Category: heart-diseases-cats

Ventricular septal defect (VSD)

Ventricular septal defect (VSD) is one of the more common congenital heart defects in cats. It is sometimes referred to as a ‘hole in the heart’. The condition is often discovered in apparently healthy cats by a vet during a routine examination (such as before vaccination).

Ventricular septal defect (VSD) is a congenital heart defect, i.e. it is caused by abnormal development of the kitten before birth. The cat’s heart, like that of humans, is a muscular pump with four separate chambers. The right side of the heart sends blood to the lungs where it picks up oxygen. The left side of the heart pumps the blood around the body. The heart is divided into left and right halves by a muscular wall (the septum). The ventricular septum separates the right and left ventricle.

In a VSD the septum doesn’t develop properly resulting in a small ‘hole’ in the septum allowing some blood to divert from the left side of the heart to the right side. The effects of this on the cat depend on the size and location of the defect. Most cats have small defects that are well tolerated. In some cases, very small VSD holes may close spontaneously. Larger defects can lead to congestive heart failure.

Although the condition is present from birth, signs of a ventricular septal defect (VSD) are usually not noticed until later in life. Many cats with VSD have no outward signs of illness. The murmur caused by a VSD is often detected by a vet (often during a routine health check). When you bring home a new kitten it is always advisable to ask your vet to check for any heart murmurs.

If the defect is large, clinical signs may be seen when the cat is less than two years of age. Severely affected animals may have stunted growth, although this can be difficult to recognise without direct comparison to their littermates. If the condition goes unrecognised and heart failure develops the affected animal may be reluctant to exercise, cough, or have difficulty breathing.

If your vet hears a murmur when listening to your cat’s heart they will want to do some other tests. Heart murmurs are caused by the sound of abnormal and high-speed blood flow and are very common findings in cats with VSD. Very quiet heart murmurs can be present in an otherwise healthy pet so a diagnosis of VSD or other congenital heart disease is not necessarily inevitable.

Ultrasound is the method of choice for finding the cause of a heart murmur. If a heart murmur is heard, an ultrasound examination is recommended. Ultrasound examination of the heart requires considerable knowledge and experience and should be performed by someone with experience in examining young cats.

X-rays are important in the diagnosis and monitoring of heart disease. In cats with severe VSD, evidence of heart enlargement on the left side is often evident. X-rays are also used to see if signs of heart failure are present, if there any signs of further heart failure treatment is usually started immediately.

If the VSD is very small, then your cat may lead a normal life with no treatment being necessary. However, if the defect is large, the outlook is worse and your cat may have a significantly reduced life expectancy. Your vet will discuss the outlook and long term management of your cat with you.

If the ventricular septal defect is small, then no treatment is needed and the hole may spontaneously close. Large VSDs may need medical management to treat heart failure if it develops. Some surgical options are available to help reduce the flow of blood across the hole but definitive repair to actually close the hole is typically not possible

Many animals with ventricular septal defect live a normal life with no signs of heart disease but this depends on the size and location of the defect. Affected cats and their parents (who could be genetic carriers of the condition) should be not be allowed to have kittens.

Cats with more severe defects are likely to develop heart failure at a relatively early age and the long term outlook is poor. Life-expectancy may be reduced and long term medication will be required.

Investigating heart disease

It is important that your vet can recognise the early stages of heart failure (and therefore when to begin therapy, if necessary). Investigations of animals with heart disease are important to identify early signs of failure and to establish the appropriate timing and type of therapy. Heart disease and heart failure are not the same thing. In the early stages of heart disease most animals are able to cope although their heart is not working as well as normal. Animals can live with some forms of heart disease without showing any signs of illness at all. Heart failure occurs when the heart disease is more severe and signs of malfunction (usually coughing or breathlessness) develop. Investigations of animals with heart disease are important to identify early signs of failure and to establish the appropriate timing and type of therapy.

When heart function is compromised there are many mechanisms that come into play to reduce the impact on the animal. Since few pets are athletic, heart disease can be present in many without their owner noticing any ill effects. Many cases of heart disease are detected by the vet at routine examination, e.g. before vaccination. Abnormalities are often detected in the first 2 years of life, if congenital lesions are present; or in middle-aged to older pets when acquired degenerative changes develop. Owners generally notice signs once heart failure has developed or if their pet has problems during periods of stress or excitement.

Clinical signs of heart disease vary according to the area of the heart affected. They range from coughing and weight loss to abdominal distension (ascites) or collapse.

Clinical examination

A full clinical examination is essential in all patients with suspected heart disease. Abnormalities in the heart may cause changes to the pulse rate, rhythm or strength. The pulse should be assessed at the same time as listening to the heart to confirm that every heartbeat generates a strong pulse. Poor heart function may also result in congestion of the veins (fluid retention). By listening to your pet’s heart with a stethoscope your vet can determine the rate and rhythm of heartbeats and hear murmurs or other abnormal heart sounds. Ideally blood pressure should be measured in all patients with heart disease.


X-rays are very important in the diagnosis and monitoring of heart disease. Assessment of the heart size and shape itself is important but the lungs and blood vessels visible on the radiograph are also examined. At least two pictures of the chest (one with the animal lying on its side and one lying on its belly) are needed for complete x-ray assessment of the heart. In almost all cases it is safer to administer a low dose of an appropriate sedative than to risk a patient struggling when x-rays are being taken.


Ultrasound is the method of choice for determining the cause of structural heart disease. It allows the vet to see the heart structures, ie myocardial thickness, the relative sizes of the heart chambers, and the position of the valves and major blood vessels. Echocardiography also allows the vet to watch the heart beating. The motion of the valves, contraction of heart walls and abnormal patterns of blood flow can be seen. Echocardiography requires considerable knowledge and experience and should always be performed by experts.

Laboratory tests

Routine laboratory tests (particularly assessment of liver and kidney function) are important for monitoring of animals with heart disease. The dose of many drugs used in the treatment of heart disease may need to be altered in patients with reduced liver or kidney function. It is important to assess kidney function in all patients before they start on long-term therapy for heart failure.

Electrolyte abnormalities are common in patients with heart failure and some drugs can make these imbalances worse. Low potassium levels can develop as a result of inappetence and the use of some diuretics, e.g. furosemide.

There are now some laboratory tests which allow us to measure substances produced by damaged hearts and these tests are likely to become increasingly important in the diagnosis and management of heart disease in the future.


The electrocardiogram (ECG) measures the electrical activity in the heart. Although measurements made on ECG can provide an approximate guide to the size of specific heart chambers, x-ray or ultrasound are better for assessing heart size. The most important use of ECGs is in the monitoring of abnormal heart rhythms (arrhythmia).

High blood pressure (hypertension)

Hypertension (high blood pressure) has long been known to be a problem in people and is being increasingly recognised in pets. Hypertension is very common in older people and is often associated with smoking, or with stressful living. In animals, hypertension is almost always caused by an underlying disease.

When the heart contracts a pulse of blood is forced through the arteries. This pulse generates the systolic blood pressure. In between the heart contractions the pressure in the arteries falls – this is the diastolic blood pressure. In animals we mostly measure systolic blood pressure.

Systolic pressure does not stay the same at all times. Arteries are constantly being constricted (narrowed) or dilated (widened) so that blood can be diverted to whichever organs are most active at the time. A dilated artery has a larger diameter, making it easier for blood to flow through. Less pressure is needed to pump blood through the dilated artery and so blood pressure is lower if arteries are dilated.

Blood pressure also tends to increase a little with age. The arteries of older pets tend not to be as elastic as in younger animals. These arteries do not dilate easily so the overall resistance to blood flow is increased, resulting in higher blood pressure.

Hypertension in animals is almost always secondary to other problems. In cats the most common link is with kidney failure, but some cats with hyperthyroidism (an overactive thyroid gland) may also develop hypertension. Any cat that has been diagnosed with one or both of these diseases should also be monitored for hypertension every 3-6 months.

Other diseases that may cause hypertension include tumours of the adrenal glands and hyperadrenocorticism (Cushing’s disease), but these diseases are very rare in cats. Diabetes mellitus (sugar diabetes) often causes hypertension in people, but although it is a common disease of cats it rarely seems to cause hypertension.

In hypertension the increased pressure in the blood vessels damages the vessel walls, causing bleeding and blood clot formation. This causes particularly severe problems if blood vessels in the eye, kidney, heart or brain are affected. In addition when blood pressure is high the heart has to pump against a greater resistance and this places increased strain on the heart muscle.

In the early stages of disease there are few, if any, signs of hypertension itself, but because hypertension is commonly associated with an underlying disease you may notice signs of that disease in your pet. Appetite may be decreased in kidney failure, or may be increased in hyperthyroidism, and both conditions can cause weight loss, excessive drinking and vomiting.

Signs related to secondary damage to blood vessels will depend upon the organ affected. Damage to the blood vessels in the eye may cause sudden onset blindness, and this is often the first recognisable indication of hypertension in cats. Damage to blood vessels in the brain can cause “strokes” and other neurological disorders, and increased blood pressure in the blood vessels that supply the kidney can cause further deterioration in kidney function.

The signs and symptoms that your cat develops may be highly suggestive of the presence of hypertension, especially if there is also evidence of kidney failure or hyperthyroidism. If your vet suspects hypertension they will want to examine your cat’s eyes for areas of haemorrhage (bleeding) or detachment of the retina (at the back of the eye). Examination of the eyes can be a very useful way to identify the disease but the best way to confirm the diagnosis, and to monitor the response to any treatment, is by measurement of blood pressure.

Hypertension must always be considered in cats that have been diagnosed with kidney disease or hyperthyroidism and blood pressure should be checked regularly in these cases.

Measurement of blood pressure is becoming more common in veterinary practice although it is not yet part of the routine examination in most cases.

The method used to measure blood pressure is very similar to that used routinely in people, but because of the small size of the arteries more specialised equipment is needed. An inflatable cuff is placed around one of the cat’s legs (or sometimes round its tail) and the vet uses a small receiver held against the arteries in the foot (or tail) to detect the pulse. The cuff is then inflated and deflated a number of times and the vet listens for changes in the sound of the pulse as the pressure in the cuff increases and decreases.

The process only takes a few minutes; does not hurt and most animals do not object at all. Blood pressure needs to be monitored regularly in animals that have been diagnosed with hypertension and most cats soon become used to the procedure.

Most healthy cats have a systolic blood pressure of between 120 and 180 mmHg. A cat with a blood pressure that is consistently over 180-190 mmHg is considered to be hypertensive, although older animals do tend to have slightly higher blood pressure than young cats.

If an underlying cause of hypertension can be identified this disease should be treated, and if the blood pressure is only slightly elevated then this may be sufficient to bring blood pressure down into the normal range. However in most cases it will be necessary to use additional treatments that are specifically aimed at lowering the blood pressure. In cats the calcium channel blocking drug amlodipine is usually most effective and can safely be used in cats with kidney failure. Other groups of drugs which may be effective include ACE inhibitors and beta-blockers.

Animals with hypertension have individual responses to treatment and it is important to monitor the blood pressure closely once treatment has been started, altering the dose of the drugs, or altering the medication as necessary. In patients with kidney failure, it is also important to monitor kidney function when using anti-hypertensive drugs.

Feeding a low salt diet may also be of value although it is unlikely to be sufficient as a sole treatment of hypertension. You should avoid feeding pet treats to cats with high blood pressure since most of these are quite high in salt. Most hypertensive cats can be fed a normal commercial cat food, although your vet may recommend the use of a prescription diet for management of underlying disease, e.g. chronic kidney failure.

If your cat has suffered sudden onset blindness emergency treatment to rapidly lower the blood pressure may be recommended. Blood pressure must be measured regularly whilst this emergency treatment is given to ensure blood pressure does not drop too low – so your cat may need to be admitted to hospital during the first stages of emergency treatment. If treatment can be started at an early stage of the disease then there is a chance that your cat may regain its sight.

If an underlying cause can be identified and treated then blood pressure may return to normal without the need for any specific medication. Unfortunately in most cases this is not possible and additional drugs are needed to reduce blood pressure. Fortunately in most cases treatment is effective, and blood pressure can be brought into the normal range within a few weeks of starting treatment. For the majority of cats treatment will then be required for the rest of the cat’s life and in all cases it remains important to continue to monitor blood pressure as accurately as possible in order to identify any recurrence of the problem.

In cats where blindness has already occurred treatment of hypertension is still beneficial as it will prevent continued damage to the brain, kidneys and heart. Affected cats often live for several years, with a good quality of life, once their blood pressure has been brought under control.

Heart rhythm disturbance (atrial fibrillation)

There are many different heart problems that can affect cats. Some of these affect the rhythm of the heart beat and one such condition is atrial fibrillation. This is most commonly seen in association with severe heart disease. Atrial fibrillation does not cause any specific signs so it is unlikely that you will identify this as a cause of illness in your pet. Signs of heart disease can also be hard to recognise in the cat but may include excessive lethargy, inappetence and rapid or laboured breathing. However, any heart disease should be taken very seriously and an early visit to your vet can help to achieve the best outcome.

Atrial fibrillation is one type of disturbance of the normal heart rhythm (dysrhythmia). To understand this condition we first need to know how the normal heart functions:

In the normal heart electrical activity is initiated from a natural pacemaker in the heart and follows a set path around the heart muscle. As the electrical activity moves through the muscle the muscle begins to contract. The electrical signals move in an ordered way like a wave over the heart surface, from the chambers at the top of the heart (atria) to the lower chambers (ventricles). As the electrical signals pass through the heart muscle contracts in a synchronised fashion, like squeezing a tube of toothpaste.

The heart is divided into a left and right side and there are 2 chambers on each side (one atrium and one ventricle, linked together through a valve). In the normal heart the upper chambers (the atria) contract first squeezing the blood out of the atria and into the main pumping chambers, the ventricles. When the ventricles are full they begin to contract to pump blood around the body (from the left side of the heart) or around the lungs (from the right side).

Atrial fibrillation normally occurs in enlarged hearts where the atrial muscle is already stretched. Damage to the muscle caused by the stretching can make it more likely that a spontaneous electrical pulse will be generated in an area outside the pacemaker.

In atrial fibrillation there is disruption of the normal electrical activity throughout the atria resulting in random and chaotic atrial muscle contractions and preventing normal atrial contraction. The electrical pathways in the ventricles are still intact, allowing the ventricular muscle to contract in an organised manner. But because the electrical signals that the ventricles receive from the atria are random and so much more frequent and chaotic than normal, the ventricles often do not have time to contract and relax before a new signal arrives telling them to contract again.

Hence the contractions are not regular and there is a variable time between each heart beat. When you listen to a heart in atrial fibrillation you hear an erratic jungle drum beat rather than the regular lup-dup sound. Some people say that atrial fibrillation sounds like shoes in a tumble dryer.

Atrial fibrillation is normally a rapid heart rhythm and because the ventricles are contracting so often they do not have time to fully fill with blood between each beat. The amount of blood leaving the heart is therefore reduced and this poor blood supply can result in signs of lethargy and exercise intolerance and even exacerbate heart failure.

Atrial fibrillation usually only occurs in cats with enlargement of the upper chambers of the heart (the atria). This enlargement can be caused by a variety of diseases. Occasionally it is caused by congenital heart disease (where the heart develops abnormally from birth). More commonly it is caused by heart muscle disease (cardiomyopathy) that develops after birth, normally during adult life.T here are several forms of cardiomyopathy, with hypertrophic cardiomyopathy (HCM) being the most common. Dilated cardiomyopathy (DCM) is now rare in cats.

A few drugs (most notably digoxin, which may be used in the treatment of some heart diseases) can also cause atrial fibrillation. If your cat is receiving any medication make sure you mention this to your vet even if you think they already know.

Your vet may suspect that your cat has atrial fibrillation when they listen to your cat’s heart. However, in order to confirm the diagnosis an ECG examination is essential. This is a simple test which records the electrical activity from your cat’s heart. If atrial fibrillation is detected then other tests are indicated to look for underlying heart disease. These tests will almost certainly include X-rays and ultrasound examination of the heart but sometimes blood tests are also required.

Even for a vet, it is very difficult to accurately assess heart rate in atrial fibrillation without an ECG. This is because some of the heart beats are so weak that they are very hard to hear with a stethoscope and do not result in a pulse that is strong enough to feel.

Although in people there are some treatments specifically aimed at converting the heart rhythm back to normal this is rarely undertaken in cats. Cats with atrial fibrillation usually have an underlying heart disease that requires management. Some drugs can be given to try to slow the heart rate and allow more time for the heart to fill properly between contractions. It is very difficult to effectively monitor your cat’s heart rate at home when they have atrial fibrillation, but your vet may well ask you to monitor breathing rate and other symptoms to ensure that the heart condition is adequately controlled.

It is unusual for cats with atrial fibrillation to ever go back to having a normal heart rhythm. Since cats with atrial fibrillation usually have a severe underlying heart disease the long term outcome is unfortunately not good in most cases.

If you have any concerns about your cat contact your own vet for further advice.

Heart disease in your cat

Heart disease is increasingly common in cats, probably because their average life expectancy has increased due to improved veterinary care. Some heart defects may be present from birth (congenital heart defects) but only show symptoms as the cat gets older. Other diseases develop later in life as a result of the effects of ageing or damage to the heart. The most common heart disease which develops later in life is hypertrophic cardiomyopathy.

The cat’s heart, like that of humans, is a muscular pump with four separate chambers. The right side of the heart sends blood to the lungs where it picks up oxygen. The left side receives blood from the lungs and pumps it around the rest of the body. The chambers are separated from one another by a series of valves that ensure that blood can only flow in the right direction around the heart.

Heart disease may affect any area of the heart:

Heart valves

The valves within the heart may fail to develop properly, e.g. mitral dysplasia, or may degenerate as a result of ageing (endocardiosis). Specific infections can affect the heart valves (endocarditis). Abnormal valves allow leakage of blood between heart chambers even when they are closed. When valves leak abnormal blood flow can be detected when listening to the heart (a murmur) and on ultrasound.

Heart muscle

In general terms the heart muscle may be either too thick or too thin. If the muscle is too thin the heart is unable to contract properly and if the muscle is thick the heart cannot relax and therefore does not fill with blood between contractions. In either case the heart is unable to pump sufficient blood out.

Electrical conduction

Abnormal electrical conduction affects the rate and rhythm of the heart. Electrical abnormalities can be caused by disease outside the heart. If the heart beats too quickly there is not enough time for it to fill properly between beats and so it pumps less blood with each beat. If the heart beats too slowly there are not enough pulses to supply enough blood to the body. Chaotic rhythms occur where contractions of different parts of the heart are not synchronised and so pulse volume is reduced.


The pericardium is a strong sac that surrounds and supports the heart. Changes to the pericardium usually result in constriction of the heart, preventing it from filling properly between contractions. The right side of the heart (because it has thinner walls) is usually more easily compressed than the left. Diseases of the pericardium are very rare in cats.

Kittens can be born with heart defects (congenital heart problems) because the heart does not develop normally. The most common problems are leaky valves and holes inside the heart that allow blood to flow in abnormal directions. Heart disease in older cats is usually caused by changes in the heart muscle.

The most common forms of heart disease in adult cats are those affecting the muscle of the heart itself (cardiomyopathy):

Hypertrophic cardiomyopathy

The most common form of cardiomyopathy is hypertrophic cardiomyopathy, in which the heart muscle becomes abnormally thick which prevents the heart from working properly and reduces the amount of blood flowing through it. Hypertrophic cardiomyopathy can be caused by an increased workload for the heart. If the heart has to do more work to pump blood out then the muscles in the heart get bigger (just like a weight-lifter’s muscles get bigger when they work-out a gym). However, hypertrophic cardiomyopathy can also occur in otherwise healthy animals and the exact cause is often unknown. It is more common in certain breeds, e.g. Persians, which suggest that it may be inherited in some cases.

Dilated cardiomyopathy (DCM)

This disease is caused by the stretching of the heart muscle walls so that the heart swells (like a balloon filled with water). The contractions of the heart muscle become very weak so blood is not pumped around the body effectively. This is often seen in cats whose diet contains insufficient amounts of a chemical called taurine. However, DCM is much less common now because pet food manufacturers add extra taurine to their cat foods. In humans, heart disease is usually the result of damage to the heart muscle caused by blood clots (myocardial infarction) – this causes the signs of a heart attack. However, cats do not get this kind of heart disease.

The truth is, you may not know your cat has heart problems until it is too late! Cats are usually good at concealing ill health and there may be no evidence of any problems until the condition is very advanced. Many of the effects of heart disease are similar to those changes that occur naturally as your cat gets older – poor appetite, low energy levels with reduced activity and longer rest periods. Your cat’s tongue or gums may turn bluish red as a result of oxygen starvation. There are lots of signs that can be associated with heart disease. If the heart starts to fail fluid may build up in the lungs or in the chest making it difficult for your cat to breathe.

Sometimes the first sign of heart disease in cats are ‘fainting fits’ or seizures (fits). Panting, weight loss, restlessness, coughing, fainting and swelling of parts of the body because of water retention are signs of very severe heart disease and are not normally seen until the disease is advanced.

Heart disease can be associated with increased blood pressure (hypertension) that may cause blood vessels to burst. If the blood vessels in the eye are affected your cat may go blind.

If the heart is not working properly the blood may start to clot inside the heart. Fragments of clot can break off and escape into the circulation where they may cause a blockage in one of the blood vessels. If the clot blocks the vessel taking blood to the hind legs it may cause sudden paralysis. This condition is very painful and cats may be found lying outside, unable to walk and very distressed. These signs are frequently misinterpreted as being the result of a traffic accident. If your cat is found like this it needs emergency veterinary treatment.

A thorough examination of your cat will often be enough to tell your vet that your cat has heart disease. When listening with a stethoscope your vet might hear changes in the heart sounds (a ‘heart murmur’) or an abnormal heart rhythm. Your vet may be able to see other changes in your cat’s appearance, which suggests that heart disease is present.

X-rays and ultrasound scans may also be used. X-rays will usually be needed to see if the heart is enlarged or abnormally shaped. Ultrasound scans allow measurements to be made of the heart muscle to see if it is too thick or has become stretched. Ultrasound can also let your vet see if the heart valves are working properly and if there are any holes in the heart.

An electrocardiogram (ECG) records the electrical activity when the heart beats, and in cardiomyopathy the heart may have an abnormal or irregular beat which can be seen on the ECG.

If your vet has identified a cause of the heart disease and this can be treated (such as removal of an over-active thyroid gland) the damage to the heart muscle might actually repair with time.

Unfortunately, it is unusual that the root cause can be treated so easily and in most cases long-term medication is needed to control the signs.

Sometimes your vet will recommend a special diet that may have low salt to reduce water retention. However some cats do not like these diets and will refuse to eat them. It is much more important that your cat eats than has any special food.

If the heart disease is diagnosed early enough long-term medication and other measures can slow the disease down but they will probably not stop it completely. It may help to change your cat’s lifestyle to eliminate stress (although most cats lead pretty stress-free lives already).

Your vet will prescribe some drugs to improve the heartbeat and others to help get rid of the excessive fluid that can accumulate in your cat’s chest and interfere with its breathing. Aspirin may also be prescribed (as in human heart disease patients) to stop the formation of blood clots. This drug can be dangerous in cats and the dose has to be carefully controlled. A single dose of aspirin may last as long as three days in your cat (not three hours as in people). Never give medication to your cat unless it has been prescribed by your vet.

It is difficult to predict how long your cat will live if it has heart disease or how good its quality of life will be. A lot depends on how far the disease has progressed. On average it is likely that your cat will survive for about six months after diagnosis but the time may vary between a few weeks and several years.

Feline aortic thromboembolism

Cats may be struck ‘out of the blue’ by a blood clot resulting in dramatic signs (sudden onset of lameness and pain) and potentially devastating damage. The back legs and the right front leg are most often affected and may be paralysed. This is an emergency – if you suspect that your cat has suffered a blood clot, you must seek veterinary help as soon as possible.

The term thromboembolism is made up of two words: ‘thrombo’ and ’embolism’. A thrombus is a blood clot within the heart or in a blood vessel; an embolus is something that travels through the bloodstream, lodges in a blood vessel and blocks it. Therefore thromboembolism is the formation of a blood clot in the circulatory system (thrombus) that breaks loose and is carried by the blood stream until it lodges in a blood vessel and blocks it.

The clot may block a vessel anywhere in the body. The more commonly recognised sites are the legs (most frequently the back legs), lungs (pulmonary embolism), the kidneys, the gastrointestinal tract and the brain (stroke).

When a blood vessel becomes blocked, the tissue it normally supplies is deprived of a blood supply (and therefore oxygen). The signs will depend on the location of the affected blood vessel. In the cat clots most often get stuck in the main blood vessels providing blood supply to the back legs. One or both back legs may be affected.

Your cat will be lame or might not be able to stand or use the affected leg or legs at all. The legs may be colder, the muscles may be firm and the nails and foot pads (if not pigmented) may be pale. In the beginning this condition is very painful and your cat might cry, look at or even chew the affected leg(s). The condition comes on very suddenly so a cat may go out of the house apparently completely healthy and later be found on the doorstep unable to stand on their back legs.

If a blood vessel in the lung is blocked, your cat may breathe more heavily, more rapidly and may start to breathe through its mouth. A clot in the brain can cause seizures or altered behaviour. Your vet will be able to identify this.

Thromboembolism is an emergency and it is important to get your cat to your vet as soon as possible.

Your vet will already have a suspicion of a blood clot if your cat has suffered a sudden onset of these signs. However, cats suffering from back injuries or involved in a road traffic accident can show similar signs. When your vet examines your cat they will have a close look at its legs and will check for pulses in the legs. If these are absent, it is very likely that your cat has a blocked artery to one (or both) of its legs.

A blocked artery is a serious problem. The reduced blood flow results in a build up of toxic substances in the tissue. This can be life-threatening and if the blood vessel to an organ is blocked the function of that organ will be impaired. This can cause changes in the blood which your vet can detect with some simple blood tests.

There are several reasons why blood clots form and although your cat may not have shown any other signs of illness it is likely that a severe, life-limiting disease is present. Your vet will investigate these to be able to provide the best treatment for your cat and to give you an indication of what the outcome is likely to be. Most blood clots in cats are the result of heart disease and your vet may take X-rays of the chest and/or perform an ultrasound investigation of the heart. Some tumours can also cause clots to form and, if your vet suspects this, they might also take X-rays and/or obtain an ultrasound of your cat’s belly.

Once treatment is started long term monitoring will be required to check that treatment is being effective and well tolerated and to follow the progress of the underlying disease.

Treatment is possible in all cases but often it is not possible to resolve the clot. ‘Clot-busting’ drugs can only be used in cats shortly after development of the clot. These drugs can have severe side effects which might even result in the death of your cat so even if your cat is presented immediately for treatment your vet will want to explain all the options available and discuss if the use of these drugs is appropriate for your cat.

However, cats have extra blood vessels, which open up to provide an alternative blood supply to the legs if the main supply is blocked. This doesn’t happen immediately and to buy the time for recovery your vet will provide your cat with pain-killers, drugs which prevent further clot formation, and medication for any underlying disease, which caused the clot in the first place.

As treatment progresses (which may take many weeks) your vet will let you know if your cat is improving as expected. Once cats are discharged from hospital they will stay on long-term treatment for the underlying disease (frequently these diseases cannot be cured). Clot-preventing medications will also need to be continued. These modern drugs are quite good and cause only very rarely cause bleeding.

It depends on the disease which caused the clot in the first instance, and the possible organ damage caused by clots. Sadly, the outlook for cats with thromboembolism is not good. Around one-third of all affected cats do not recover from the initial episode (although in many cases this is because their owner elects for euthanasia). However, depending on the severity (and your vet can advise you on this) it is worth starting treatment to buy some time.

If your cat improves over the first few days, there is a chance that further improvement will occur and your cat may recover and have a good quality of life. In cats that do recover, complete function of the affected leg(s) is often not achieved. As most of the underlying diseases which cause clots cannot be cured, your cat is at risk of developing another clot (even if on long-term treatment with anti-coagulation drugs) and around 50% of cats will develop a further thrombembolism.

Thromboembolism is a serious, life-threatening disease, which can come out of the blue. Affected cats will have a serious underlying disease which can be treated, but almost certainly not cured. If affected your cat will be on treatment for the rest of its life and is at risk of developing another clot. Your vet will be able to give you an idea of how badly affected your cat is and whether treatment is worth trying. Cats that do respond to treatment can have a good quality of life, and may live for several months or sometimes even years.