3.5.1 Classification of cardiac murmurs
Murmurs are classified in order to aid their identification and as a useful reference for subsequent examinations. Functional murmurs and murmurs associated with various pathological conditions have certain characteristics. Classification of murmurs helps to identify functional from pathological murmurs and to distinbetween the different types of pathological murmur. The classification is according to the timing and duration of the murmur, its quality (pitch, change in intensity), intensity (loudness), point of maximum intensity and radiation.
Timing and duration
Murmurs should be identified as being systolic or diastolic (or rarely, continuous).
The duration of systole and diastole can then be subdivided into the following
1. Early, mid or late (proto, meso or telo) systolic or diastolic. 2. Pansystolic (from the beginning of S1 to the end of S2). 3. Holosystolic (from the end of S1 to the beginning of S2). 4. Holodiastolic (from the end of S2 to the beginning of S1). 5. Presystolic (between the A sound and S1).
Diagrammatic representation of these murmurs helps to clarify their timing and acts as a useful reference for subsequent examinations (Figures 3.3-3.10).
The term 'character' or 'quality' of a murmur refers to its change in intensity, pitch and other subjective qualities. The changes in intensity of a murmur depend on the change in velocity of flow and, therefore, on the pressure gradient which is generating flow. The change in intensity is best illustrated by phonocardiography; however, it can also be appreciated on auscultation. Murmurs associated with AV valve regurgitation usually have a fairly constant intensity throughout systole because the pressure gradient between the ventricles and atria remains high throughout systole, and are termed plateau-type or band-shaped murmurs. The murmur associated with aortic valve incompetence gradually declines during diastole because the pressure gradient between the aorta and the LV falls away due to run-off into the systemic circulation and ventricle. These are termed decrescendo murmurs. In the great arteries, the velocity of blood flow rises and falls rapidly, and is greatest in early-mid systole. Consequently, murmurs assowith normal flow in these vessels increase and then decrease in intensity, are loudest in early-mid systole and are termed crescendo~decrescendo mur(although the crescendo period is short and can be difficult to identify clearly on auscultation). The murmur associated with a patent ductus arteriosus (PDA) is described as a waxing and waning murmur because of the changes in intensity caused by variation in the pressure gradient across the shunt.
Descriptive terms such as harsh, soft or musical can be used to describe a murmur. Additional characteristics such as variability in the intensity of a murmur at different heart rates may also be noted. The majority of murmurs are noises, i.e. they are a mixture of different frequencies of sound. A musical murmur is not a vague descriptive term, but a specific description of a murmur in which a specific frequency or frequencies of sound are detected. Musical murmurs are usually caused by resonating structures such as valve leaflets which vibrate and produce a a pure note of sound.
It is also possible to classify a murmur according to its variability during the course of auscultation. Some murmurs vary in intensity or quality on a beat to beat basis, others may vary depending on the heart rate.
The intensity of a murmur is graded on a scale of 1-6 (or 1-5). Each grade should be given in relation to the range used (e.g. grade 3/6). The grades are:
Grade 1: A quiet murmur that can be heard only after careful auscultation over a localised area. Grade 2: A quiet murmur that is heard immediately once the stethoscope is placed over its localised PMI. Grade 3: A moderately loud murmur. Grade 4: A loud murmur heard over a widespread area, with no thrill palpable. Grade 5: A loud murmur with an associated precordial thrill. Grade 6: A murmur sufficiently loud that it can be heard with the stethoscope raised just off the chest surface.
In general terms, loud murmurs are more likely to be significant than quiet murmurs. However, some murmurs are not associated with a large volume of abnormal flow but are very loud. This most often occurs when a structure is made to vibrate rapidly in the jet of abnormal blood flow. Often these murmurs are musical. Theoretically, a large abnormal communication may carry a substantial amount of low velocity blood flow generating a murmur which may be of low intensity, or even absent. However, this is rare and clinical signs are usually very severe before this stage is reached in the horse.
Point of maximal intensity and radiation
Confusion has arisen over the position of the point of maximal intensity (PMI) of sounds associated with the different valve areas. The point at which sounds are loudest does not correspond accurately with the anatomical position of the valves because sounds have to be transmitted to the body surface to be heard. Systolic sounds associated with the AV valves are best heard towards the apex beat area because they are transmitted by the stiff ventricular wall to the point at which it touches the chest wall. However, external anatomical landmarks are a rough guide to the location of underlying cardiac structures. The mitral valve area is located in the fourth or fifth intercostal space on the left side of the chest just dorsal to the level of the olecranon. The aortic valve area is located in the fourth intercostal space on the left side of the chest just below the point of the shoulder. To hear sounds in this area, the stethoscope must be pushed forward under the triceps muscle. The pulmonary valve area is located in the second or third intercostal space on the left side of the chest just ventral to the aortic valve area. The tricuspid valve area is located in the third or fourth intercostal space on the right side of the chest approximately midway between the level of the olecranon and the shoulder. Sounds associated with the tricuspid valve may also be heard on the left side of the chest in the second intercostal space at a similar level in some animals. These areas reflect the cranio-caudal orientation of the LV and the right-left orientation of the right ventricle (see section 1.2.1).
Although external landmarks are a general guide to the position of valves, there is a slightly variable relationship between these landmarks and the posiof the heart. In addition it is not always easy to identify the exact intercostal space on each occasion. For this reason, it is helpful to identify the location of the valves by identifying the characteristic sounds heard in each area. The mitral valve area is defined as the area where S1 is loudest, on the left side of the chest. The heart base is the area where S2 is most easily heard, although S1 may still be relatively loud in this area. The same principles apply on the right side of the chest. The tricuspid valve area is usually well cranial to the caudal aspect of the triceps muscle, if the horse is standing level. The stethoshould be advanced well into the axilla, with the right leg ideally posiin advance of the left.
The radiation of murmurs can be a helpful guide to their origin and is a rough guide to the severity of the condition. Functional murmurs are usually localised to a limited area. Murmurs associated with pathological processes can be localised or widespread. In general terms, the more widespread they are, the more sigthe degree of valvular incompetence. The radiation of a murmur often reflects the direction of the jet. The holodiastolic murmur of aortic regurgitation usually radiates ventrally on the left side of the chest, but may also radiate to the right side in some animals if the jet projects towards the interventricular septum. Murmurs of mitral regurgitation may radiate dorsally and caudally if the jet runs up the caudal aspect of the left atrium, dorsally and cranial if they are running up the cranial wall. In the latter case the PMI may appear to be towards the base of the heart because the jet is causing vibrations along the atrial wall where it borders the aorta.
In some cases, a fine vibration of the chest wall will be apparent on palpation. This is known as a precordial thrill. A thrill is associated with a high velocity jet, and is most commonly found in animals with a ruptured chorda tendinea of the mitral valve, when the jet may strike the left side of the left atrial wall (see section 6.4.1 and section 6.4.2) with aortic regurgitation, and in animals with a ventricular septal defect (Chapter 5). A precordial thrill is indicative of significant disturbed flow.
Using the classification described above, it is possible to categorise each murmur so that its probable origin can be decided. There are a limited number of conditions which commonly affect the equine heart, so a few, typical murmurs will usually be found (Figures 3.3-3.11). The identification of murmurs is sumin Table 3.4.