What is a Heart Murmur?

Heart murmurs are audible successive sounds with a distinct duration, as opposed to normal heart sounds, which are short transitory events. Cardiac murmurs result from turbulence created in blood flow. When the flow velocity of fluid within a vein/artery exceeds a certain value, turbulence develops and energy is dispelled this in turn generates audible vibrations. Turbulence may also arise when blood passes through a small hole that partially occludes a vein/artery.  When the diameter of the vein/artery abruptly changes, or when blood strikes a surface turbulence is created. The characteristics of the murmur depend upon the velocity of blood flow & the surrounding structures that are caused to vibrate.

There are several causes of heart murmur, some of the commonest are listed further down the page.

Heart function - how the heart works

The heart is a pump which pumps oxygenated blood (from the lungs) to all cells in the body via the arteries, veins & capillaries. As circulation completes its journey, deoxygenated (carbon dioxide rich blood) returns to the heart where it is once again pumped to the lungs. Gas exchange occurs in the lungs, carbon dioxide is expired & oxygen is inspired & the process of circulation continues ad infinitum.

The major vessel (artery) connected to the heart is the aorta which carries blood from the left side of the heart. Normally, the blood that is pumped to the body from the left side of the heart is fully saturated with oxygen. The oxygen is extracted from the blood for use in the various tissues and then the deoxygenated blood is returned to the right side of the heart. It is then pumped to the lungs to discharge carbon dioxide & pick up oxygen, and then is delivered back to the left side of the heart, from which it is pumped out to the tissues again.

Diagnosis of heart murmurs

Heart murmurs are diagnosed during auscultation (via stethoscope). Heart murmurs may be classified as (1) innocent, (2) functional, or (3) pathologic and this is based on the turbulence heard & not necessarily the severity of the condition.

At the time of auscultation, clinical signs are also observed i.e. the colour of the mucus membranes, capillary refill time, listening for crackles/wheezes from the lungs and whether a cough is present or can be induced. It is also helpful if a vet monitors pulse (femoral artery of the rear legs) at the same time as listening to heart sounds, this will help determine which phase is which; the pulse should be evident during systole.

Once a murmur is diagnosed, it is classified according to standard criteria. Firstly, they are categorized according to their timing in the cardiac cycle, accordingly, murmurs are identified as being systolic, diastolic, or continuous.

The next feature noted is the intensity. Unfortunately, murmur intensity does not necessarily indicate flow volume. Third, the frequency, or pitch, of the murmur over each of the heart valves and at the thoracic inlet should is characterized. The fourth aspect of the characterization of a murmur is the modulation or shape of the murmur i.e. a crescendo murmur starts softly & gets louder before it stops; a decrescendo murmur does exactly the opposite.

A crescendo-decrescendo murmur starts softly, becomes louder and dies away softly. Finally the location of the murmur where the intensity is loudest & the area of radiation should be succinctly noted, location may help pinpoint the valve involved.

If a murmur is detected at routine examination further diagnostic tests may be warranted, these include:

• Electrocardiogram (ECG) - This is an assessment based on the records of heart electrical activity over time. It is used when murmur is suspected to determine the accurate heart rate, extended problems or damage & diagnose any other type of diseases that can cause abnormal rhythms.
• Cardiac ultrasound (echocardiogram) - This is the standard ultrasound techniques that can slices the image of the heart into 2D or 3D real-time imaging. This procedure allows vet to determine valves problems, functions, contractions, pumping efficiency, chamber's size and thickness of your dog heart walls.
• Chest X-rays - Vet's use this method to examine the lungs & diagnose abnormalities of heart problems when viewing radiographic images. Many types of heart diseases have the potential to enlarge the size of the heart (particularly the left side of the heart).

Classification/Grading of murmurs

Innocent systolic ejection murmurs are produced by normal turbulent flow through the proximal great arteries at the time of ventricular ejection (blood leaving the heart each time it contracts). The intensity of the murmur, as influenced by stroke volume or velocity of ejection, & proximity of the great arteries to the chest wall determine whether the murmur is audible with the stethoscope.

Such murmurs are typically heard when tachycardia (rapid heartbeat) exists, and in puppies and young slim dogs; these animals tend to have a brisk circulation, a smaller thoracic cage with a narrow transverse diameter additionally, young pups are likely to have increased heart rate due to the excitement/stress of visiting the vet for the first time.

Puppy murmurs tend to be of medium frequency, peaks in early to mid-systole, usually ends before the second heart sound, and is best heard on the left side over the mitral or aortic valve or at the thoracic inlet. The murmur may be the result of turbulent flow into the pulmonary artery or aorta, or both. This type of murmur is commonly diagnosed at the time of the first injection or if a breeder has pups checked pre-sale. Innocent murmurs (if heard) are usually present until a pup is around 16 weeks old before disappearing spontaneously. They are quite common in pups (due to physical immaturity) and should usually cause no alarm.

Systolic ejection murmurs imply turbulent blood flow at the time of right or left ventricular ejection into its corresponding artery (coronary arteries). They are typical of the murmurs produced by congenital aortic and pulmonic stenosis.

Functional systolic murmurs are produced by increased velocity of blood within the cardiovascular system and by extracardiac factors i.e.  Pleural or pericardial effusion, anaemia, fever, hyperthyroidism. Tachycardia of any cause may produce functional murmurs. Such murmurs are often detected during anaesthesia when a sinus tachycardia exists due to atropine administration.

A continuous murmur extends from systole (the rhythmic contraction of the heart, by which blood is driven through the arteries) into diastole (the period of time when the heart fills with blood after systolic contraction). Such a murmur results from blood flow continuing from a high-pressure to a lower-pressure area, despite aortic semilunar valve closure. Patent ductus arteriosus is the only common example in dogs of a continuous murmur that peaks in intensity at the second heart sound.

Illustration courtesy of NIAAA/Chung, M.K., and Rich, M.W.

History Checklist

The precise diagnosis of which type of heart defect is causing a murmur clearly needs to be established by an owners vet or if required by a cardiologist, however there are some broad generalisations.

Puppies 4 - 16 weeks (approx.) often have Grade 1 - 2 murmurs which are deemed innocent, these are generally due to physical immaturity (no surprise really, they are young and still developing physically!).

Dogs that are less than six months old with Grade 3 murmurs almost certainly have a congenital (present from birth) cardiac defect which needs further investigation to reach a diagnosis as there are several defects responsible. The majority of middle-aged dogs (over 5 years old) will likely have mitral valve insufficiency or dilated cardiomyopathy. Dogs with cardiomyopathy tend to be diagnosed around age 7.

Signs & symptoms

The signs of heart murmurs will vary according to the underlying cause & extent of the problem, in mild cases there will be no symptoms. Commonly seen signs are:

• Chronic hacking cough
• Laboured breathing
• Poor exercise tolerance - tires easily
• Lack of stamina
• A bluish tinge to the tongue or gums
• Fainting

The symptoms presenting directly relate to the effect of the condition causing the murmur rather than the actual grade of the murmur.

Causes of heart murmur:

Patent Ductus Arteriosus

This is the most common heart defect found in cocker spaniels. The ductus arteriosus is a blood vessel that connects the two main arteries of the body - the aorta & the pulmonary artery.

This blood vessel is normal in the foetus as during the time in the womb, blood is not routed via the lungs to collect oxygen/discharge carbon dioxide as the placenta provides this function. Whilst in the womb a pups blood passes through the pulmonary artery through the ductus arteriosus to the aorta. However, shortly after birth (within a few days), the ductus arteriosus should close to allow pulmonary circulation to commence (the word 'patent' means the duct is open). When the ductus arteriosus remains open after birth, this abnormal communication between the aorta & pulmonary artery allows some blood to bypass the lungs. This defect is commonly referred to as a 'hole in the heart'.

Some PDA's arise due to a late closure of the ductus arteriosus and given a little time the defect will spontaneously resolve and at a subsequent routine examination no heart murmur can be identified. However this is not always the case. Any symptoms associated with PDA are not likely to be noted by owners until the dog is 12 months old or so. Therefore, if a puppy is found to have PDA it is advisable to monitor heart sounds to ensure the defect has closed. If the defect does not self resolve quickly after diagnosis it is a recommendation that an operation to close the PDA with a suture is performed shortly after diagnosis as unfortunately upwards of 60% of dogs diagnosed will die within a year of diagnosis.

Symptoms of Patent Ductus Arteriosus

Generally, there are no serious symptoms of PDA (other than the presence of a murmur) unless congestive heart failure has caused fluid build-up in the lungs. The condition is typically identified in puppies during a routine veterinary visit for vaccinations. If symptoms develop they are usually delayed onset (occurring sometime during the first 12 months of life rather than immediately obvious at birth or whilst a puppy is very young) and include:

• Fast breathing, working hard to breathe, or shortness of breath
• Poor appetite & poor weight gain
• Tiring easily

Pulmonary stenosis

Pulmonic stenosis is one of the more common causes of heart murmur found in cocker spaniels (as is dilated cardiomyopathy).

Pulmonic stenosis is a congenital narrowing in the region of the pulmonary valve which lies between the right ventricular chamber of the heart & the pulmonary artery. This artery carries deoxygenated blood from the heart to the lungs & the narrowing impairs normal blood flow into the artery.

Because of the obstruction, the right side of the heart has to work harder to pump blood to the lungs. This causes an increase in the mass of the heart muscle, or right ventricular hypertrophy, one of the hallmarks of this disorder. The narrowing can occur within the valve itself (valvular); below the valve at the conus arteriosus or infundibulum (subvalvular); or above the valve in the pulmonary artery itself (supravalvular) - uncommon. Sometimes more than one form is seen in an individual and, although usually an isolated lesion, pulmonic stenosis can occur with other defects and it is one of the components of another congenital defect Tetralogy of Fallot.

Symptoms of Pulmonary Stenosis

Usually, affected dogs show no external signs of the disease & it is usually diagnosed during routine examination of the heart with a stethoscope, However, if blood flow is seriously impaired signs associated with right-sided heart failure may be seen including:

• Poor exercise tolerance
• Feinting
• Fluid accumulation in the abdomen (due to ascites, an accumulation of serous fluid in peritoneal cavity)
• Blue-tinge to the gums especially with exertion

Tetralogy of Fallot

Tetralogy of Fallot is the term for a serious combination of defects that are the result of abnormal development of the embryo during the period of gestation when the heart & major cardiac blood vessels are formed. Tetralogy of Fallot consists of four heart defects. These are pulmonic stenosis, ventricular septal defect, overriding aorta & right ventricular hypertrophy.

In pulmonic stenosis, there is a partial obstruction of blood flow from the right side of the heart through the pulmonic valve.

A ventricular septal defect is a hole in the muscular wall of the heart (the septum) that separates the right & left ventricular chambers.

The result of the defects that make up the Tetralogy of Fallot is that poorly oxygenated blood is delivered to the body. This causes general cyanosis or a blue-grey tone to tissues that would normally be pink. Tetralogy of Fallot is the most common cyanotic heart defect.

Symptoms of Tetralogy of Fallot

• Weakness
• Failure to thrive & grow
• Reduced tolerance for exercise
• General cyanosis (blue-grey instead of pink mucous membranes)

Treatment of heart murmurs

Canine heart murmurs could be either benign or acquired. For the proper treatment of the disorder, a thorough examination of the dog is carried out. It determines whether the heart murmur is benign or acquired. The benign heart murmur doesn't need any kind of treatment. This minor form of heart murmur is treated by providing a low sodium diet to the dog. Medications can used to lower the blood pressure and to increase the strength of contractions in the heart. In critical situations, surgery remains the only option to treat the damaged valves present in the heart.

For dogs suffering pulmonary stenosis, routine treatment/medication is not usually required. Those showing clinical signs of heart failure are treated with diuretics (e.g. frusemide) as an excess of water (oedema) is a common side effect of heart failure.  This is due to the hearts inability to pump well and the body holds on to salt and water which in turn increases the amount of blood circulating which unfortunately means the heart has to work harder to pump blood around the circulatory system. Diuretics remove excess water from the system via the kidneys/urination & make it easier for the heart as there is less fluid to pump around the body.

For dogs with Tetralogy of Fallot β-adrenergic medication can be used to reduce the dynamic component of right ventricular outflow obstruction and to attenuate β-adrenergic-mediated decreases in systemic vascular resistance or surgery to create a shunt between & aorta and pulmonary artery so that sufficient blood can enter the lungs.

Prognosis

The long-term outlook for the dog varies and this is governed by the exact cause of the problem and the responsiveness to treatment (if treatment is required). It is difficult to generalise, as clinicians are aware that the grade of a murmur often does not correlate to the severity of the condition. However, despite not wishing to generalise, it is normal for dogs graded 1 - 2 to have no symptoms, suffer no ill effect and to in all probability 'grow out' of the condition. Dogs graded 3 - 4 usually live completely normal lives with a normal lifespan & tend to have no or few symptoms. Indeed even dogs graded 5 or 6 can live quality lives which are not sufficiently shortened or plagued with health problems.

The exact cause and severity of a heart murmur can really only be determined by a Veterinary Cardiologist who may or may not offer surgical or medical treatment if the dog isn't maintained satisfactorily using standard treatments via GP vets.

Heart murmurs are often very much about managing symptoms & this is one of the reasons why the grade of a murmur often does not reflect the seriousness or lack of seriousness of the condition in individual cases. Many dogs are fortunately asymptomatic & this accounts for the wide variation in long-term outlook for each dog diagnosed with a heart murmur.