3.5.3 Cardiac murmurs associated with pathological conditions
Holosystolic and pansystolic plateau-type murmurs
Holosystolic and pansystolic plateau-type murmurs are associated with blood flow between two chambers when there is a fairly constant pressure gradient throughout systole. This occurs with AV valve regurgitation and ventricular septal defects (VSDs). In both of these situation, blood flows from a high-pressure chamber to a low-pressure chamber and produces a murmur of relatively constant intensity ('plateau' or 'band' type). A holosystolic murmur is one which lasts from the end of S1 to the beginning of S2 (Figure 3.7). A pansystolic murmur is one which lasts from the beginning S1 to the end of S2, and therefore obscures these heart sounds (Figure 3.8). The difference between them is academic in terms of the diagnosis. Pansystolic murmurs are often louder and more significant.
Late systolic crescendo-type murmurs
Late systolic crescendo-type murmurs are associated with AV valve regurgitation when the valve does not become incompetent until well after the beginning of systole. They may be associated with valve prolapse. They often have a musical element (Figure 3.9).
Holodiastolic murmurs last from the end of S2 to the beginning of S1 (Figure 3.10). They are associated with incompetence of the semi-lunar valves. Because the pressure gradient between the great arteries and the ventricles gradually falls during diastole, the murmurs fade in intensity and are decrescendo. Blood flow through an incompetent pulmonary valve seldom generates sufficient vibrations for a murmur to be heard at the body surface so, when a holodiastolic murmur is heard, it is almost certainly caused by aortic valve insufficiency. The PMI is over the left base, and the murmur usually radiates ventrally. It may be heard on the right side of the chest also. The character of the murmur can be quite variable. It may be low or high in pitch, and frequently has a musical element. It often has a cooing, rasping, blowing or buzzing nature. The murmur may have a distant quality. In some cases there will be an increase in intensity at the time of atrial contraction, associated with the jet of blood striking the anterior septal leaflet of the mitral valve (see section 6.5.3).
Continuous machinery murmurs
Continuous machinery murmurs are associated with blood flow through a ductus arteriosus. Usually this flow has ceased within a few days of birth; if it is patent after this period it is known as a PDA (see section 5.10). The murmur waxes and wanes in intensity during systole and diastole and is continuous because of the persistence of a variable pressure gradient between the aorta and the pulmonary artery throughout the whole of the cardiac cycle (Figure 3.11). The PMI of the murmur is over the left heart base, but the murmur can be almost as loud over at the right heart base. If a ductus arteriosus remains patent, it may lead to CHF. As pulmonary hypertension develops, the diastolic element of the murmur is lost because the diastolic pressures in the aorta and the pulmonary artery begin to equilibrate. The murmur is quite common in neonates, but PDAs are very rare in animals over a few days of age.
Murmurs in horses with colic
Murmurs typical of MR and functional murmurs may also be detected in horses during episodes of colic. The reason for this is not clear; however, it seems likely that it is related to altered fluid balance rather than to myocardial depression or primary valvular disease. In some cases, the murmur may have been present but undetected for some time. It is wise to re-examine animals in which murmurs are detected in these circumstances, after other clinical problems have been resolved, before any judgement is made as to their significance.