Myxomatous mitral valve disease (MMVD) in dogs is a slowly progressive disease. Until recently, focus was aimed at the symptomatic phase of the disease, when congestive heart failure (CHF) develops. However, since the publication of the EPIC trial, which showed that treatment with pimobendan delays the onset of CHF, the focus has now shifted onto the early diagnosis of MMVD. This article considers the tests necessary to diagnose MMVD and aims to provide vets with the knowledge and tools to undertake these successfully.
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Nuala Summerfield, Virtual Veterinary Specialists, www.vvs.vet e-mail:
Nuala Summerfield graduated from the Royal (Dick) School of Veterinary Studies and completed a residency in veterinary cardiology at the University of Pennsylvania, USA. She has spent the past 15 years working as a veterinary cardiologist in academia and private practice. Nuala recently created Virtual Veterinary Specialists (VVS). VVS is a live, interactive and educational telemedicine service, designed to support vets in general practice.
MYXOMATOUS mitral valve disease (MMVD) develops gradually, often over a period of years, before clinical signs develop. During this pre-clinical phase the patient with MMVD will seem outwardly normal and asymptomatic. It is only in the later stages of the disease, when the body’s cardiovascular compensatory mechanisms can no longer maintain normal cardiac output and blood pressure, that clinical signs of congestive heart failure (CHF) develop. These include dyspnoea, tachypnea and exercise intolerance.
Following the publication of the EPIC study in 2016, the focus has shifted to diagnosing MMVD earlier in the course of the disease, before the patient shows signs of CHF (Boswood and others 2016). The EPIC study provided strong evidence that administration of pimobendan to dogs with MMVD that have echocardiographic and radiographic evidence of heart enlargement, significantly prolongs the preclinical (asymptomatic) period. We now know that in order to achieve optimal longevity and quality of life for canine patients with MMVD, they must be correctly diagnosed and the disease accurately staged at the first detection of a heart murmur, even though the patient is asymptomatic.
However, early diagnosis of preclinical MMVD often requires a conscious effort by vets to look for the disease, because the patients will have no clinical signs. This emphasises the importance of cardiac screening. Screening for MMVD should ideally be performed annually in dogs identified as having certain risk factors that might predispose them to MMVD.
MMVD risk factors are based on breed, bodyweight and age, which is why patient signalment is a key starting point in MMVD diagnosis. MMVD is the most commonly acquired heart disease in dogs. It tends to be a disease of middle-aged to older dogs, but can develop at any point during a dogs’ lifetime. MMVD typically affects dogs weighing less than 15 to 20 kg, so can be considered a disease of smaller dogs. It can affect both pure breeds and mixed breeds, but certain pure breeds are predisposed, such as cavalier King Charles spaniels (CKCS), dachshunds, miniature schnauzers, poodles and chihuahuas.
Physical examination findings will depend on whether the dog has preclinical, asymptomatic MMVD or symptomatic MMVD with CHF (Table 1).
Cardiac auscultation is the most important part of the physical examination in a patient suspected of having cardiac disease. The degeneration and thickening of the mitral valve leaflets that occurs in dogs with MMVD results in leakage of blood across the mitral valve, which is called mitral regurgitation (MR). MMVD is characterised by the presence of a typical left apical systolic heart murmur of mitral regurgitation on cardiac auscultation. Cardiac auscultation is an effective screening tool for MMVD in at-risk dogs (middle- to old-aged dogs of small breeds) (Clinical tip 1).
Cardiac auscultation technique
Place the palm of the hand over the left apex to identify the precordial impulse (left side of the chest between fourth and fifth intercostal spaces at the level of the costochondral junction). Place the diaphragm of the stethoscope over this region, which is the location of the mitral valve. This is where a murmur of MR is heard best. The first heart sound (S1) is also loudest here. Then, slowly move the stethoscope from the left apex to the left base. The left base is approximately two rib spaces cranial from the left apex and slightly dorsal. The left heart base is where the second heart sound (S2) is heard louder than the first heart sound. Next, place the palm of the hand over the right apex beat and then place the diaphragm of the stethoscope over this region. This is the tricuspid valve region, where a murmur of tricuspid regurgitation is most easily detected. Then slowly move the stethoscope cranially to listen over the right heart base (Clinical tip 2).
Murmurs are characterised by their intensity (loudness), location and timing.
In dogs with MMVD, an approximate correlation exists between the loudness of the murmur and the volume of blood leaking backwards across the mitral valve (the louder the murmur, the larger the leak across the mitral valve). The intensity of the murmur is typically graded on a scale of one to six, with a grade 1 murmur being the softest and a grade 6 murmur being the loudest (Table 2).
In a dog with MMVD and a loud heart murmur, the detection of a normal resting heart rate and a sinus arrhythmia is a reliable indicator that the dog is not in congestive heart failure (CHF). A normal resting heart rate and a sinus arrhythmia indicate that vagal tone is higher than sympathetic tone at rest. When CHF develops, sympathetic tone increases and the sinus arrhythmia is replaced by a resting sinus tachycardia.
The position where the murmur is heard the loudest is called the point of maximal intensity (PMI). The PMI of a MR murmur is the left apex.
The murmur is described relative to its timing within the cardiac cycle. MR is a systolic murmur caused by blood leaking from the left ventricle to the left atrium when the left ventricle contracts in systole (Clinical tips 3 and 4) (Audios 1 and 2).
Audios 1 and 2
Two audio files of a grade 2 systolic murmur (Audio 1) and a grade 4 pansystolic murmur (Audio 2) are provided with the online version of this article at inpractice.bmj.com. The author would like to thank Boehringer Ingelheim for use of its audio files.
Always perform cardiac auscultation with the patient in a standing position if possible. Recumbency changes the heart’s position within the chest, resulting in incorrect localisation of heart sounds.
Confirming MMVD and staging disease
Following detection of a heart murmur, further diagnostic tests are required to confirm the diagnosis of MMVD and stage the disease.
Thoracic radiography and echocardiography (cardiac ultrasound), both play an important role in the diagnostic evaluation of a dog with MMVD to establish whether there is evidence of heart enlargement. Dogs with MMVD and heart enlargement are more likely to develop CHF within one to two years than those that have a normal heart size. Therefore, diagnosing heart enlargement not only allows you to provide the owner with a prognosis for their dog but also enables you to design an appropriate treatment plan if the dog meets the EPIC study criteria.
As the severity of MR progresses, left atrial size dependably increases. This parameter can be used to track progression of disease in an individual dog over time. Left ventricular enlargement is also a typical finding in dogs with moderate to severe MR.
It is good practice to palpate the femoral pulses simultaneously while performing cardiac auscultation to ensure there is a palpable pulse for each heartbeat. The timing of the murmur can also be confirmed, as systolic murmurs such as the murmur of mitral regurgitation, will be heard synchronously with the palpation of the peripheral pulse.
Although both radiography and ultrasound are used to identify heart enlargement, they provide different information about the cardiac status of the patient and so are considered complimentary diagnostic tests.
Thoracic radiographs are a useful tool to check for heart enlargement in a dog with a heart murmur suspected of having MMVD. Typically, asymptomatic dogs with a large enough volume of MR to cause enlargement of the left atrium and ventricle (MMVD stage B2 dogs) will have a murmur of grade 3/6 or louder. The cardiac silhouette might be normal in dogs with a small volume of MR (MMVD stage B1 dogs), as there might be no left atrial enlargement yet in these dogs.
Radiography allows evaluation of the pulmonary parenchyma, airways and pulmonary vessels, which is important in a dog with MMVD and clinical signs of tachypnoea, dyspnoea or cough. Radiography is the gold-standard diagnostic test for confirming pulmonary oedema associated with left-sided CHF in dogs with advanced MMVD (stage C or D).
It is important to remember that radiography provides no information about the heart muscle function or the heart valves. This can only be assessed with ultrasound. Additionally, sedation or general anaesthesia might be required to obtain good quality thoracic radiographs.
Vertebral heart score
Measuring the vertebral heart score (VHS) is a useful tool to establish whether a dog with MMVD has an enlarged heart. Obtaining a VHS from a lateral thoracic radiograph is a straightforward procedure. Good radiographic technique and positioning are important to prevent rotation from affecting VHS measurements and so that serial imaging comparisons of an individual patient are more reliable. Good visibility of the thoracic vertebrae is vital. If you have a digital x-ray, it is recommended to use the software to assist with taking measurements. If you have a film x-ray, you can use either calipers or a ruler to do this.
When performing cardiac auscultation, always count the heart rate and record it in the patient’s record. Listen for long enough to be able to assess the heart rhythm for any irregularities; listening for a minimum of 30 to 60 seconds is ideal as the heart rhythm may be intermittently irregular.
VHS can be measured from either a right or left lateral thoracic radiograph. However, when taking serial images for VHS comparison in an individual animal over time, VHS should be always be measured using the same lateral recumbency. This is because slightly larger VHS measurements will be obtained from right lateral positioning.
The VHS uses two measurements of the heart, a long-axis measurement and a short-axis measurement (Fig 1). To obtain the long-axis measurement, measure the distance between the carina, or the tracheal bifurcation, and the cardiac apex. The short-axis measurement should be made at the widest part of the heart and should extend from the cranial to the caudal border of the cardiac silhouette. The short-axis measurement should be made perpendicular to the long-axis measurement.
These short- and long-axis measurements then need to be compared to the dog’s thoracic vertebrae. Start by identifying the fourth thoracic vertebra (T4) on your radiograph. The first thoracic vertebra, or T1, is the first vertebra with a long spinous process. T4 can be identified by counting caudally from T1. Using the software, calipers or a ruler, transpose both the long- and short-axis measurements onto the vertebral column, extending caudally from the cranial edge of T4. Count the number of individual vertebral bodies traversed by each of the measurements in turn, estimated to the nearest 10th of a whole number. Add these two values together to obtain the VHS (Fig 1) (Clinical tips 5, 6).
In dogs with MMVD with significant left atrial enlargement and elevation of the left main stem bronchus, the long-axis measurement should be made from the cardiac apex to the ventral border of the elevated bronchus. This ensures inclusion of the enlarged left atrium within the long-axis measurement, which would otherwise not be accounted for if the typical tracheal carina landmark was used. In this situation, the perpendicular short-axis measurement should be made at the dorsal border of the caudal vena cava (Fig 2).
Overweight dogs often have a large amount of fat in the pericardial sac that may elevate the cardiac silhouette from the sternum. When measuring vertebral heart score, it is important not to include this apical fat in the long-axis measurement, as it will overestimate the VHS.
An abnormal VHS is more than 10.5. In dogs with MMVD, an increased VHS strongly suggests cardiac enlargement. The degree of cardiac enlargement in a dog with MMVD will depend on how advanced the underlying disease is and this will be reflected by the increase in VHS measurement. A VHS of 11 suggests mild cardiac enlargement, a VHS of 12 suggests moderate cardiac enlargement and a VHS of greater than 13 suggests severe cardiac enlargement.
The VHS was originally designed to be applicable across dog breeds of differing sizes and chest conformations. Many breeds studied individually do fall within the normal VHS range. However, it is important to be aware that a number of breeds normally have VHS values that would suggest cardiomegaly using the original scale. Of the breeds evaluated to date the French and English bulldog, pug, Boston terrier, Pomeranian, CKCS, boxer, Labrador retriever and whippet have been found to have average VHS values that are significantly higher than other dog breeds. Sets of breed-specific normal values have been developed and published for use in these breeds (Table 3).
Echocardiography (heart ultrasound) is the gold-standard test to confirm the diagnosis of MMVD and to stage the disease (assess the degree of individual cardiac chamber enlargement). Ultrasound enables the cardiac muscle function and the cardiac valves to be assessed in detail, which provides information that is important to both diagnosis and treatment.
A normal heart size on thoracic radiographs does not rule out underlying mitral valve disease (MVD). In the early stages of the MVD (MVD stage B1) the volume of mitral regurgitation might not be sufficient yet to cause radiographic evidence of left atrial or ventricular chamber enlargement. Further investigation with cardiac ultrasound may be necessary to check for the characteristic mitral valvular lesions and therefore confirm and stage the disease.
Veterinary surgeons are typically more familiar with interpreting thoracic radiographs than cardiac ultrasound images. However, with some focused, practical training, the ultra-sound images necessary to diagnose and stage MMVD in dogs (as per the EPIC criteria) can be reliably obtained and interpreted in a general practice setting.
In the EPIC study, echocardiographic evidence of advanced MMVD was defined as:
Characteristic valvular lesions of the mitral valve apparatus;
MR on the colour Doppler echocardiogram (Video 1);
Echocardiographic evidence of left atrial and left ventricular dilatation defined as:
Left atrial-to-aortic root ratio (LA:Ao) greater than or equal to 1.6;
Bodyweight normalised left ventricular internal diameter in diastole (LVIDDN) greater than or equal to 1.7.
Measuring left atrial size
Measuring the left atrial size is a useful tool to establish whether a dog with mitral valve disease has heart enlargement. Therefore, it is important to understand how left atrial size can be measured from a heart ultrasound in patients with MMVD.
Currently, the most commonly used and simplest method to assess left atrial size in dogs with mitral valve disease is the two-dimensional left atrial to aortic root ratio (or the LA:Ao ratio). This provides a bodyweight-independent measurement of left atrial size.
The dog should be lying in right lateral recumbency on a table designed for heart ultrasound (with a cut-out area, over which the dog’s right cranial thorax is positioned). The right parasternal window is the first location to image, and is located between the fourth and sixth intercostal spaces. Palpation of the area of the strongest apical beat typically identifies the most optimal position to image from. Position the probe at the level of the costochondral junction or slightly closer to the sternum. The right parasternal short-axis views are obtained by positioning the probe so that the transducer mark is oriented towards the right elbow. Start by obtaining a right parasternal short-axis view of the heart base at the level of the aortic valve and left atrium.
Once you have this view, the next step is to measure the internal short-axis diameter of the aorta from the middle of the right coronary aortic valve cusp to the opposite commissure between the non-coronary and left coronary aortic valve cusps. This should be done in early diastole on the first frame after the aortic valve shuts. Ideally, the ‘Mercedes-Benz sign’ of the closed aortic valve should be visible as this confirms good cross-sectional alignment and the correct phase of the cardiac cycle (early diastole, when the aortic valve is closed).
Next, measure the internal short-axis diameter of the left atrium in the same frame. This measurement should be taken from the commissure between the non-coronary and left coronary aortic valve cusps, across to the left atrial free wall, along a line that extends from the aortic diameter measurement (Fig 3).
In some images a pulmonary vein may be visible entering the left atrium, resulting in loss of continuity of the left atrial lateral margin. The margin of the left atrium should be approximated by extending the visible edges of the left atrium in a curved fashion, so that left atrial diameter can be accurately measured (Fig 4).
Once you have both measurements, compare the LA:Ao measurements as a ratio. The normal LA:Ao ratio is less than 1.6 when measured using the short-axis view. So, if your LA:Ao ratio is more than 1.6 this suggests that your patient does indeed have left atrial enlargement. This is important prognostic information and will help you to develop an appropriate management plan.
Measuring left ventricular size
Taking the LA:Ao view as a starting point, angle the probe slightly ventrally to obtain a right parasternal short-axis view of the left ventricle, ensuring the left ventricular cavity is as symmetrical as possible. Use the track-ball to move the M-mode cursor over the real-time image. Position the M-mode cursor between the left ventricular papillary muscles, perpendicular to the interventricular septum and left ventricular free wall at the level of the chordae tendinae, just below the level of the mitral valve. The M-mode will show the right ventricular chamber at the top of the image, followed by the interventricular septum, left ventricular chamber and left ventricular free wall at the bottom of the image (the right ventricular wall in the near-field may not be clear). The normal interventricular septum and left ventricular free wall move away from each other in diastole as the left ventricle is filling, and towards each other in systole as the left ventricle contracts to eject blood into the aorta.
Using the echocardiogram for timing and measurements assures greater accuracy when comparing measurements from serial examinations in the same patient. All diastolic measurements should be made at the onset of the QRS complex. Systolic measurements should be made at the point of the smallest left ventricular diameter on M-mode. Measurements should be made using the ‘leading edge to leading edge’ principle, in other words from the leading or top edge of one structure to the leading edge of the next structure. At least three to five cardiac cycles should be used and averaged for each measurement, to overcome the effects of respiration and changes in cardiac filling secondary to sinus arrhythmia. From an left ventricular M-mode, the routine measurements are septal and left ventricle free wall thickness in systole and diastole, left ventricle chamber diameter dimensions in systole and diastole, and fractional shortening (Fig 5) (Clinical tip 7). Fractional shortening is used as an estimate of myocardial contractility.
Fractional shortening (FS):
(LVIDdiastole – LVIDsystole / LVIDdiastole) x 100 = per cent Normal canine FS per cent = approximately 25 to 40 per cent.
Be aware that in dogs with mitral valve disease (MVD) that have large volumes of mitral regurgitation (MR), the FS per cent will tend to increase above the normal range. This does not reflect an improvement in the ability of the heart to contract. It is a result of the fact that the leaky mitral valve allows the left ventricle (LV) to ‘offload’ blood into the left atrium during systole so the LV systolic dimension (LVIDs) decreases. Coupled with the fact that the LV diastolic dimension (LVIDd) will increase due to the extra volume it has to accommodate as the MR volume increases over time, it is easy to see why the FS per cent value will increase when calculated with the above equation. In advanced stages of MMVD when the heart is very enlarged, myocardial failure develops secondarily to chronic volume overload, and the FS per cent value will fall.
In a dog with MMVD and no clinical signs of CHF, it is typically recommended to monitor for an increase in loudness of the murmur with cardiac auscultation every six months. Thoracic radiographs and heart ultrasound can be repeated annually to monitor for changes in heart size. Once the heart is significantly enlarged (advanced MMVD stage B2), then closer monitoring is prudent as CHF may be imminent.
The owner should be educated to count resting/sleeping respiratory rate at home as this is a very sensitive indicator of the first development of pulmonary oedema in advanced stage B2 dogs, as well as breakthrough episodes of pulmonary congestion in those patients with MMVD stage C that are already receiving CHF therapy.
To achieve optimal longevity and quality of life, dogs with MMVD must be diagnosed before clinical signs of CHF are evident. Cardiac screening for MMVD is recommended for ‘at risk’ dogs. Once a characteristic heart murmur is detected, further investigations with radiography and cardiac ultrasound enable the presence of MMVD to be confirmed and the disease to be staged. With some focused, practical training, the ultrasound images necessary to diagnose and stage MMVD in dogs (as per the EPIC criteria) can be reliably obtained and interpreted in a general practice setting.
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