6.2.2 Clinical examination
The murmur associated with MR is typically holo- or pansystolic. This is because regurgitation starts as soon as the pressure of the left ventricle (LV) rises above that of the LA and lasts until the end of systole. The character of the murmur is usually described as plateau or band-shaped (see section 3.5.1). This is due to the fact that a relatively constant pressure gradient exists between the left ventricle and the LA throughout systole. In some animals, particularly those with MV prolapse, the murmur may be restricted to a limited period of systole. Murmurs associated with valve prolapse are typically loudest in late systole, are crescendo in character, and may have a musical or honking quality.
Typically, the point of maximal intensity of a murmur (PMI) associated with MR is over the apex beat area of the heart, but it may radiate dorsally and craup towards the base, or sometimes in a dorsal and caudal direction (Figure 6.1). The pattern of radiation of the murmur will depend on the position of the regurgitant jet within the LA. Some very loud murmurs of MR are loudest over the cardiac base in the region of the aortic valve. This may lead to some conas to their origin. The pansystolic plateau-type characteristics remain, and the localisation of the PMI to this area is likely to be because of a cranially and dorsally directed jet running up the aortic aspect of the LA.
The intensity of murmurs which result from MR may be relatively quiet or very loud. They are usually at least grade 2/6, are most often grade 3 or 4, but may be associated with a precordial thrill and murmurs of grades 5 or 6/6.
Some cardiologists have described diastolic murmurs which they have attribto mitral stenosis. There is, however, no documented evidence of a true stenosis seen on M-mode, two-dimensional or Doppler echocardiographic (DE) examination of these animals. Volume overload of the LV and LA, which may result from significant MR, can cause an increase in the intensity of S3 due to the increased volume of blood flowing through the MV. This finding can be a useful clinical observation, although S3 may also be loud in animals with LV eccentric hypertrophy caused by exercise training.
Additional characteristics of the murmur associated with MR can be identified in some animals. Quiet musical murmurs are heard in some horses, often in younger animals with no other signs of cardiac disease. The significance of these murmurs is often particularly hard to define. They are unlikely to be of any clinical significance at the time of examination, but it is unclear whether they progress to cause significant valvular incompetence later in life. In animals with rupture of the chordae tendineae the murmur may have a vibrant, rasping quality and may be associated with a precordial thrill.