HEART MURMURS: DECIPHERING THEIR CAUSE AND SIGNIFICANCE

Transcription

1 Vet Times The website for the veterinary profession HEART MURMURS: DECIPHERING THEIR CAUSE AND SIGNIFICANCE Author : Pedro Oliveira Categories : Vets Date : May 27, 2013 PEDRO OLIVEIRA provides guidance for classifying heart murmurs and what causes them, and warns their detection always warrants further investigation Summary Cardiac auscultation is part of the daily routine of all veterinary clinicians. Every day we detect heart murmurs and try to decipher their cause and significance so we can act accordingly. Our ability to perform this task is invaluable since a heart murmur is often the only indication of heart disease in most of our patients. Unfortunately, auscultation is a skill that cannot be developed by just reading a book. It takes experience to recognise the sounds. A few guidelines are presented in this article to help with the classification of heart murmurs and their use to try to understand the underlying cause. The most important concept to keep in mind is that a heart murmur is nothing more than vibrations emanating from the heart and blood vessels perceived by us as sounds and that these sounds shouldn t occur in a normal heart. Hence, detection of a heart murmur always warrants further investigation. Key words cardiology, murmur, auscultation, dog, cat IF we listen to hearts as a regular part of our clinical examination, we will detect lots of murmurs. However, do we know what clues they are giving us? Several questions may 1 / 12

2 arise. Is the murmur severe? Should I treat? What should I treat with? Should I refer to a cardiologist? Sometimes we may even find ourselves in doubt as to whether the murmur really exists. Murmurs are nothing more than vibrations emanating from the heart and blood vessels, perceived by us as sounds and ones that shouldn t occur in a normal heart. A logical assumption is that a heart murmur signals an abnormal heart or heart function. Indeed it does: detecting a heart murmur is often the first step in the diagnosis of cardiovascular disease. Unfortunately, auscultation is not a skill that can be developed through simply reading a book it takes experience and guidance. However, once a specific heart murmur is heard for the first time, it is never forgotten. The first rule to cardiac auscultation is to listen carefully in a quiet room with the patient in a standing position and not panting. Auscultation in a recumbent patient is fraught with difficulty and prone to error. Both sides of the thorax and the whole area of projection of heart sounds must be examined, as well as the cranial left thorax under the axillary region. If a patent ductus arteriosus is present, for example, and this area is not examined the murmur might be missed. How are heart murmurs classified? Heart murmurs are classified according to intensity, duration and area of maximal intensity. They can also be classified according to their acoustic characteristics into ejection or regurgitation murmurs (although a clear distinction may be difficult). Intensity Heart murmurs can be extremely loud in intensity or not at all loud, and are graded accordingly from one to six in intensity as follows: 1/6 murmurs are very soft and are only detected after close auscultation in a quiet room; 2/6 murmurs are soft, but easily detected; 3/6 murmurs are moderately loud; 4/6 murmurs are loud; 5/6 murmurs are very loud and accompanied by a palpable thrill; and 2 / 12

3 6/6 murmurs are also accompanied by a thrill and can be heard with the stethoscope close to the chest, but not in contact with the chest wall. Murmurs are generated by turbulence of the blood or vibration of cardiac structures, hence it would be logical to assume the louder the murmur the worse the condition causing it. However, this is not always the case and interpreting the severity of heart disease based on the intensity of the murmur may be greatly misleading. For example, in dilated cardiomyopathy (DCM) a mitral regurgitation murmur may be heard due to dilation of the heart and poor coaptation of the mitral valve cusps. This murmur is often soft because the heart muscle simply does not have the strength to create much pressure in the left ventricle compared to the left atrium. In fact, it may even reach a point where the difference in pressure in both chambers is so low a murmur is no longer audible, despite the presence of severe disease. Figure 1 In mitral valve the intensity murmur is usually proportional severity there aredisease, some exceptions. Asof in the DCM, myocardial failure will result to in disease a reduction of the( ), but murmur s intensity, despite more severe disease. Similarly, rupture of a tendinous cord may result in a markedly larger regurgitating orifice, which may, in turn, lead to a reduction in intensity of the murmur despite more severe disease. The smaller the regurgitating orifice and the higher the pressure difference between the left ventricle and the left atrium, the greater the turbulence created and the louder the murmur. In contrast, in some patients a very loud (5/6 or 6/6), often called musical murmur, may be noticed. These murmurs are associated with mild to moderate disease and are usually not present in severe cases. They are the result of strong vibrations of the mitral valve apparatus. So, the intensity of a murmur can be misleading, with soft murmurs present with severe disease and loud murmurs with mild disease. It is more prudent to assume a murmur signals an abnormality in heart function and that further investigation is needed to assess its significance. Duration Heart murmurs may be classified into systolic or diastolic murmurs (Figure 2). This is an important clue as to the underlying cause. Diastolic murmurs signal either semilunar (aortic/pulmonic) valve regurgitation or atrioventricular (mitral/tricuspid) valve stenosis. Systolic murmurs may be the result of obstruction to flow through the semilunar valves or outflow tracts (ejection murmur), atrioventricular valve regurgitation (plateau murmur) or a left-to-right shunting ventricular septal defect (plateau murmur; Figure 3). Ejection murmurs are characterised by an increase and subsequent decrease in intensity during systole; as such, they are also called crescendodecrescendo murmurs. Plateau murmurs have the same intensity throughout systole. Some murmurs are heard throughout systole and diastole. In 3 / 12

4 some cases, they are caused by a systolic murmur followed by a diastolic murmur (to-and-fro murmurs). The most common example is a systolic murmur caused by a left-to-right shunting ventricular septal defect accompanied by aortic regurgitation. Other examples are aortic or pulmonic stenosis, accompanied by aortic or pulmonic regurgitation respectively. A patent ductus arteriosus flow of blood through this communication creates a murmur that is always present (continuous murmur) and that increases in intensity throughout systole and tapers in diastole. This murmur is often described as a machinery murmur, as it resembles the noise of a machine working (Figure 4). Location Assessing the area of maximal intensity of a murmur is also important, as it may help to identify the affected valve. For example, a murmur most audible over the left apex is most probably related to abnormal mitral valve function. If this murmur is also systolic and has a plateau conformation, mitral regurgitation is the likely cause. In contrast, if it occurs during diastole with a tapering intensity it might be due to mitral stenosis. The same logic applies to the left heart base for the semilunar valves or the right heart apex for the tricuspid valve. What to do after classifying a murmur? As mentioned before, a murmur signals abnormal heart function. Not all murmurs are caused by heart disease, though. Blood flow within the heart may become turbulent in certain conditions, such as anaemia, high metabolic states (fever, hyperthyroidism, pregnancy) or in puppies throughout growth. The murmurs heard in these cases are commonly defined as functional, physiologic or innocent murmurs and have certain characteristics: An innocent murmur is soft (1-3/6), occurs during early systole and is audible over the left heart base. As tempting as it may be to automatically classify these murmurs as innocent, especially when dealing with otherwise healthy puppies, it is impossible to be sure there is no underlying cardiac disease. A few years ago, I examined the clinical records of all the canine patients aged below one that had been referred for investigation of a heart murmur. From 373 dogs, 74 presented with murmurs that could be classified as innocent. Interestingly, only 36 per cent of these dogs were indeed free of heart disease, which I found surprising. Surely, this data is from a cardiology referral centre and may not reflect the general population? These numbers might be very different if collected from a first opinion practice, but, nevertheless, they clearly illustrate that if we assume no underlying disease is present merely based on the characteristics of a murmur we will fail to detect heart disease in a significant number of patients. As such, it is my opinion, despite all the information and clues we might obtain from analysing heart murmurs, that further investigation is always warranted and should be offered to every client. 4 / 12

5 I always tell my clients a murmur means there is something abnormal in the heart itself or in its function. It might be significant, so it is very important to determine the cause of the murmur, whether there is reason to be concerned, what treatment (if any) is required, and lastly what the prognosis and best ways to monitor progression are. References Fox P R, Sisson D and Moïse N S (1999) The physical examination. In Textbook of Canine and Feline Cardiology, W B Saunders, Philadelphia. Kvart C and Haggstrom J (2002). Heart sounds and murmurs in dogs and cats. In Cardiac Auscultation and Phonocardiography in Dogs, Horses and Cats, VIN Publications. Haggstrom J, Kvart C and Hansson K (1995). Heart sounds and murmurs: changes related to severity of chronic valvular disease in the cavalier King Charles spaniel, J Vet Intern Med 9(2): / 12

6 Figure 1. Three examples of mitral valve disease and the resulting murmur: mild (A), moderate (B) and severe (C). Usually with this disease, the intensity of the murmur is proportional to disease severity, but there are exceptions. 6 / 12

7 Figure 1. Three examples of mitral valve disease and the resulting murmur: mild (A), moderate (B) and severe (C). Usually with this disease, the intensity of the murmur is proportional to disease severity, but there are exceptions. 7 / 12

8 Figure 1. Three examples of mitral valve disease and the resulting murmur: mild (A), moderate (B) and severe (C). Usually with this disease, the intensity of the murmur is proportional to disease severity, but there are exceptions. 8 / 12

9 Figure 2. Murmur duration. Heart murmurs may be classified into systolic or diastolic murmurs. They are usually further classified as to where in systole or diastole they occur. The terms proto, meso and tele are used for murmurs that occur in the beginning, extend into mid or end of systole or diastole respectively. Murmurs that are present from the first heart sound (S1 atrioventricular valve closure) to the second heart sound (S2 semilunar valve closure) for systole and vice versa for diastole are termed holo. If S1 and S2 are no longer distinguishable and only the murmur is heard the term pan is used. 9 / 12

10 Figure 3. Examples of the murmur shape. 10 / 12

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12 Figure 4. Continuous murmur. Echocardiographic image illustrating the continuous flow through a patent ductus arteriosus assessed by spectral Doppler (above) and the resulting continuous machinery murmur registered with a phonocardiogram (below). 12 / 12 Powered by TCPDF (