Discussion

Introduction:
The ability to accurately diagnose the cause of a cardiac murmur significantly improves with training, although the ability to hear the physical properties of the murmur remains unchanged. The challenge in diagnosis is not due to a failure of hearing, but an inability to locate an abnormality in the live animal and relate this to the underlying pathophysiology. Getting the most out of your stethoscope involves learning how to identify normal heart sounds and rhythms and developing a thorough knowledge and understanding of cardiovascular physiology.

Identification of normal heart sounds and rhythm:
It is necessary to identify the normal heart sounds to accurately time and therefore understand the likely origin of any abnormal sounds. Palpation of the left cardiac area especially in slim athletic horses will reveal the apex beat (mitral valve area). Auscultation at the mitral valve area will reveal the relatively long first (S1) and shorter, higher pitched second (S2) heart sound in all horses, marking the start and end of systole respectively. Time spent initially counting the heart rate and appreciating the cardiac rhythm will avoid misdiagnosis. The fourth (S4) and quieter, low frequency third (S3) heart sound are commonly heard over the mitral valve area in horses. The S4 (atrial contraction) occurs just before S1 giving the le lubb sound at the start of the cardiac cycle; S3 marks the end of the rapid filling phase of the ventricle and occurs shortly after S2. Appreciation of the timing, origin and intensity of these sounds helps in the diagnosis of dysrhythmias, physiological murmurs and the severity of mitral regurgitation.
Starting from the mitral valve area it is possible to identify all other valve areas by auscultation. When the stethoscope is slid dorsal and cranial under the triceps mass S2 becomes very short and distinct (aortic valve area). Sliding the stethoscope more ventral so that it can be pushed more cranial under the triceps mass will reveal the pulmonary area where the 2 components of S2 (of pulmonary and aortic origin) can often be distinguished.
The tricuspid valve is located over the right hemithorax slightly more cranial to the mitral valve area on the left. An apex beat is not usually palpable on the right side of the chest except in very narrow chested horses or in horses with ventricular enlargement.

Differentiation of normal flow (physiological) murmurs from abnormal murmurs:
The timing of the murmur within the cardiac cycle indicates which valves are open, which are closed and what the normal direction of blood flow is. The location of the murmur or point of maximum intensity (PMI) on the chest wall indicates the likely valve area which is the source of the murmur. Valvular stenosis is very rare as an isolated finding in horses and can be discounted as a differential diagnosis.

Left sided systolic murmur:
Is it mitral regurgitation (MR) or a murmur caused by the normal flow of blood out of the ventricle? This can easily be decided if the point of maximum intensity (PMI) of the murmur is localised either over the mitral valve or the aortic valve. If however the PMI of the murmur cannot be clearly identified, the different timing of these 2 systolic murmurs will aid diagnosis. At the start of systole, the pressure in the ventricle rises steeply, exceeds atrial pressure and seals the mitral valve. The ventricular pressure remains higher than the atrial pressure until after S2. Therefore, if the mitral valve leaks, it can do so, for as long as ventricular pressure exceeds atrial pressure, i.e. from the onset of S1 until immediately after S2 (pansystolic). Unfortunately not all murmurs of MR fit this description, as it is possible for the valves to leak for only part of systole. The murmur caused by normal outflow (ejection) can only start when the ventricular pressure exceeds aortic pressure and will end when these pressures are reversed in the last third of systole. Therefore the timing of an ejection murmur is early to mid-systolic, always ending before S2. If the murmur of mitral regurgitation also ends before S2, these murmurs can be difficult to differentiate and diagnosis is then based on the PMI and the variable nature of functional murmurs with changes in sympathetic tone. Functional murmurs do not radiate widely and are not associated with a thrill. NB Horses with colic may have loud ejection murmurs which are audible throughout systole.

Right sided systolic murmur:
Is it tricuspid regurgitation (TR) or a ventricular septal defect (VSD)? The PMI of the most common type of VSD is cranial to the tricuspid valve area rather than directly over the tricuspid valve. Unlike TR, the murmur of the VSD radiates ventrally towards the sternal border. Depending on the volume of flow through the VSD, a systolic ejection murmur may be audible over the pulmonary valve. The murmur of the VSD is always present throughout systole therefore a short (early, mid or late) right sided systolic murmur must be TR.

Diastolic murmur:
Is it a physiological filling murmur or aortic insufficiency? The physiological filling murmurs are only audible when the ventricle is filling; during the rapid filling phase between S2 and S3 (early diastolic murmur) and following the atrial contraction between S4 and S1 (presystolic murmur). The filling murmurs are normal findings in young athletic horses. They are variable, tending to come and go, are always localised and are never associated with a thrill. The early diastolic filling murmur often presents as a distinct squeak audible at S3. Loud early diastolic filling murmurs may indicate severe mitral regurgitation. The murmur of aortic valve regurgitation (AR) is a distinct decrescendo shaped murmur with the PMI over the aortic valve area. As AR becomes more severe, the murmur becomes audible on the right side of the chest. The murmur will extend beyond S3 and may increase in intensity after S4. This murmur is often very musical and unlike with nonmusical murmurs the radiation and intensity will not relate to severity.