3.5.2 Functional cardiac murmurs
Early-mid systolic ejection-type murmurs
Early-mid systolic ejection-type murmurs are heard in up to 60% of normal horses. They occur in all breeds and types, but are often louder in athletic horses and foals. There is no evidence that they are associated with any abnormality. They are caused by high velocity blood flow during the ventricular ejection phase. They are classically described as crescendo-decrescendo, however, the crescendo element may be lost in S1. In some animals the murmurs sound like a rather long and fuzzy S1. The most important features in detecting these murmurs are that:
- They finish before S2, 50 a silent period can be heard between the end of the murmur and the beginning of S2 (Figure 3.4).
- They are usually medium- or high-pitched in character.
- They are loudest over the heart base.
- They are localised in radiation.
- They may be variable in intensity at different heart rates, different R-R intervals and on different occasions.
These murmurs are most often heard on the left side of the chest, but may also be heard on the right side.
Early diastolic murmurs
A distinctive functional murmur can be heard in early diastole in some horses. This is most often found in fit young animals and has been described as a 'two-year-old squeak'. The murmur is squeaky is quality, but can be found in any age of horse. The exact cause of the murmur is unclear. It may be related to rapid early diastolic flow, to a small leak at the aortic valve as the valve closes, or to another unknown cause. The murmur is high-pitched, with a distinctive 'whoop' sound. It may be more obvious at slightly increased heart rates, and for this reason may be detected after light exercise in some animals when it was absent at rest. The important features which identify it and distinguish it from pathodiastolic murmurs are that it is very short in duration, occurring between S2 and S3 (Figure 3.5). It may be heard on the left or right side of the chest, or both. There is no evidence that it is associated with any valvular or myocardial abnormality.
The phase of presystole occurs between atrial systole and ventricular systole. Because this period is relatively short, the murmur may be difficult to distinguish from the A sound and S1 (Figure 3.6). The murmurs are short and grating in quality, and may be heard on either side of the chest. The cause of presystolic murmurs is unclear. Evidence from Doppler echocardiography suggests that they may be due to leakage of blood into the atria at the time of AV valve closure. The amount of blood leaking at this time is of no physiological significance, so they are not considered to be a cause for concern.