Understanding the circulatory system in dogs

Without a beating heart, none of us would get very far and the same is true for canines. Here we look at how the system functions to keep our companions alive, and then consider two of the most common heart diseases that affect dogs – mitral valve disease and dilated cardiomyopathy.

THE CIRCULATORY SYSTEM

All mammals have a similar circulatory system. The heart takes blood rich in oxygen from the lungs and pumps it under pressure through the blood vessels to all areas of the body. The first blood vessels that the oxygenated blood enters from the heart are the arteries. These arteries gradually branch into smaller and smaller arteries (called arterioles) until they reach tiny capillaries in the tissues. The capillaries reach almost every cell in the body. At the capillary level, nutrients and oxygen are transferred into the cells of the body, and in return, the capillary takes up carbon dioxide and other waste products from the cells. The flow of blood continues from the capillaries to the small veins (venules) and into the larger veins until it reaches the largest vein, the vena cava, and returns to the heart.

The heart then pumps the blood back into the lungs for reoxygenation, and it returns to the heart, ready to start the whole circulatory cycle again.

In a dog at rest, it takes about a minute for all the blood in the body to make this journey.

THE HEART

The heart is made mainly of a specialised type of muscle called the myocardium. This muscle is not under voluntary control, i.e. you don't have to tell your heart to beat in order for it to do so. The muscle cells in the heart are tiny forked cells, and each is connected to the next one by small gaps in the cell membrane. This allows the cells to communicate very quickly and work together in a coordinated fashion. In terms of anatomy, the heart is divided into four chambers – two atria (upper chambers) and two ventricles (lower chambers) – but the heart is best thought of in two halves. The right half of the heart receives the deoxygenated blood from the body and pumps it to the lungs, and the left side of the heart receives the oxygenated blood from the lungs and pumps it to the body. This process occurs as follows:

• Deoxygenated blood enters the right atrium of the heart from the body.

• Once filled, the atrium contracts at the same time as the right ventricle beneath it relaxes.

• Blood is pushed down into the right ventricle through a one-way valve called the tricuspid valve.

• Once the ventricle is full, it contracts, creating high pressure that pumps the blood to the lungs for oxygenation.

• At the same time as the right atrium fills with blood, the left atrium fills with oxygenated blood from the lungs.

• As the right atrium contracts, the left atrium also contracts, forcing blood into the left ventricle.

• The right and left ventricles then both contract at the same time, and the left ventricle forces blood out into the arteries to be sent to the rest of the body. Blood does not flow back from the left ventricle into the left atrium due to the presence of the mitral valve. The left ventricle is much thicker and larger than the right side because it has to send the blood much further (i.e. to the whole body, not just the lungs).

THE HEART'S ELECTRICAL SYSTEM

Throughout the heart are collections of cells that are responsible for the timing of the heartbeat – they are the heart's inbuilt pacemaker. The most important of these are the sino-atrial node which is found in the atria, and the atrioventricular node, which straddles the atria and ventricles. The sino-atrial node has an inbuilt 'countdown' to depolarisation, the name given to the moment the cell electrically signals the muscle cells to contract. Each muscle cell reacts quickly to the electricity by contracting, and the contraction spreads through the muscle cells. In order to ensure that the blood is pushed in the correct direction, extra-fast connections called Purkinje fibres make sure that the cells at the base of the heart receive the message first and so contract first, causing a squeezing action in an upwards direction, pushing blood to the arteries.

HEART DISEASE IN DOGS

MITRAL VALVE DISEASE (MVD)

The most common heart disease we find in dogs is called Mitral Valve Degeneration, and it makes up about 75% of clinical cases of heart disease. It is most common in small dogs, particularly Miniature Poodles, Miniature Schnauzers, Cocker Spaniels, Dachshunds, small terriers and Cavalier King Charles Spaniels, and the risk of the disease increases with age. The mitral valve is the one- way valve between the left atrium and left ventricle, which allows blood to flow from the left atrium to the left ventricle but prevents it from flowing back in the reverse direction, thereby forcing the blood to leave the heart and enter the arteries. Over time the valve can thicken, stiffen, change shape and become leaky so that instead of preventing the backflow of blood when the ventricles contract, a small amount leaks back into the atria. This backflow causes a noise we call a heart murmur.

The problem with this is twofold:

• Firstly, as some blood is lost backwards, instead of entering the arteries, the body receives less oxygen, and the heart compensates for this by beating faster.

• Secondly, as less blood is leaving the heart, lower blood pressure is sensed by the liver and kidneys. In response, the liver releases a protein which goes through a complicated conversion system called the renin-angiotensin-aldosterone system ('RAAS'), eventually causing the kidneys to retain salt to increase the blood pressure. Unfortunately, this system tends to overcompensate, and high blood pressure results causing damage to the heart. Eventually, the heart struggles to cope and damage to the other valves results. The high pressure then backs up into the lungs, causing fluid to build up, at which point the dog is described as being in congestive heart failure.

Symptoms of MVD

Symptoms include:

• Increase rate of respiration at rest

• Restlessness, particularly at night

• Coughing and gagging

• Weakness

• Exercise intolerance

• Syncope (fainting)

• Decreased appetite and weight loss

Diagnosing MVD

Diagnosis usually occurs because your vet hears a new murmur upon examining your dog. They will then take a blood pressure as a baseline and might advise a chest X-Ray to assess whether or not the heart is enlarged, and an echocardiogram (an ultrasound scan of the heart) to look at the blood flow through the valves. It will also be important to take bloods to assess for damage to other organs.

DILATED CARDIOMYOPATHY (DCM)

DCM is far rarer than MVD but occurs in large breed dogs with a possible genetic component. Commonly affected breeds include Irish Wolfhounds, St Bernards, Afghan Hounds, Great Danes, Dobermans and Boxers. It may also be related to diet, and there has been a recent increase in cases. In DCM, the muscle of the heart is affected directly, and the heart walls become thinner and less able to contract effectively. As the heart doesn't function properly, it doesn't move blood around well, so there is a back-up of blood in the heart and lungs. This causes the heart to attempt to expand to move more blood and compensate, but as the cells aren't functioning correctly, all that happens is the heart dilates – it takes up more space in the chest but has no more pumping power. The thin heart walls also don't conduct electricity normally, so there may be skipped heartbeats, an abnormally fast heart rate, fainting or even sudden death. Ultimately, DCM will cause fluid build-up, and congestive heart failure develops.

Symptoms of DCM

Symptoms of DCM tend to be less evident than those of MVD but include the following:

• Increase rate of respiration at rest

• Lethargy

• Weakness

• Sleeping more than usual

• Loss of weight

• Collapse

• Respiratory distress

Diagnosing DCM

DCM is often diagnosed in dogs that collapse suddenly. Vets will sometimes hear an arrhythmia on auscultation with the stethoscope. They will also take a blood pressure measurement as a baseline. A chest X-Ray in these dogs will show a very dilated heart (the dog appears to have 'swallowed a basketball'), and an echocardiogram may be carried out to assess the level of muscular functionality. As the arrhythmias in these dogs tend to be episodic, vets may equip the dog with a portable monitor to assess the electrical activity in their heart for a few days. They will then analyse this electrocardiogram (ECG) to assess the severity of the condition and the level of electrical disruption.

X-Rays of a healthy dog

Symptoms of congestive heart failure developing from MVD or DCM

• Coughing

• Rapid respiratory rate

• Restlessness which is worse at night

• Open mouth breathing

• Extended neck posture

• Weight loss

• Blueish tint to gums and tongue

• Pitting oedema of the limbs (skin remains dented if you press your fingers into it)

• Pericardial effusion (leakage of fluid into the sac around the heart)

• Abdominal fluid pooling

OTHER CARDIAC DISEASES

Other generally rarer cardiac diseases in dogs include cardiac tumours, pulmonary hypertension and arrhythmias.

COMMON MEDICATIONS

Pimobendan

Pimobendan is one of the most commonly prescribed heart medications for canines with both MVD and DCM. It increases the heart's ability to contract effectively, increasing the amount of blood pumped and reducing the backlog of blood that causes high pressure. It also causes blood vessels to dilate, which reduces blood pressure. This reduces the workload for the heart making congestive heart failure much less likely.

ACE inhibitors

ACE-Inhibitors such as benazepril and enalapril work to decrease blood pressure. They interrupt the RAAS system by preventing it from creating angiotensin II, a chemical which causes the narrowing of blood vessels. The blood vessels remain open, causing reduced blood pressure and making it easier for the heart to pump.

Diuretics

Furosemide is one of the oldest diuretics but is still one of the most useful. It binds to cells in the kidney, causing them to stop retaining salt and potassium and instead to lose them in urine. Where salt goes, water follows, so water is lost from the body in urine. With less water in the body, the blood pressure is lower, and the fluid is less likely to pool in the lungs and cause the symptoms of congestive heart failure. Spironolactone is another old diuretic that is seeing an increase in popularity. This drug also works on

the kidney but interrupts the RAAS by blocking the effect of aldosterone, thereby reducing the body's ability to reabsorb sodium from the urine, reducing water retention.

Take home message

Cardiac disease is reasonably common in dogs, so being on the lookout for changes in your dog's behaviourparticularly in their respiration rate - is key. If you have any concerns about your dog or suspect cardiac disease, chat with your vet as soon as possible.