Overview of Feline Heart Murmurs
Heart murmurs are sounds created by high velocity, turbulent or disturbed blood flow in the heart or blood vessels. Heart sounds and murmurs are detected by listening to the heart with a stethoscope, an examination called “cardiac auscultation”. Murmurs are associated with a variety of causes including congenital malformations (birth defects), heart diseases acquired during life such as cardiomyopathy, or by diseases of other organs that affect the heart indirectly, as with hyperthyroidism. Importantly, nearly half of all heart murmurs in cats are caused by the sympathetic nervous system (or adrenaline) acting on an otherwise normal heart. Heart murmurs may affect cats of any age, sex, or breed although some cats breeds are more prone to certain heart diseases.
From a practical standpoint, feline heart murmurs can be classified into four broad categories:
1) Functional murmurs associated with a normal heart;
2) Murmurs due to congenital heart disease;
3) Murmurs associated with cardiomyopathies or heart muscle disease; and
4) Murmurs related to diseases of blood vessels.
Unlike dogs, heart murmurs in cats are rarely caused by degenerative valve problems or by infection of the valve.
The term “functional murmur” indicates that the heart is structurally and functionally normal based on a cardiac ultrasound examination. Some functional murmurs detected in kittens are probably due to an immature heart and these murmurs disappear as the pet reaches about 4 months of age. These functional murmurs are often called “innocent” by veterinarians.
However, the majority of functional murmurs are thought to develop from the cardiac effects of the sympathetic nervous system. Most people recognize the effects of adrenaline and sympathetic nerve chemicals (nor-epinephrine) on their own hearts. These increase with stress, exercise, and illness, making the heart beat faster and stronger. The increase in strength of contraction causes a faster ejection of blood from the ventricles, often creating a murmur. Since most cats removed from their home experience some degree of stress, functional murmurs are frequently detected during veterinary examinations. Functional murmurs can also develop with some illnesses, including fever, anemia, and high levels of thyroid hormone (hyperthyroidism). These murmurs are indistinguishable from those caused by congenital or acquired heart diseases.
Congenital heart disease means the heart did not form properly prior to birth. Collectively these are called “cardiac malformations”. Most common in cats are holes in the heart (ventricular and atrial septal defects) and malformations of the mitral valve. Congenital heart defects allow blood to move in an abnormal direction or cause a valve to malfunction, resulting in high-velocity and turbulent blood flow. These are usually detected during a kitten’s wellness examination. The majority of cat owners have no idea there is anything wrong with their cat’s heart.
Cardiomyopathies are common and represent the most important acquired heart diseases in this species. The word “cardiomyopathy” simply means that the heart muscle is diseased. The most common form is genetically determined and results in a thickened left ventricle (hypertrophic cardiomyopathy). There are other forms of cardiomyopathy as well, including some “secondary” cardiomyopathies associated with chronically high blood pressure (hypertension), hyperthyroidism, or diabetes mellitus (sugar diabetes). These cardiomyopathies are discussed under separate topics on this site. Cardiomyopathies can induce heart valves to leak or create obstruction to blood flow within the ventricular chambers, dysfunctions that can generate a murmur. Cardiomyopathies in some cats are very serious, leading to heart failure, dangerous blood clots (thromboembolism), or sudden death. This is the main reason veterinarians are so concerned when they detect a heart murmur in a mature cat and why they recommend additional diagnostic tests.
Diseases centered in the blood vessels sometimes lead to heart murmurs although the precise reasons are not completely understood. As in humans, systemic hypertension (high blood pressure) is common in older cats. This can lead to dilation of the aorta, thickening of the left ventricle, and a heart murmur. In older cats the aorta sometimes dilates for uncertain reasons (a condition called “idiopathic aortic dilation” or “aortoannular ectasia”): many of these cats also have a cardiac murmur. Pulmonary hypertension (high pressure in the arteries of the lung) can also cause a murmur. This condition can follow heartworm infection or chronic lung disease.
Murmurs are often classified based by their location, timing, duration, character and intensity or grade. For example:
- Location refers to the area over the heart in which the sound appears to originate or has maximal intensity. While this is classically described by proximity to a valve area, in cats the heart is so small that isolating valve areas is difficult. Accordingly, most describe the location relative to the left or right sternal border and indicate if the location is caudal (apical) or more cranial.
- Timing simply indicates when in the heart beat cycle the murmur is heard. Murmur can develop during ventricular pumping (systolic murmurs) and rarely during filling (diastolic murmurs). Continuous murmurs most often indicate a congenital heart defect, patent ductus arteriosus, which can be corrected surgically. Some systolic murmurs are shorter whereas others are detected throughout pumping (holosystolic).
- Duration refers to how long the sound lasts within the timing phase.
- Character of the murmur refers to the quality of the sound. Sometimes functional or innocent murmurs are vibratory or musical, but most have a harsh (mixed-frequency) character.
- Grade refers to the intensity or loudness of the murmur with murmurs graded from 1 (softest) to 6 (loudest). Some murmurs are accompanied by a “thrill” which indicates that the murmur is so loud it causes a vibration that can be felt over the chest wall. Except for very loud (grade 5 to 6) murmurs, the grade does not predictably relate to the cause of the murmur or to the severity of any underlying heart disease.