Physiologic flow murmurs are murmurs associated with normal blood flow through the heart and great vessels. Physiological flow murmurs over the aorta and pulmonary artery have been detected in a large percentage of horses and may be difficult to distinguish from murmurs of valvular regurgitation or from murmurs associated with other cardiac disease. The advent of echocardiography has enabled the veterinarian to precisely determine the cause of the murmurs and their significance.

Murmur Characteristics

Careful auscultation of each horse should be performed and the murmur(s) detected described in terms of their timing, duration, intensity, quality, pitch, shape, point of maximal intensity (PMI) and radiation. These characteristics of the murmur will help to distinguish the physiological flow murmur from the murmur associated with cardiac disease. The murmur's characteristics are particularly helpful in distinguishing a functional murmur from a murmur of valvular insufficiency when the murmur is of lower intensity. These findings will determine when echocardiographic evaluation is indicated to help determine the significance of the murmur detected. The phase of the cardiac cycle (systolic, diastolic or continuous) occupied by the murmur is its timing. The duration is the length of time the murmur is detectable during the cardiac cycle (early, mid, late, holo or pan systolic or diastolic). The murmur's intensity, 1-6/6, is determined by the blood flow through the murmur's origin (quantity and velocity), its distance from the stethoscope and the acoustic properties of the thoracic wall. The radiation of the murmur is usually from the murmur's origin in the direction of abnormal blood flow. The murmur's radiation is also determined by the murmur's intensity and physical characteristics of the chest. The shape of the murmur auscultated is determined by the phonocardiographic depiction of the intensity of the murmur over time. The quality of the murmur is harsh, coarse, rumbling, scratchy, musical, honking or blowing and may be high, medium or low pitched. Musical or honking murmurs are usually created by the vibration of an intracardiac structure such as a valve leaflet or chordae tendineae and have a narrow frequency range while harsh murmurs are usually mixed high and medium frequency murmurs.

The systolic physiological flow murmur is a low intensity (grade 1-3/6) murmur that occurs in early, mid or late systole or is a holosystolic ejection murmur. The murmur has its point of maximal intensity over the aortic (usually) or pulmonic valve with little or no radiation of the murmur. The murmur is usually low frequency and crescendo-decrescendo in quality, often described as soft and blowing. These murmurs are generated by ejection of blood through the great vessels (aorta and pulmonary artery) during systole and are common in horses because of their large diameter great vessels. The larger the diameter of the vessel, the more likely that flow through the vessel will be nonlaminar. Increased blood flow velocity and decreased blood viscosity also will increase the likelihood that nonlaminar blood flow will occur. Physiological systolic murmurs typically become more intense after exercise, although in some horses they are not detectable after exercise. Functional systolic murmurs also occur in horses with severe anemia or fever. In contrast, most murmurs associated with cardiac disease change little with excitement or exercise.

Early or late diastolic murmurs are physiologic but not murmurs that occur throughout diastole. These murmurs are decrescendo or squeaky, low intensity (grade 3/6 or less), with their point of maximal intensity over the aortic valve area. Early diastolic murmurs are more frequently ausculted in racehorses than in other types of performance horses and are more common in younger horses.

Mitral Regurgitation

Mitral regurgitation murmurs are usually harsh, plateau (band) shaped, > grade 3/6 (but can be a grade 1-2/6), holosystolic or pansystolic with their point of maximal intensity at the left atrioventricular valve area radiating dorsally and cranially or caudally. Mitral valve prolapse usually produces crescendo mid to late systolic murmurs while honking or harmonic musical systolic murmurs are found in horses with ruptured chordae tendineae. Mid or late systolic clicks may also occasionally be heard in horses with mitral valve prolapse. The loud crescendo mid to late or holosystolic murmur detected in colicky horses is usually associated with mitral valve prolapse and volume depletion. This murmur usually resolves or improves significantly with correction of the colic.

There is no direct correlation between the intensity of the systolic murmur and the severity of the mitral regurgitation present. Murmurs of mitral regurgitation have also been reported to increase in prevalence and intensity in horses in race training. In general, however, murmurs of mitral valve prolapse that are crescendo in quality and only present in mid to late systole have only mild mitral regurgitation associated with them and tend not to progress or the amount of mitral regurgitation increases very slowly over many years. The pansystolic band shaped mitral regurgitation murmurs that are grade ≥4/6 are usually associated with significant mitral regurgitation (at least moderate). Murmurs of mitral regurgitation often become softer as the mitral regurgitation becomes more severe. An echocardiogram is always indicated in all horses with a grade 4-6/6 holosystolic or pansystolic murmur is detected, when the murmur is band shaped and holosystolic or pansystolic (regardless of its intensity), when the horse is in atrial fibrillation, when there are clinical signs of cardiovascular disease or when the horse has poor performance or respiratory signs consistent with lower airway disease. The valvular insufficiency most likely to cause clinical signs is mitral regurgitation. Once the mitral regurgitation becomes hemodynamically significant, it is likely to affect performance, particularly for more rigorous types of athletic endeavor.