It is a commonly held belief that murmurs which are inapparent after exercise are insignificant. In the author's view, this rule of thumb, while not wholly without foundation, is dangerous. Functional murmurs associated with ejection of blood through the semilunar valves are often variable in intensity at different heart rates. They are dependant on stroke volume and afterload, consequently they may vary if there is a change in heart rate. Sometimes these murmurs increase in intensity after excitement or exercise and may even become apparent in animals which are excited or exercised although they were absent at rest. Often ausafter exercise is hampered by extraneous noise and respiratory sounds. Under these circumstances it is possible to miss murmurs which may be of significance to the future performance of the horse. For example, a quiet holosystolic plateau-type murmur associated with mitral insufficiency may be difficult to hear at higher heart rates. While a quiet murmur of this type may not always be significant at the time of examination, it is important that it is detected and not attributed to normal flow.
High-pitched early diastolic murmurs are particularly likely to be increased in intensity at heart rates in the range of 3~60 bpm. Sometimes murmurs will be less apparent after exercise, but it is not clear whether this is due to more difficult auscultatory conditions, particularly respiratory noise and wind blowing on the stethoscope if the examination is performed outside. Decreased murmur intensity may also be related to other changes such as increased blood viscosity due to splenic contraction.
It is important that all the characteristics of a murmur are considered in evaluating its origin and significance. If murmurs are characterised according to the guidelines described above (see section 3.5.1) it should be possible to identify the source of most cardiac murmurs, to make a specific diagnosis, and then to make a reasoned judgement of the significance of the condition.