DIAGNOSING HEART FAILURE IN DOGS

Transcription

1 Vet Times The website for the veterinary profession DIAGNOSING HEART FAILURE IN DOGS Author : Mike Martin Categories : Vets Date : November 7, 2011 Mike Martin offers advice on distinguishing respiratory disease from heart failure in dogs, and looks in more detail at the commonest conditions RECOGNITION of the clinical signs of heart failure is important, not only for diagnosis, but also to differentiate from dogs with respiratory disease. It is also needed to assess the severity of heart failure (which is a progressive disease) and for the purposes of monitoring response in order to optimise and balance therapy. This is particularly important when it comes to diuresis, where overdosing can be as detrimental as underdosing. Heart disease in adult dogs is dominated by mitral valve disease (MVD) which accounts for more than 80 per cent of cases seen in general practice followed by dilated cardiomyopathy (DCM). However, there are a few other less common causes of heart disease, such as pericardial effusion which leads to cardiac tamponade and right-sided heart failure. Other causes are cor pulmonale associated with pulmonary hypertension secondary to pulmonary pathology, which also leads to right-sided heart failure, and primary arrhythmia cases, which are fortunately rare, but can be difficult to diagnose or manage, and often necessitate referral to a cardiologist. Pathophysiology of left-sided heart failure The normal heart In the normal heart, oxygenated blood returns from the pulmonary vascular bed through the pulmonary veins and drains into the left atrium. As the left ventricle relaxes (diastole), it sucks 1 / 7

2 blood from the left atrium through the mitral valve (a non-return valve). Just prior to ventricular contraction, the left atrium contracts to fill the ventricle a little more and prime it for contraction. During ventricular contraction (systole) the mitral valve is pushed closed, the aortic valve pushed open and blood is ejected into the aorta ( Figures 1 and 2 ). Mitral valve disease Mitral valve disease (MVD) results in valvular incompetence, with regurgitation of blood back into the left atrium during systole. This squirt of blood creates the sound of a murmur, occurring with each systolic contraction. MVD is a myxomatous degeneration of the valve cusps and degeneration of the chordae tendineae, which also have the potential to rupture ( Figure 3 ). MVD tends to begin in middle or older age in dogs, initially detected as a quiet systolic murmur. However this insidiously and progressively becomes louder over time (typically several months to years). The volume of regurgitation back through the mitral valve into the left atrium progressively increases. This results in dilation of the left atrium (and auricle) and impedes return of blood from the pulmonary veins, resulting in pulmonary venous engorgement. If the progression is slow, the left atrium can become remarkably dilated, with further impediment to pulmonary venous flow and, ultimately, impediment of the return of interstitial fluid from the lung tissue into the capillary bed. Interstitial fluid thus gradually accumulates (interstitial oedema). This progresses to fluid being squeezed out into the alveoli, resulting in alveolar oedema (evident on radiography; Figures 4 and 5 ). At the same time as there is dilation of the left atrium, the left ventricle also dilates as it tries to cope with the increased volume of blood circulating between itself and the atrium. Thus the diastolic volume of the left ventricle increases. Initially, the ventricle maintains its systolic strength and contracts to near-normal, assisted by blood exiting not just through the aorta, but backwards through the mitral valve, thus reducing its resistance to contraction. All this results in the impression of the ventricle having hyperdynamic contractility on echocardiography. However, the ventricle eventually cannot cope and systolic dysfunction insidiously progresses too. Dilated cardiomyopathy Dilated cardiomyopathy (DCM) is a failure of left ventricular systolic function. The ventricular muscle becomes weak and unable to contract down fully to normal in systole. Therefore, the chamber becomes dilated and globoid, as well as being poorly contractile. Ventricular arrhythmias are not uncommon in DCM ( Figures 6 and 7 ). As a consequence of the ventricle not emptying fully, the left atrium dilates on a par with the ventricle, exacerbated by stretching of the mitral valve, so that they also fail to coapt properly and 2 / 7

3 regurgitation develops. However, the volume and velocity of regurgitation is small compared to MVD thus the murmur often remains quiet or even inaudible. As the left atrium dilates, then it impedes return of blood from the pulmonary veins, which, like MVD, gradually become engorged. This results in the build-up of interstitial fluid in the lung tissue, leading ultimately to alveolar oedema ( Figure 8 ). Radiographic diagnosis of left-sided congestive heart failure Both MVD and DCM lead to left-sided congestive heart failure (CHF). In both cases, if pulmonary oedema is present, then there should also be left atrial dilation and pulmonary venous engorgement evident on the radiographs. It can be difficult to decide if the pathology seen in the lungs on chest radiographs is oedema due to heart failure maybe because of less than ideal positioning or inspiration or x-ray tube power. However, in most cases it should still be possible to see evidence of left atrial dilation and pulmonary venous congestion to support the possibility that the lung pathology could be oedema and thus heart failure. Pathophysiology of right-sided heart failure The most common visible clinical sign of right-sided heart failure in dogs is ascites. The fluid of ascites due to heart failure is a modified transudate (clear to straw-coloured, with a specific gravity (SG) between and 1.036). Thus, a very useful and simple diagnostic test is to perform abdominocentesis to check the fluid is not blood or a pure transudate, for example. Heart failure is not the only cause of a modified transudate, but it should place heart disease high on the differential list, before an exploratory laparotomy is considered. Right-sided heart failure alone can be seen with pericardial effusion and cor pulmonale. However, more often it is seen in combination with left-sided heart failure in dogs with DCM, which often have bilateral failure, or dogs with both MVD and tricuspid valve disease ( Figure 9 ). How to recognise heart failure clinically The three common presenting histories by an owner are: exercise intolerance; breathlessness; and coughing. 3 / 7

4 However, these signs are also common in dogs with respiratory disease, which is why it can be difficult to decide which body system is the culprit. Breed Undoubtedly, breed is the biggest clue. Certain breeds are predisposed to heart disease and others to respiratory disease. Of course, a few get both. In general, dogs with MVD are small breeds, such as cavalier King Charles spaniels and terriers, and, often, mixed smallbreed dogs. Importantly, dogs with MVD have a loud murmur. If they do not have a murmur, they cannot have MVD. Because the murmur is typically loud, it would be difficult to miss. In general, dogs with DCM are medium to large breeds (more than 15kg). Cocker spaniels are the smallest breed predisposed to DCM. It is rare in breeds smaller than this. Common breeds predisposed to DCM are Dobermanns, boxers, great Danes, German shepherd dogs, Labradors and Irish wolfhounds ( Figure 10 ). In contrast to MVD, it is very rare to see DCM in a mixed breed, as it is primarily a disease of pure-bred dogs. However, some large breeds are also predisposed to MVD, particularly in older age. When might symptoms be due to respiratory disease? Symptoms might be due to respiratory disease when there is a strong predisposition in that breed. In particular, brachycephalic breeds, such as the bulldog, pug and bichon frise, are more likely to present with respiratory problems. Breeds predisposed to tracheal collapse, such as Yorkshire terriers, or those predisposed to pulmonary fibrosis, such as westies and cairns, are often less likely to necessarily have heart failure, even when there is a murmur. But, importantly, if there is no murmur in any of these breeds then they cannot have heart failure due to MVD (and small-breed dogs don t get DCM). But even if an older dog (which is predisposed to respiratory conditions) does have a MVD murmur, this might be an incidental finding and not necessarily the cause of the symptoms. What clinical findings might be indicative of heart failure? Pallor of the gums or conjunctiva can indicate anaemia or shock, but is also seen in dogs with heart failure due to poor peripheral perfusion. In dogs with a history of collapse, it can be useful to teach owners to examine the mucosal colour during a collapse. A rapid heart rate (tachycardia) is common in dogs with heart failure, as they try to maintain cardiac output by increasing the heart rate. In dogs with respiratory disease (and after they have relaxed in the consulting room), the heart rate often slows or there is a sinus arrhythmia, which suggests 4 / 7

5 vagal dominance (and the absence of a sympathetic drive). As cardiomegaly increases with progression of the heart disease, the apex beat becomes easier to palpate (increased apex beat). This will be less obvious in dogs with respiratory disease. Arrhythmias are common in dogs with DCM, with ventricular ectopics and atrial fibrillation being the most common. These will usually result in auscultation of an irregular rhythm, with a pulse deficit, which would indicate the need for ECG recording. A gallop sound is a third heart sound (S3 or S4) in addition to the normal lubb-dubb (S1 and S2). All four heart sounds can be heard on auscultation of the normal horse, but, normally, only the two main heart sounds are heard in dogs. Thus if S3 or S4 becomes loud enough to hear in a dog, it usually indicates cardiac strain. A gallop sound can be difficult to learn to recognise on auscultation. In essence, it creates a triple sound on each cardiac beat. It is usually only audible in dogs with DCM, whereas in MVD the superimposed sound of the murmur makes it very difficult to recognise a gallop sound. A gallop sound should not be confused with occasional premature beats, which creates a tripping in the rhythm rather than a triple-sounding heart beat. Ascites and jugular distension are features of right-sided heart failure. While respiratory disease can lead to cor pulmonale (right-sided heart failure), this is uncommon. Thus the presence of ascites and jugular distension would place heart failure high on the differential list. What clinical findings might indicate respiratory disease? A dog with dyspnoea during inspiration is indicative of an upper airway obstructive problem which then places heart failure low on the differential list. Inspiratory dyspnoea is recognised as a prolongation in the duration of inspiration, often associated with a noise suggesting a narrowed airway passage. A narrowed nasopharynx due to a fat/long soft palate results in stertor (snoring-like sound) during inspiration. A laryngeal obstruction (from paralysis, for example) can produce stridor (a roaring or whistling-like sound during inspiration). A respiratory sinus arrhythmia would tend to place heart failure low on the differential list and many dogs with airway disease often seem to have an exaggerated sinus arrhythmia. The heart rate seems to speed up on inspiration and slow down on expiration. Cyanosis indicates a very low level of oxygenation in the blood. Respiratory diseases often result in poor oxygenation and thus cyanosis, whereas mild to moderate heart failure does not. It is only in severe heart failure with fulminant alveolar oedema that dogs will go blue. In these cases they often cough up a pink-tinged frothy oedema fluid ( Figure 11 ). So in the absence of that, a respiratory cause is more commonly associated with cyanosis. 5 / 7

6 A wheeze (whistling-like sound) during expiration (when auscultating over the lungs and/ or trachea) implies the bronchi are narrowing during expiration. This is typically associated with lower airway diseases, such as bronchitis, and would not be heard in dogs with heart failure. Pulmonary crackles can be caused by both moisture in the airways and air bubbling through that, or the snapping open of very small airways that have closed at end-expiration. It is near impossible to distinguish wet from dry crackles on auscultation. However, as for cyanosis, for heart failure to cause crackles it has to be very severe, with oedema fluid within the airways and thus usually coughed up and seen. More often, pulmonary crackles are associated with the snapping open of airways associated with lower airway disease, most commonly pulmonary fibrosis such as seen in westies and cairn terriers. So, does your dog have heart failure or a respiratory condition? By the end of a history and clinical examination, you should have a reasonable idea whether a dog has cardiac or respiratory disease, and, therefore, whether you need to pursue a cardiac or respiratory work-up. If you are still undecided, a few options are available at this point, which also depend on the owner s finances. A serum pro-bnp test is reasonably accurate to help rule in or out cardiomegaly, provided the blood is taken in the correct manner and using the correct tubes. Chest radiography is common to both work-ups and is potentially a useful first diagnostic test if there is still some uncertainty (provided the chest radiographs are of good quality). Echocardiography is an important test to confirm the presence and the aetiology of heart disease, but is an expensive method to rule out heart disease if it turns out to be a respiratory condition. An ecg is generally of very limited value in deciding whether a case is respiratory or cardiac. although often seen as a fairly simple and less expensive test to perform, its value for money is actually low. Finally, there is also the option of referral to a specialist, knowledgeable and experienced in cardiorespiratory medicine, who is likely to choose the most useful diagnostic pathways. while this may be the most expensive option, it can often be the best value because of the high likelihood of an accurate diagnosis and thus treatment plan. 6 / 7

7 Powered by TCPDF ( A FEW GOLDEN RULES For a small-breed dog believed to have symptoms of heart failure due to mitral valve disease (MVD), there must be a loud murmur of mitral regurgitation. If lung pathology on chest radiography is believed to be pulmonary oedema, there must be cardiomegaly with left atrial dilation and there should also be pulmonary venous enlargement. The presence of sinus arrhythmia would tend to imply that heart failure is unlikely. If ascites is due to right-sided heart failure, the jugular should also be distended, the hepatic veins engorged on ultrasound scan, and the ascitic fluid should be a modified transudate (SG=1.018 to 1.036). If a dog is in heart failure due to either MVD or dilated cardiomyopathy, then the left atrium should be dilated on ultrasound examination. 7 / 7