HEART DIAGNOSTICS

I was asked to write this article to try and clear up some confusion regarding how to interpret cardiac testing results. Many of you were sent a brochure in May, 1997 from the Orthopedic Foundation for Animals (OFA) entitled: “OFA Congenital Heart Disease Registry: General Procedures”. That reference, as well as others cited at the end, were used to construct this article.

All veterinarians will have been schooled in auscultation, which in this context is listening to your dog’s heart with a stethescope. The vet is listening to the heart rate and rhythm, for any extra or unusual heart beats, and for the sound of murmurs or other abnormal sounds. In dogs, two heart sounds are normally heard. The first sound (known as S1) is associated with the closure of the atrioventricular (AV) valves, which separate the atria from the ventricles. The AV valve on the right side of the heart is known as the tricuspid valve, and the corresponding valve on the left side is called the mitral valve. The second sound (known as S2) is audible with the closure of the semilunar valves. The semilunar valves separate the right ventricle from the pulmonary artery (leading to the lungs) and the left ventricle from the aorta (taking blood back out to the rest of the body). These valves are designed to be one-way, and to force the blood to flow in only a single direction (forward) during n ormal cardiac function.

Murmurs are described as gentle, blowing auscultatory sounds. They are caused by changes in the way blood flows through the heart and are by far the most common sign of heart disease.

Mitral valve disease (MVD) is classified as an acquired heart disease. The definition of an acquired heart disease is one that is not present at birth, but instead develops gradually over time. Cavaliers have the highest rate of early onset MVD of all breeds.

Many diseases affecting the mitral valve and its supporting structures result in partial backflow of blood through the valve (regurgitation). The most common primary disease of the mitral valve is valvular degeneration. In this disease, the leaflets or cusps comprising the valve contract and curl back on themselves, allowing the valve to leak. In mitral regurgitation, blood leaks back from the left ventricle into the left atrium. To accommodate for this increased quantity of blood, the left atrium enlarges. The left ventricle grows larger (hypertrophies) so that it may pump a larger quantity of blood to compensate for the leak. In primary mitral regurgitation, the amount of regurgitation correlates directly with the size of the left atrium and ventricle. Severe mitral regurgitation can produce a tremendous increase in left heart size together with varying degrees of congestive heart failure.

When no heart murmur is heard, the dog is said to be clear on auscultation. If a heart murmur is heard, it is supposed to be graded as follows:

  • Grade 1 – a very soft murmur that can only be detected after very careful auscultation – the lowest intensity that can be heard
  • Grade 2 – a soft murmur that is readily evident (restricted to a localized area)
  • Grade 3 – a moderately intense murmur not associated with a palpable precordial thrill (vibration) – immediately audible when auscultation begins
  • Grade 4 – a loud murmur; a palpable precordial thrill is not present or is intermittent
  • Grade 5 – a loud cardiac murmur associated with a palpable precordial thrill; the murmur is not audible when the stethoscope is lifted from the chest wall
  • Grade 6 – a loud cardiac murmur associated with a palpable precordial thrill; the murmur is audible even when the stethoscope is lifted from the chest wall

Still and all, the grading of a murmur is subjective, based on how it sounds to the listener. It is not that unusual for different clinicians to give the dog a different grading – maybe because of background noise, their level of training, etc. – but in the author’s experience, anyway, those differences are usually within one level – one person calls the dog a Grade 3, another a Grade 4, for example.

Far more accurate measurements are available to us through the use of cardiac ultrasound, or echocardiology. The next section of this article is taken from “Echocardiology: the heart as never before” by W. Bradford Swift, DVM, Dog World, April, 1993, pages 121-126.

In recent years, Doppler echocardiology has made it possible not only to see the structures of the heart, but also to obtain important information regarding the flow of blood through it. With the use of two different methods of Doppler echocardiology – color flow and spectral – it is possible to obtain a color-coded image of blood as it flows through the heart, including areas of abnormal turbulence. Then one can determine the blood flow velocity in specific areas of the heart to access the extent of the abnormality.

According to Dr. Mark Kittleson, professor of medicine at the University of California at Davis, and a diplomate of the American College of Veterinary Internal Medicine, “With two-dimensional ultrasound, you can see the anatomy of the heart: the chambers, the valves moving, the relationship of them to each other. You can see lesions, holes between the chambers of the heart, abnormal valves, chambers that have enlarged.”

Such detail is not available by other diagnostic means. Conventional radiology is limited to outlining the heart, lungs and other structures of the chest cavity. Although changes in the shape and size of the heart can be discerned by X-ray, it is impossible to see the inner workings of the heart.

Electrocardiograms measure the electrical output of the heart muscle. Although they can point to abnormalities of the heart’s size, shape and function, EKGs cannot give the detailed information of a cardiac ultrasound.

Echocardiology uses ultrasonic sound waves, which are reflected off the structures being examined and can distinguish objects that are less than one millimeter apart. The reflected waves are then converted into a picture that can be viewed on a monitor, photographed or videotaped for future study.

With conventional echocardiology, there are two techniques commonly used: M-mode and two-dimensional. The newest innovation, Doppler echo-cardiology, also has two different techniques: spectral and color flow. All four techniques give important information but in a different way.

With two-dimensional echocardiology, the clinician gets a clear picture of the heart and can then accurately measure the sizes of the chambers of the heart as well as the thicknesses of the walls with M-mode echocardiology. Chamber size is important in diagnosing many heart conditions since the heart often enlarges one or more of its four chambers to compensate for an abnormality.

Doppler echocardiology is used to measure blood flow in the heart. Color flow echocardiology gives the clinician a color display of the blood as it circulates through the heart. With this technique, blood flowing away from the measuring device is colored blue and blood moving toward the device is colored red. Also, the faster the blood is moving, the brighter the hues. Turbulent blood flow, which is an indication of heart disease, is often characterized by high-velocity blood flow with many different velocities and will show up as a mixture of various reds and blues.

As with conventional echocardiology, Doppler echocardiology’s two techniques give different information. Spectral Doppler echocardiology gives very specific blood velocity measurements of isolated areas, while color-flow Doppler echocardiology gives an overall picture of blood flow.

In most cases, the echocardiogram is performed with the dog fully awake. Before the ultrasound, the dog’s side may be shaved, although in many cases the only preparation needed is the application of the ultrasound gel to the chest. The animal is placed on his right side on a table which has areas cut out. The dog’s chest is placed over one of the areas so that the echocardiograph transducer can be applied between the ribs on the surface lying on the table. The transducer must be applied to the chest in an area where the heart is in contact with the chest wall.

With the two-dimensional echocardiogram, the ultrasonic beam “slices” through the heart, resulting in two-dimensional pictures. Depending on what is found with the two-dimensional study, M-mode echocardiology may be employed to determine the sizes of the heart chambers. If the equipment has Doppler capability and blood flow data is needed, color flow Doppler is performed next. If hemodynamic irregularities are detected, more specific measurements may be made using the spectral Doppler technique.

Since the mitral valve is easily visualized with echocardiology, this procedure is useful in diagnosing the condition as well as determining the severity of the damage.

In September, 1996, veterinary cardiologists Dr. Michael O’Grady and Dr. Sandra Minors came to the Sunday brunch following the National CKCSCC specialty. A three-page handout composed by Dr. Minors was distributed at that session. It is worth repeating some of the points made in that handout.

There is no apparent sex predilection for MVD. Both males and females are equally affected.

Colour flow doppler is clearly superior to auscultation for the detection of mitral valve insufficiency and thus a better screening tool for discovering mild to moderate MVD (emphasis mine).

A significant proportion of dogs screened as young as 1 year old (42%) were found to have detectable mitral valve insufficiency with colour flow Doppler. This percentage was doubled in dogs screened between the ages of 3-4 years. Thus young dogs that are cleared by auscultation alone, may not really be clear of MVD.

Having made a great case for the use of colour flow Doppler to screen for mitral valve disease, its limitations should be clarified:

Colour flow Doppler is a pictorial colour map superimposed on a 2-dimensional image of the heart that represents the velocity (speed) of blood flow. It is NOT a map of the volume (amount) of blood flow.

Thus, mitral regurgitant jets seen on colour flow Doppler are pictorials of high velocity/turbulent blood flow. The size of the colour jet is influenced by a number of factors independent of the actual volume of regurgitant jet, the gain settings of the ultrasound machine, the compliance of the left atrium, the heart rate etc. The size of the colour flow Doppler jet is, therefore, only crudely quantitative, that is a rough estimate of the amount of regurgitation.

Based on these limitations, colour flow Doppler should be used as a sensitive tool for the detection of mitral regurgitation, however, breeding practices should not be based on size of mitral regurgitant colour jet. For example, a dog with a “trivial” jet may have just as much or more regurgitation than a dog with a “mild” jet! When breeding a bitch, one does not ask, “how pregnant is she?’. The important question is, “Is she pregnant or not?”.

The quantitation of the volume of regurgitation has more importance for the individual dog, from a clinical perspective. From a breeding perspective, the presence of any mitral regurgitation in this breed must be considered significant at this time. While small or “trivial” mitral regurgitation may be considered inconsequential in other breeds, it would be dangerous to assume this is so in a breed that has more dogs affected and at a younger age than any other dog breed in the world.

Other References:

The Merck Veterinary Manual

UC Davis Book of Dogs

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