blood towards the heart

The walls are thinner than arteries and they contain less muscle

and elastic tissue

Blood flows slowly under low pressure and valves may be present to

prevent pooling of the blood

Most veins carry deoxygenated blood from the tissues except for

the pulmonary veins which carry oxygenated blood from the lungs

back to the heart

Systemic and pulmonary circulation

The circulation in the mammal is referred to as a Double circulation as

blood passes through the heart twice during a complete cycle

There are two parts to the circulatory system:

1. Systemic circulation – carries oxygenated blood around the body

and returns deoxygenated blood to the heart

2. Pulmonary circulation - Carries deoxygenated blood from the

heart to the lungs where it is oxygenated and returned to the

heart

Systemic circulation

Arterial supply

Oxygenated blood leaves the left ventricle of the heart in the

major artery known as the aorta

The aorta gives off a number of arteries that then supply various

parts of the body

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Venous return

Deoxygenated blood returns to the heart from the tissues in veins

which follow the pattern of the arteries and often have the same

name (renal artery and renal vein)

The caudal vena cave empties into the right atrium of the heart, venous

blood returns:

From the head in the Jugular vein

From the neck and forelimbs in the cephalic veins, brachial veins

and subclavian veins which drains into the cranial vena cava which

empties into the right atrium of the heart

Hepatic portal system

The liver has its own modified circulatory system within the

systemic circulation

Its function is to carry blood straight from the digestive system

to the liver so that the products of digestion can be used

immediately rather than transporting them around the body

Pulmonary system

Deoxygenated blood is pumped from the right ventricle of the

heart and is carried to the lungs in the pulmonary artery

Within the lung tissue the artery then divides into lots of

capillaries that wrap around the thin walled alveoli of the lungs

Oxygen in the inspired air diffuses into the blood and carbon

dioxide in the blood diffuses into the air in the alveoli

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Exotic animals

Small mammals

The cardiovascular system of small mammals is similar to cats and

dogs except for the ferret which only has one vessel arising from

the aorta

Birds

The avian heart is four chambered as with mammals but is

proportionally larger with respect to its body size

The major artery (aorta) curves to the right side as it leaves the

heart rather than to the left in mammals

There are several other differences in the blood vessels in birds

Reptiles

The heart is generally considered to be three chambers in lizards,

snakes and chelonians as there is no physical division between the

right and left ventricle

They function as a four chambered heart with deoxygenated blood

being directed towards the lungs and oxygenated blood to the rest

of the body

Lizards, snakes and chelonians have paired aorta leaving the heart

one going to the right one to the left. It then fuses dorsally to

become one aorta

Fish

The heart has only one atrium and one ventricle.

Blood returns to the heart via two blood vessels the common

cardinal vein which receives blood from the head and body and the

hepatic veins

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These enter into the a small chamber which then empties into the

ventricle

The ventricles then enter into a vessel like the aorta which divides

into brachial arteries and supplies the gills with blood

These then move on to the dorsal aorta which moves caudally to

supply the rest of the body with oxygenated blood

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Circulatory Disorders

The primary function of the cardiovascular system is to supply

adequate blood flow to meet the body’s metabolic demands.

Cardiac output is tightly controlled which maintains blood pressure

in normal circumstances.

Cardiac output depends on the heart rate and the amount of blood

ejected from the heart with each contraction

Common Circulatory disorders

Congestive heart failure

Acute heart failure

Congenital heart diseases to include, Patent ductus arteriosis,

pulmonic stenosis, Aortic stenosis, ventricular septal defects

Acquired heart disease to include, Dilated cardiomyopathy,

hypertrophic cardiomyopathy, endocardiosis, pericardial effusion

Vascular disease – hypertension

Dysrhythmias to include; atrial fibrillation, ventricular premature

contractions, heart block

Questions to ask an a owner

Has the animal previously had signs of heart disease?

Heart disease often goes undetected for many years before

clinical signs develop

Is the animal able to exercise normally?

Heart disease will often cause exercise intolerance or collapse at

exercise

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Is the animal eating and drinking normally?

Patients in heart failure often go off their food and are cachexic

(loss of weight, fatigue, general poor condition)

In early stages they may become polydipsic

Have there been episodes of weakness or collapse?

This could be a sign of poor circulation (reduced cardiac output)

Does the animal seem unsettled at night?

Nocturnal restlessness is common in congestive heart failure, they

find it difficult to breathe when lying down

Are there any breathing issues or is the animal coughing?

Respiratory issues may be associated with pulmonary oedema

Coughing is more commonly a sign of respiratory disease but could

be caused due to increased pressure of a large heart on the

bronchus

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The most common type of heart failure is congestive heart failure. This

can affect the left or right sides of the heart but usually progresses to

involve both sides

Right sided heart failure results in congestion of the venous

circulation in the liver and spleen which causes ascites

Left sided CHF causes congestion of vessels in the lungs and fluid

leaking from here causes pulmonary oedema. Oedema and

congestion reduce oxygen transfer in the lungs

Causes of CHF include:

Pooling in the venous system due to:

Systolic failure (reduced efficiency if the myocardium to contract

Diastolic failure (reduced ability of the myocardium to relax and

fill with blood)

Volume overload, excess volumes of blood entering the chamber

causing myocardial stretching

Pressure overload where resistance to the outflow of blood which

increases the force of contraction needed and muscle

hyperatrophy increases pressure in the ventricle

Dysrhythmias which reduce efficiency of the pumping action

Signs of CHF include:

Exercise intolerance

Cough

Dyspnoea

Syncope/collapse

Weight loss

Pale mucous membrane (due to poor supply to the peripheral

circulation)

Ascites

Restlessness

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Diagnostic testing:

Auscultation of the heart for the presence of murmurs

Increased heart rate and loss of sinus dysrhythmia

Compare heart and pulse rate

Pulse quality and strength

MM colour – Cyanotic with pulmonary oedema or pale if output is

reduced

Thoracic radiographs to assess size and shape of the heart and

presence of pulmonary oedema or pleural fluid

Ultrasonography for internal cardiac anatomy – valves, septa,

vessels and myocardium

ECG provides information about cardiac chamber size but also for

detection, recognition, classification and monitoring dysrhythmias

Treatment options:

Early heart disease (no signs but a murmur is detected) require the

underlying cause to be diagnosed and corrective treatment started

If corrective treatment is not possible it is likely that CHF will

progress so the patient should be checked every 3-6 months

A reduced calorie diet should be started to control weight

Exercise restrictions should be imposed, if pulmonary oedema is

present then ACE inhibitors and diuretics should be started

Supportive care such as draining pleural effusions and oxygen

therapy should be started

Tania Yelland VN

Congenital abnormalities

Patent ductus arteriosus (PDA)

The ductus arteriosus is a blood vessel that connects the two main

arteries of the body - the aorta and the pulmonary artery.

This blood vessel is normal in the foetus, but shortly after birth, it

should close.

When the ductus arteriosus remains open or patent after birth,

this abnormal communication between the aorta and pulmonary

artery passes extra volumes of blood into the lungs.

Patent ductus arteriosis (PDA) is a birth defect representing the

second most common congenital heart defect of dogs.

Approximately seven out of 1000 live birth puppies are affected.

Generally, there are no serious symptoms of PDA unless congestive

heart failure has caused fluid build-up in the lungs.

The condition is typically identified in puppies during a routine veterinary

visit for vaccinations. Continual blood flow through the PDA into the lungs

produces a continuous heart murmur.

Even when a PDA has been identified, most people believe their dog is

normal. In some cases, the dog can be smaller than littermates or play

less vigorously. However, the situation can be very misleading as

symptoms usually occur within a year of diagnosis. If untreated, about 60

percent of affected dogs die within a year of diagnosis.

When caught early, and following treatment with successful closure

of the PDA, most dogs live a normal life.

Unless there are complications from other heart defects or heart

failure has already developed, there is rarely any future need for

medication.

While special circumstances can influence the prognosis, most

cases are straightforward.

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What to look for:

Breathing difficulties

Coughing

Exercise intolerance

Lethargy

Diagnosis

Various diagnostic tests are needed to recognize PDA, and exclude other

diseases.

Some of the necessary tests may include:

Complete medical history and physical examination including

auscultation (stethoscope examination) of the heart and lungs. The

heart murmur of PDA is characteristic and most experienced

veterinarians learn to make the diagnosis simply by listening. Since

other birth defects can also cause heart murmurs, a veterinary

cardiologist may be consulted if the diagnosis is in doubt.

A chest X-ray (radiograph) can help determine the severity of the

problem

.

An electrocardiogram (ECG) can assist with the diagnosis.

An echocardiogram with Doppler (cardiac ultrasound) is the

definitive diagnostic test. This may require referral.

Routine blood tests may be performed prior to any anaesthesia

Treatment

The conventional treatment is an operation done shortly after

diagnosis. The PDA is closed with surgical suture.

Surgery should not be delayed by waiting for symptoms to develop.

Medical treatment may be necessary before surgery if symptoms

(coughing, difficult breathing) are present.

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In some referral centres, the PDA may be closed using special

catheterization techniques.

PDA is common in:

Miniature poodle,

collie,

Maltese

Shetland sheepdog,

German shepherd dog,

cocker spaniel,

Pomeranian, and

Labrador retriever.

Female dogs are predisposed.

Pulmonic stenosis

Cause:

A congenital narrowing of pulmonary valve or the artery leaving the

heart

Signs:

A murmur is often detected at routine examination, this can then

progress to right sided heart failure

Diagnosis

Thoracic radiographs show enlargement of the right side of the

heart

Ultrasound examination shows thickening of the right heart wall

Doppler ultrasound shows narrowing of the vessel which can

identify the severity of stenosis

Treatment

Severe cases require dilation of the artery. Symptomatic medical

treatment in non surgical cases

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Aortic Stenosis

Cause:

A narrowing of the outflow of the left side of the heart (or

abnormal development of the aortic valve). This causes resistance

to outflow resulting in hypertrophy of the left side of the heart

Congenital aortic stenosis is probably the most common heart

defect seen in large breed dogs. Newfoundland dogs have the

highest risk for this disorder. It is also seen in the golden

retriever, Rottweiler, and boxer.

There is a mildly increased risk in:

the German shepherd,

German short-haired pointer,

Great Dane,

Samoyed

bulldog.

Signs

Depending on the severity signs may be present at a young age or

develop into adulthood

Fainting

Collapse

Sudden death

Diagnosis

Thoracic radiographs may show dilation of aorta after the point of

stenosis

Ultrasound can be used to visualise the stenotic area

Doppler to measure blood flow through the area

ECG examination shows cardiac arrythmias

Treatment

Symptomatic treatment with anti-dysrhythmic drugs and beta

blockers

Mild cases don’t require treatment

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Venticular septal defect (hole in the heart)

A failure of the heart to develop properly which results in an

opening in the division between the left and right sides of the

heart.

Blood flows from left to right meaning that too much blood is

returned and the left side is overloaded.

VSD is a congenital disease in dogs and affected dogs should not be

used for breeding. When a dog is diagnosed with ventricular septal

defect, its parents should also not be used for breeding any longer.

As a dog embryo develops, the heart begins as a single tube. This tube

will then gradually separate into four different chambers. Abnormalities

can appear at several different steps in this intricate process and this

can lead to ventricular septal defect in a dog

Symptoms

The nature and extent of the symptoms will depend on both the

size of the abnormality and exactly where it is located. In mild

cases of ventricular septal defect, most dogs will display no

symptoms except heart murmur.

It is even possible for a small defect to close on its own as the dog

matures.

In a dog with a larger defect, the pressure in the left side of the

heart will be higher than the pressure in the right side, and there

will be a blood flow from left to right via the defect.

The left side of the dog’s heart will be forced to work harder than

normally and more blood will circulate to the lungs. This will in turn

cause a higher than normal work load for the lungs.

The symptoms can develop over the course of several months or

even years.

Common Symptoms

exercise intolerance

Increased respiratory effort

In severe cases, abnormal heart rhythm can lead to premature

death.

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Dilated cardiomyopathy

Dilated cardiomyopathy (DCM) is a disease affecting the heart

muscle. It is the second most common heart disease in dogs (after

mitral valve disease).

In DCM the myocardium is thinned resulting in loss of

contractibility

It affects:

Mainly middle-aged large and giant breed dogs

some spaniels

Dobermans are the main breed affected.

Cats and small breeds

Male dogs are more likely to be affected than females.

What causes DCM?

Cardiomyopathy literally means disease of the heart muscle (cardio

= heart and myopathy = muscle disease).

In DCM the heart muscle becomes thin and weakened. The heart muscle

can be damaged in a number of ways including:

viral infections

Dietary deficiencies of taurine (an essential amino acid only found

in meat protein)

Dietary deficiencies in carnitine have been reported as causes of

DCM in some groups of dogs.

In most cases of DCM there is no apparent cause of the damage to the

heart and this is termed idiopathic cardiomyopathy.

The heart muscle is damaged it becomes weak and so does not

contract well. Because heart contractions are weak the heart does

not empty with each contraction and the blood supply to the body

is reduced.

Pulses are weak and the paws may feel cold. With time the heart

muscle stretches and heart becomes a flabby sac.

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DCM eventually results in heart failure with fluid build-up in the

lungs (pulmonary oedema), the chest (pleural effusion) and abdomen

(ascites).

What are the signs of DCM:

an irregular heart beat on a routine examination

exercise intolerance

Increased respiratory effort

Anorexia

Depression

Polydipsia

Coughing

Collapse.

Cardiac cachexia.

ascites

Sudden death

How do you diagnose DCM?

Thoracic radiographs show increased cardiac size

Pulmonary oedema

Ultrasound shows thin myocardium and poor contractibility

ECG can show dysrhythmias particularly atrial fibrillation

Treatment

In almost all cases there is no treatment for the underlying muscle

disease.

Signs of heart failure can be managed according to its severity.

Taurine supplementation in some cases

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Endocardiosis

Endocardiosis is the most common acquired disease in dogs

particularly in Cavalier King Charles Spaniels

Causes:

Degenerative condition of the atrioventricular valves (usually the

mitral valve) which causes faulty valve function and the leakage of

blood through valves when they are closed

Signs

Cardiac murmur progressing over years to CHF (usually left sided)

Diagnosis

Thoracic radiographs shows left sided heart enlargement (atrium)

Ultrasound shows distortion of valves and turbulent blood flow with

leakage

Treatment

Symptomatic management of CHF

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Pericardial effusion

Often seen in middle sized and large breed dogs.

Fluid accumulates inside the pericardial sac which prevents the

heart from filling with blood.

Signs

Lethargy

Dyspnoea

Muffled heart sounds

Weak pulses

Pale mucous membranes

Muffled heart sounds

Ascites

Treatment

Drainage of the fluid (pericardiocentesis)

Pericardectomy

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Drugs commonly used in the treatment of cardiac conditions

Positive inotropes e.g. digoxin

– Increase the force of contraction of the heart muscle

– Slow the heart rate

– Can cause nausea at high doses

Local anaesthetics

Given by I/V injection to treat some dysrhythmias

They reduce the sensitivity of the heart muscle so that abnormal

contractions are reduced

Only used for short term control of severe dysrhythmias

Diuretics

Promote renal excretion of fluids and reduce oedema

Inhibits the resorption of salt and water filtered by the kidneys so

that more fluid is lost in urine

Vasodilators

Reduce the workload of the heart by:

Dilating systemic veins which reduces pressure making oedema less

likely to form

Dilating arterioles making it easier for the heart to pump blood

Betablockers

Slow heart rate

Reduce the force of contraction so not used in DCM

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Nursing the patient with congestive heart failure:

Choose a suitable kennel which can be lined with absorbent

material. This should be in a calm quiet area

Monitor and record TPR, urinary and faecal output, abnormalities

etc

A low salt/fat diet, which has a protein of a high biological value

and is highly digestible. This will reduce oedema and ascites

Water should be available at all times

Medication should be correctly administered

Exercise should be kept to a minimum