Diseases Of Different Organs
Heart problems usually relate to either valve disease or deterioration in the heart muscle. Both make the heart less efficient in maintaining circulation, with consequent congestion due to accumulation of abdominal fluid in severe cases. Some dogs develop a characteristic dry cough on exercise. Hot summer days mean added stress on dogs with faulty heart function, and fainting and collapse can easily occur if the dog stays in the sun, especially if the dog is also obese.
Some infectious diseases result in deterioration of the heart muscle, but heart valve deterioration is usually associated with age, although the two diseases may occur simultaneously. Drugs are available for many heart conditions and suspected cases need veterinary attention sooner rather than later. With proper treatment and revised life style they need not be fatal.
Parasitism caused by heart worm (Dirofilaria immitis) can bring serious heart disorders. The parasite is widespread in the United States and in tropical areas of Asia and Africa. A mosquito is needed to complete the life-cycle of heart worms. Treatment and prevention are specialized areas of medicine, sometimes requiring surgical intervention.
When the heart muscle becomes inflamed it is known as endocarditis and is usually the result of bacterial infection. It is accompanied by high temperature, and the dog may be in a state of collapse .
Heart disease in dogs
Approximately one dog in tvvo hundred that survived to maturity had congenital defects in an American survey. These included septal defects (“holes in the heart”), valve deformities, and tetralogy of Fallot (“blue baby”). All cause heart murmurs in young dogs Defects may be innocent, or they can cause weakness, poor growth, or heart failure, according to the severity of the lesion. The heart disease most commonly encountered in older dogs is a degeneration of the valves (endocardiosis). This affects primarily the left atrioventricular (mitral) valve, and causes progressive leakage as this dog ages. Smaller breeds are mainly affected with Cavalier King Charles Spaniels, Chihuahuas. Miniature Poodles and pokes being over-represented among British breeds. Affected dogs typically develop a severe valve leakage by the age of ten or twelve, with an obvious heart murmur audible with a stethoscope, and heart failure can develop within a year or so. Cavalier King Charles Spaniels appear to be affected prematurely by this disease – as young as five or six years of age. Heart valves can also be damaged by infection (endocarditis), though is uncommon in dogs. Signs are those of sepsis – fever, loss of appetite, rapid weight loss and often lameness. Larger breeds of dog are affected more often by serious disease of the heart muscle (myocardiurn). Various forms of this cardiomyopathy are encountered, but the congestive type most commonly seen affects giant breeds (e.g. Great Danes, Irish Wolfhounds, Newfoundlands and St. Bernards), together with some large breeds (e.g. German Shepherds, Dobermanns, Old English Sheepdogs. Irish Sellers), being at risk in middle age. The cause is not known, and the disorder very rapidly causes distressing and severe heart failure. It is arguable that it is finally weakness of the muscle (myocardium) that causes death, whatever the original disease of the heart. The myocardium is affected by disease processes in many other parts of the body. Hormonal disturbances, especially those involving the adrenal, parathyroid and thyroid glands, affect its function. Toxic diseases such as kidney or liver failure, and anaemia can also weaken the myacardium. It can also be damaged by infections (including parvo- virus), cancer or trauma (causing bruising or even rupture of a heart chamber).
Myocardial disease causes loss of strength (contractility) of the heart muscle, which can give rise to weakness or collapse with excitement or exercise, or it may contribute to heart failure. Additionally. it can disturb the regular rhythm of the heart beat (causing “dysrhythmias”). Rhythm is normally initiated by the sinus node (” pacemaker”), whose tale of discharge is controlled by nerves and “adrenaline”. Each impulse from the pacemaker sends a ripple of contraction across the upper chambers of the heart (the atria) to be received by another subsidiary control centre, the atriaventricular node. From this the signal is transmitted to the lower chambers (ventricles) by specialised conduction tissue (a form of internal electrical wiring). This process of conduction of the electrical signal that stimulates the heart beat can be interrupted at a number of sites by myocardial disease, causing “heart blocks”. Alternatively, the myocardiurn may become irritable when diseased, and subsidiary (“ectopic”) Pacemakers in either atria or ventricles can trigger heart beats that are additional to normal beats, or take their place. Alternatively, regular contractions may cease, to be replaced by an incoordinated writhing (“fibrillation”). This is particularly common in large and giant breeds with cardiomyopethy. Heart blocks can also be due to inappropriate regulation of the heart beat – excessive slowing, stimulated by the vagus nerve, This is notably common in brachycephalic breeds (Boston Terriers, Boxers, Bulldogs, Pekingese), Heart blocks can cause weakness or collapse if the heart rate becomes too slow (below 60 per minute). Excessive heart rate (“tachycardia”) also causes loss of output by the heart (usually if over 180 beats per minute). Few myocardial diseases can be cured, but drugs now used either to strengthen the heart beat, or to off load the rnyocardium in an attempt to “rest” it. Additionally a number of rhythm disturbances can be corrected or suppressed. The sac surrounding the heart (“pericardium”) is not commonly affected by serious disease in dogs, but haemorrhage (usually spontaneous – especially in large breads of dog) into this cavity restricts filling of the heart chambers and this causes heart failure.
When these diseases significantly affect the function of the heart as a pump, the heart and blood vessels react (“compensate”) with a number of complicated reflexes: the heart is stimulated to beat faster, the myceardium is urged to beat with greater strength, the kidneys retain water and sodium (main constituent of common salt) and the network of small capillary blood vessels shuts down to maintain pressure in the main central blood vessels. The heart and circulation can cope very well with some defects in pet animals, so that the presence of heart disease may not be very relevant : signs may only be noticed by an owner when heart failure develops, when the reflex responses are no longer adequate. Heart failure takes a number of forms, of which the most common are output failure (“heart attack” or fainting) and congestive heart failure. “Heart attacks” usually occur with excitement or exercise. Common causes include valve leakage (occurring especially when excitable small dogs are affected), myocardial disease and dysrhythmias (especially with pulse rates below 70 or above 180 per minute). Congestive heart failure is a complicated process and it is often gradual and insidious in onset. Compensatory mechanisms that are appropriate to a sudden loss of circulation of blood – e.g. blood or fluid loss (“Shock”) – may be inappropriately regulated in chronic heart failure. The heart rate becomes excessive, the tension in the walls of the heart chambers becomes excessive overstretching restricts the blood and oxygen supply to the heart muscle), excessive sodium and water are retained in the large central blood vessels (” volume overload”) and tissues are poorly supplied by blood because the small vessels (capillaries) are shut down. Congestive heart failure can be mainly due to disease of the left side of the heart (the pressure pump which receives blood from the lungs and drives it round the body) or the right side (volume chambers vvhich receive blood from the main veins of the body and pass it to the lungs for oxygen). Left sided failure is most commonly caused by a leaking mitral valve or myocardial disease. Right-sided failure is usually caused by leaking valves in the right side of the heart, by cardiomyopathy, by congenital malformations or by pericardial disease.
Left-sided failure, causes .”damming back” of blood in the lungs. Which effectively “squeezes” fluid into and around the air passages. This causes coughing and/or breathlessness. Additionally, failure of the heart as a pump restricts the supply of oxygen to muscles, the kidneys, the brain and even the heart itself. This tends to weaken the dog at exercise and to cause lethargy. Right-sided failure causes “damming back” of blood in the main veins of the body, which causes swelling of the liver, spleen and other organs, finally “squeezing” fluid into the abdomen(ascites), sometimes the thorax (hydrothorax) or even occasionally under the skin of the lower parts of the body (oedema). Congestion and excessive accumulation of fluid in tissues is exacerbated by the retention of sodium and water that follows diminished blood flow through the kidneys. Additionally, swelling of the liver, intestines and other vital tissues leads to poor function of these vital organs, and the processing of proteins and hormones is affected sufficiently to cause further illness and weight loss. Congestive heart failure is therefore a complicated process, affecting much of the body.
Diagnosis of Heart Disease and Failure
Since the development of left-sided failure following valve leakage is often a slow, insidious process, and since the heart and blood flow can compensate for a long lime (in a pet dog) for a slightly leaking valve, it is important for a clinician to assess the stage of development of heart failure. Additionally, since many signs of heart failure (exercise intolerance, faintirg, coughing. breathlessness, enlargement of the abdomen) mimic those of other diseases, it is important for a vaterinary surgeon to assess that it is heart failure that is causing one or more of these signs. Some heart disease that he finds in a dog may be quite incidental and insignificant. Evidence of heart disease (e.g. murmurs and rhythm abnormalities) is readily detected with a stethoscope. A stethoscope can also help an experienced clinician to assess the site and type of disease. Whether the heart is enlarged (as in failure) and whether the heart rate is increased by failure. A veterinary surgeon may also make use of the failure of the pulse and the appearance of the blood-flushed areas of the skin (mucous membranes) in the mouth or eyes to assess the efficiency of circulation of the blood. He may feel for enlargement of the liver, spleen or abdomen, and listen to the lungs for further signs of failure. Useful further aids in the diagnosis of heart failure include radiography (“X-rays”), from which the size and shape of the heart can suggest the site of disease, and the side of the heart that is primarily affected. Radiographs can also indicate congestion of blood in main veins of the body and the lungs. congestion in the lungs. liver or spleen, and the presence of fluid in lungs, the chest or the abdomen They are therefore very valuable in the assessment of congestive heart failure. Still more valuable in the assessment of the heart’s function, but too expensive for general use in animals are echocardiography (using an ultrasonic signal, detector and screen, as used in pregnant women for examining unborn babies) and nuclear imaging, using radioactive tracer substances with detectors. In a few cases, catheterisation of the heart (with plastic tubes) is valuable to allow the use of dyes to demonstrate heart defects on radiographs, the measurement of pressure in heart chambers and major blood vessels, and the sampling of blood from within the heart. Electrocardiograpns (ECGS) assess the electrical activity of the heart muscle. However, they neither assess heart failure or the strength of the myocardium nor give any direct indication of congenital malformations or valve leakage. They record the heart rate, rhythm and conduction of the impulse through the heart, and may indicate enlargement of the heart’s chambers.
Few disorders of the heart can be cured, although most congenital malformations in dogs have been corrected surgically, even using cardiac bypass (“heart-lung machine”) in a few cases. However, correction of most malformations is very expensive, and of little value when congestive failure has developed. Certain defects (especially patent ductus arteriosus and pulmonic stenosis) ran be relatively easily corrected with routine chest surgery. If these are diagnosed, surgery should be carried out as early as possible to avoid the development of heart failure. If blood or fluid accumulates in the pericardial sac, drainage can effect a cure in some cases. As congestive heart failure develops, so the dog may require long-term treatment with drugs and careful management over a number of years. This requires understanding and co-operation from the dog’s owner. At all stages, a reduction in the body’s requirements of the heart is essential. Rest is of paramount importance: even kennelling a dog for 24 to 48 hours with no drugs can allow relief of some signs of heart failure, and it cannot be overemphasised that any activity or exercise may overtax the heart, not so much in causing weakness or collapse, but by stimulating further excessive retention of sodium and fluid in the body as blood is diverted away from the kidneys. For similar reasons, obesity should be rigorously reduced, pregnancy prevented and excessive heat and excitement avoided. Other concurrent diseases – for example in kid neys, liver or lungs – may cause sudden deterioration or death Surprisingly, anaesthetics may not be as much of a threat in heart failure as often imagined. Since sodium and water are retained excessively in congestive failure, intake of the former should be reduced to a minimum: most dogs receive far more sodium in the form of salt in their diet than required for normal life. Since most prepared foods (tinned, biscuits meal, even bread) contain added salt for palatability, it may be advisable for the owner of a dog with heart failure to cook food (e.g. meat with pasta, rice or potatoes) without adding salt, or to obtain commercial sodium free food.
Traditional management of heart failure included the use of digitalis (Foxglove extract) one of a group of drugs (glycosides) which have been employed in dogs for nearly 200 years. Glycosides strengthen the heart beat and slow the heart which helps to control it in heart failure. However, they readily cause side effects, and they are difficult to dose accurately and potentially dangerous. Additionally, the emphasis in modern treatment has been towards relieving the heart of overload, rather than to attempt to urge it to beat more strongly when it is already over-stressed. Of prime value are diuretics (fruse-mide, thiazides), drugs which very effectively encourage the kidneys to shed sodium and fluid from the body, directly counteracting some of the effects of heart failure. These drugsare potent and remarkably safe in dogs, even in long-term use. When serious congestion has been reduced, dosage may be reduced to once daily or occasio nal use, to minimise side effects (such as puddles on the floors). Other drugs coming into use are vasodilators, which reduce the shutting down of capillary blood vessels that is a component of heart failure. These drugs can reduce the central pooling of blood and congestion in major blood vessels, improve the blood flow through tissues, and reduce the stretching and overload of a failing heart. However, in a few dogs or at excessive doses, the blood pressure can fall too low, causing the blood pressure can fall too low, causing the dog to faint. In a few cases, drugs that block some of the effects of adrenaline (“beta bleckers”) can control excessive heart rate and muscle tension, Once again, these must be used with care as they may remove too much drive to the heart muscle, and exacerbate heart failure. It is increasingly possible to control rhythm abnormalities that cause weakness or collapse: many drugs are now used, and artificial pace- makers are now being fitted successfully to some dogs with excessively slow hearts. Hovwever, it will be many years before this type of technology can be applied to many cases. It is unfortunately not possible to advise on the prevention of heart disease in dogs, other than to recommend that dogs with most types of heart disease should not be used for breeding. It is known that certain congenital malformations are transmitted hereditarily and there is a suspicion of heredity in some disorders that develop later in life. Additionally, obesity, very common in pet dogs, should not be tolerated. It should be emphasised that early diagnosis, management and treatment of heart failure is most likely to result in successful long-term survival, before the heart becomes irreversibly overstretched, and other vital tissues too diseased. Survival rates are improving greatly with careful modern management, with well-treated dogs often leading happy, rested lives as good companions for their owners.