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Mitral valve disease in practice

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01 September 2010, at 12:00am

The diagnosis and treatment of mitral valve disease was the title of this fascinating evening talk in Worcester, which was given by cardiology specialist Dr Mark Patteson and sponsored by Ceva Animal Health.  

Mitral valve disease is more common in small breed dogs such as Cavalier King Charles Spaniels, although large breed dogs can also be affected. 

The mitral valve connects the left atrium and ventricle and should close and prevent the backflow of blood into the left atrium during systole. With mitral valve disease, however, the valve becomes thickened and unable to form a proper seal. Blood, therefore, shoots back from the high-pressure left ventricle into the low-pressure left atrium, creating a left apical pan-systolic murmur.

Dr Patteson started by emphasising the importance of the clinical examination in assessing these patients. Mitral valve disease is a progressive condition and many dogs will have a heart murmur for 3-4 years before the signs of congestive heart failure (CHF) begin.  

Routinely recording the respiratory rate at rest (ideally when least stressed such as on the floor before putting the dog up onto the table), the heart rate, the strength of the apex beat and the murmur grade/intensity can help to assess whether the disease is worsening and the stage at which therapy is required.  

Although stress in the veterinary clinic can cause an increase in heart rate, if the heart rate in the clinic is monitored over time and increases, for example, from 110bpm to 140bpm, then this may suggest that congestive heart failure is on its way. 

Other signs that may indicate congestive heart failure include exercise intolerance and breathlessness. Dr Patteson likened the lungs to a sponge that, in the normal situation, quickly springs back. However, in a dog with congestive heart failure and pulmonary oedema, the lungs “absorb” the fluid, making them stiff and slower to return to normal. This results in rapid, shallow breathing (tachypnoea).  

Coughing is easily observed by owners and can be caused, for example, by atrial enlargement pushing up on the airways (harsh cough) or severe pulmonary oedema (soft cough). It is important, however, to differentiate whether the cough is caused by heart failure or respiratory disease as, for example, a dog coughing due to respiratory disease may have an incidental heart murmur. 

In addition to clinical examination, Dr Patteson described other diagnostic tests which can also help to determine the presence of congestive heart failure and monitor disease progression, in particular radiography and echocardiography. 

Radiography is a useful way of assessing heart size, the presence/degree of pulmonary oedema and the presence of concurrent respiratory disease. Signs that may be noted include a large left atrium (which can cause tracheal elevation) and pulmonary oedema, which often appears as a “fuzzy” interstitial pattern on the peri-hilar (dorsocaudal) region of the caudal lung fields.  

The Buchanan Vertebral Heart Scale can be a useful way of assessing and monitoring heart size over time. Most dogs with CHF have a score of 12 or more.  

Measure the atrium

Echocardiography was also discussed by Dr Patteson who explained that “if you only measure one parameter, measure the size of the atrium”. This is because, as a rule, the larger the atrium, the worse the prognosis and if, for example, a dog is coughing but has a normal left atrial size, then this rules out congestive heart failure as the cause.  

Left atrial size can be measured using a right parasternal long-axis view by comparing its size to the size of the aorta (left atrial:aortic ratio). The ratio should be <1.6:1 but is often >2:1 in congestive heart failure cases. 

When treating mitral valve disease, Dr Patteson explained that there is currently no evidence that any treatments are beneficial during the asymptomatic period, regardless of whether cardiomegaly is present. 

He therefore reserves treatment for symptomatic patients in heart failure where he has a pragmatic approach, explaining that there are two reasons to use a drug for treating heart failure: improving quality of life and improving survival. 

Firstly, Dr Patteson stated that the diuretic furosemide is mandatory for treating the congestion that accompanies heart failure. It is important, however, to try and get down to the lowest possible dose as side-effects of furosemide may include electrolyte imbalances (for example, hypokalaemia) and activation of the renin-angiotensin-aldosterone system (RAAS).  

In addition to furosemide, Dr Patteson explained that three other complementary drugs (spironolactone, pimobendan and ACE inhibitors) have evidence-based indications for the treatment of symptomatic heart failure caused by mitral valve disease. 

ACE Inhibitors have a variety of beneficial properties and a number of studies (for example, the COVE study and the BENCH study) demonstrate that their addition makes dogs feel better and live longer. Similarly, the QUEST study demonstrated that the addition of pimobendan (Vetmedin, Boehringer) significantly improves survival time thanks to a number of beneficial effects, including its action as a positive inotrope by increasing the sensitivity of contractile proteins to calcium.  

Spironolactone (Prilactone, Ceva) was then discussed in further detail. Dr Patteson explained that, despite historically being classified as a diuretic, spironolactone is actually a very weak diuretic and should be classified as an aldosterone receptor antagonist (ARA).  

During heart failure, aldosterone levels are increased and cause a number of harmful effects, including myocardial and vascular fibrosis. Spironolactone helps to protect against these harmful effects and has been shown to improve outcomes in heart failure patients.  

In human patients with heart failure, for example, a large study demonstrated that the addition of spironolactone to standard therapy (which included an ACE inhibitor and furosemide) resulted in a 30% reduction in the risk of mortality.  

Similarly, a recently published study in dogs with heart failure caused by mitral valve disease demonstrated that the addition of Prilactone as part of standard first-line therapy caused a dramatic 55% reduction in the risk of cardiovascular mortality/morbidity as well as improvements in clinical signs. 

To conclude, Dr Patteson explained that, once dogs develop heart failure caused by mitral valve disease, the condition will inevitably deteriorate and, despite therapy, the survival for dogs is often less than one year. However, advances in diagnosis and treatment now mean that, more than ever, we can extend and give affected dogs an excellent quality of life in their final months.