The canine and feline cardiovascular system

The cardiovascular system comprises the heart (cardio) and veins (vascular) that pump and transport oxygenated and deoxygenated blood throughout the body. While the respiratory system is responsible for bringing oxygen into- and carrying carbon dioxide out of the blood, it’s the heart and veins that circulate the oxygen throughout the body. Just like humans, dogs’ and cats’ cardiovascular system performs the same function, is also controlled by the autonomic nervous system and is also susceptible to disorder and disease.

In this article, we’ll explore the organs of the cardiovascular system, its function and how it works, as well as problems that arise in the cardiovascular system and how they affect our pets. We’ll look at the symptoms of cardiovascular diseases, how the vet diagnoses them and what kinds of treatments are available.

Organs of the cardiovascular system

Central to the cardiovascular system is the heart. Branching off from the heart are arteries (transporting oxygenated blood into the body), veins (carrying deoxygenated blood back towards the heart and lungs), and capillaries (which carry blood between arteries and veins).

A range of arterial and ventricular valves ensure that blood flows in one direction around the body. These valves along with the thick arterial walls and thinner ventricular walls, as well as the positioning of the arteries, veins and capillaries help to maintain blood pressure (and therefore equilibrium) in the body.

How does the cardiovascular system work?

The heart is the body’s muscular ‘pump’ that relaxes and contracts in regular beats, and is controlled by a chemical reaction that generates its own electrical impulses to keep it beating.

When the heart muscle relaxes, deoxygenated blood arrives through veins (venae cavae) and fills the right atrium and ventricle. When the blood enters the right ventricle, the tricuspid valve prevents it from flowing back into the atrium when it is pumped through the pulmonary valve into the pulmonary artery to carry it to the lungs.

In the lungs, gaseous exchange takes place (carbon dioxide is exhaled and oxygen is inhaled) and this newly oxygenated blood is transported back to the heart. Oxygenated blood enters the heart through pulmonary veins and fills the left atrium. The blood flows through the mitral valve down into the left ventricle – the mitral valve ensures the blood doesn’t flow back into the left atrium. The left ventricle then forces the oxygenated blood through the aorta, where it is transported into the body. The aortic valve ensures the blood doesn’t flow back into the heart.

When each part of the heart muscle contracts, the valves open in one direction and blood is pushed into now dilated arteries and carried into the lungs and body. When the blood has been evacuated, the heart muscle relaxes, and the process starts again. All of the arteries, veins, valves, heart chambers and heart muscle work together to ensure enough oxygen is pumped into the body and that blood pressure is maintained to keep the cardiovascular system functioning optimally.

What causes a heartbeat? When the vet listens to your pet’s heartbeat with a stethoscope, they hear the sound of the valves between the heart chambers closing (diastole) and then the sound of the pulmonary and aortic valves closing (systole). Diastole = blood into the heart. Systole = blood out of the heart. The number of beats per minute (bpm) is slower for larger animals like large breed dogs, and faster for smaller animals like toy dogs and cats.

Cardiovascular issues in dogs and cats

When the vet listens to your dog or cat’s heart, they will listen for the type of heartbeat that indicates a healthy heart. This heartbeat is usually quicker on inhalation and slower on exhalation, but there are anatomical, functional and electrical abnormalities that can affect pets’ heartbeats. These abnormalities will be detectable via stethoscope, and may prompt the vet to investigate further with X-rays, ultrasound, electrocardiogram (ECG), and blood tests.

The normal circulation of blood through the heart, lungs and ventricular system is dependent on the valves, chambers and heart muscle all functioning in the correct time and order. If, however, there is a structural change or valves that leak or fail, it interrupts the flow of blood through the heart’s structures and causes turbulent blood flow, and a vibration or murmur that the vet can hear through the stethoscope.

Some examples of cardiovascular issues in dogs and cats include:

Valvular disease in pets

When the valves between the heart’s atria and ventricles aren’t able to close properly, there is some backflow of blood. Since it is not able to exit the heart with the proper force, this excess blood enlarges the heart’s chambers and decreases the heart’s capacity to supply enough oxygenated blood to the body. There are many different valve disorders, but the most common ones include:

  • Degenerative mitral valve disease (DMVD) is a valvular disease that affects nearly half of all cavalier King Charles spaniels, as it was present in the small gene pool from which most modern day cavvies were bred, and is the leading cause of their death. Many other small breed dogs are also susceptible to DMVD, including miniature poodles, Maltese, Dachshunds, mini schnauzers, Chihuahuas and Pomeranians.

In cats, the mitral valve can be affected as a complication of hypertrophic cardiomyopathy (HCM), which we’ll discuss below.

  • Subaortic stenosis refers to the condition in which the aortic valve cannot open all the way because of fibrous bands that reduce its elasticity. This means that less blood is able to flow through the aorta and a greater pressure is required to supply the normal volume of blood to the body. This higher pressure causes the muscle around the left ventricle to thicken, which also reduces its capacity. Subaortic stenosis is rare, but certain large breed dogs are susceptible to it such as Samoyeds, Newfoundlands, bullmastiffs, bouviers, Rottweilers, German shepherds and boxers.

Myocardial disease in pets

The myocardium is the heart muscle, so myocardial diseases refer to problems with the heart muscle’s form and function. It can be weak, flaccid, enlarged or scarred enough to perform poorly and not keep up with the body’s oxygen requirements.

  • Dilated cardiomyopathy

Dilated cardiomyopathy (DCM) is a progressive disease (gets worse over time) that reduces the heart’s ability to contract and feed the body with oxygenated blood. The heart muscle becomes weak and flaccid, and because there is not a strong emptying of the heart’s chambers (particularly the left ventricle), the heart becomes enlarged. When middle- to old-aged large breed dogs are brought into the vet with heart trouble, it’s very often the result of DCM. Giant breed dogs are particularly at risk for DCM.

DCM is present, but quite rare in cats. When it does appear in cats, it is usually the result of a taurine deficiency. 

  • Hypertrophic cardiomyopathy

Hypertrophic cardiomyopathy (HCM) is the type of heart disease most commonly found in cats (compared to dogs). It’s caused by a birth defect, with some cats showing signs of illness early on, while others can go years without showing any symptoms. This disease is characterised by the thickening of the heart muscle, its inability to relax properly and enlargement of the heart. It can include valve defects. Maine coons, Persian and Siamese cats are most at risk for HCM.

HCM can also be a complication of another disease such as hypertension, hypothyroidism or aortic stenosis.

  • Restrictive cardiomyopathy

Restrictive cardiomyopathy (RCM) is characterised by scar tissue in the ventricles affecting the heart’s ability to relax and empty all the way or to fill up to capacity. The ventricular scar tissue is restrictive, causing the atria to enlarge, and affecting the heart’s ability to function properly. RCM mostly affects elderly cats and can also be caused by blood clots or infection that damage the myocardium.

  • Myocarditis

Myocarditis refers to inflammation of the heart muscle, which can cause cells in the myocardium to die off. The heart muscle can be attacked by viruses, bacteria and protozoa, which cause it to become inflamed. Mineral deficiencies, antibiotics and poisons can also cause myocarditis, which typically presents with symptoms similar to congestive heart failure.

Symptoms of heart problems in dogs and cats

Many heart problems in dogs and cats have the same result: the inability of the heart muscle to expand, relax or pump to capacity means less oxygen is circulated in the body. These heart valve and heart muscle failures can be asymptomatic for a long time, but when the symptoms do show, they can include:

  • increased respiratory rate
  • panting/laboured breathing
  • persistent cough
  • distended abdomen (from excess fluid)
  • lethargy/weakness
  • exercise intolerance
  • reduced appetite
  • weight loss
  • vomiting
  • pale or blue lips and gums
  • collapse
  • sudden death

Cats may develop hind-leg paralysis caused by saddle thrombus – a blood clot in the lower regions of the body that blocks blood flow to the back legs. This is a medical emergency and the cat will need immediate medical attention.

When to see the vet about your pet’s cardiovascular issues

If you notice physical changes in your dog or cat’s appearance or behaviour, don’t wait to see if they resolve. Take your pet to the vet immediately as they may already have been suffering with a heart condition for a while, with the symptoms only becoming visible in the later stages of heart disease.

The vet has a range of diagnostic tools available to test your pet for a variety of heart conditions. The stethoscope will reveal any arrhythmias, blood turbulence or murmurs. From there, the vet can perform:

  • Echocardiogram

Echocardiography is akin to an ultrasound for the heart, and gives the vet a real-time view of the structure of the heart as it is functioning. The vet can measure the size of the heart and the thickness of the myocardium and chambers, and also see how the valves are functioning and calculate specific measurements such as pressure and blood flow volume. The echocardiogram can give the vet a lot of very specific information that is crucial to the diagnosis of heart disease.

  • Chest X-rays

X-rays will show the vet if the pet’s heart is enlarged, if there’s fluid in the lungs and show symptoms of congestive heart failure.

  • Electrocardiogram (ECG)

Electrocardiography is the measurement of the heart’s electrical activity, which is imperative for identifying arrhythmias and the heart beating slower (bradycardia) or faster (tachycardia) than usual. The vet can also identify whether the heart’s various chambers are enlarged, just by looking at abnormal patterns on the waveform – these can even indicate which chambers are not functioning properly.

  • Blood tests

Blood tests can be ordered to check the levels of certain hormones in the blood, which indicate how hard the heart is working. Higher amounts of B-natriuretic peptide (BNP) mean the animal may be close to heart failure. In cats, the levels of thyroid hormone in the blood can indicate that their heart disease is caused by hyperthyroidism.

Treatment for pets’ heart disease

Heart disease cannot be completely reversed, but its progression can be slowed with a range of different approaches to treatment. Medication, changes in your pet’s diet and exercise, as well as lifestyle adjustments can be made in conjunction to manage your pet’s heart disease. Medications such as ACE inhibitors are used to open up and relax blood vessels, helping pets to circulate more oxygenated blood in their bodies; diuretics help to eliminate retained fluid; and antiarrhythmic medications help to normalise pets’ heart rates.

If you have any concerns about your pet’s heart health or have noticed troubling symptoms in your pet, make an appointment to see the vet to rule out – or diagnose and treat – heart disease.  

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