What is a heart murmur?

Normal heart sounds are often described as ‘lub dub’ and are caused by the valves between the chambers of the heart snapping shut to prevent blood leaking backwards as the heart beats.

The term ‘murmur’ refers to any abnormal sound that may occur either between or over the normal, rhythmic heart beats and is detectable when listening to the heart with a stethoscope. The presence of a murmur generally indicates turbulent blood flow somewhere within the heart. Many (but not all) murmurs are harmless, and they are very common in newborn foals in particular.

Why are murmurs common in

newborn foals?

During pregnancy the lungs of the foetus are inactive, as the foal does not yet need to inhale air to oxygenate its blood and organs. Instead, each time the foetal heart beats it pumps blood through the arteries in the umbilical cord to the placenta. Within the placenta the foal’s and dam’s red blood cells flow through tiny blood vessels that run adjacent to each other. In this way the blood of these two individuals does not mix, but the thin-walled vessels are so close to each other that oxygen is able to pass from the mare’s circulation into that of the foal.

The newly oxygenated blood flows back to the foal’s heart via the umbilical veins and is pumped around the body to nourish the developing organs. The oxygen is used up and the blood then returns to the foal’s heart. Once the foal is born and its lungs become functional, it will pump the deoxygenated blood from its heart to its lungs to pick up more oxygen. This system is not yet working in the foetus however: instead, there’s a small vessel called the ductus arteriosus running between the main arteries and veins of the lungs. The foal’s blood diverts through the ductus arterious, bypassing the lungs and makes its way back to the placenta via the umbilical cord to be oxygenated by the dam’s circulation again. Once the foal is born and takes its first breath, its lungs will inflate more or less instantly within the sealed chest cavity, causing the pressure within the chest to rise (as the space is now fully occupied by the enlarged lungs). The thin walls of the ductus arteriosus can’t withstand this rise in pressure and it begins to collapse in on itself, resulting in the blood being redirected to the lungs. The umbilical cord typically ruptures shortly after the foal begins to breathe for itself, causing the blood vessels within it to recoil and snap shut. Nature is amazing, as all this means that within a matter of minutes the foal has transitioned from relying on the placenta for oxygen to being able to breathe and exist independently.

Is a heart murmur always serious?

The ductus arterious normally takes 2-3 days to close completely and a small proportion of the blood being pumped from the heart to the lungs may continue to flow through it. If you listen to a foal’s heart with a stethoscope during this period, this flow may be detected as quite a loud murmur low down on the left hand side of the chest between the 3rd and 4th ribs. As long as the foal is otherwise alert and healthy, a murmur of this nature is considered a normal finding that typically disappears within 72 hours of birth. Occasionally a foal may be born with a congenital abnormality such as an opening between the left and right cardiac chambers (a ‘hole in the heart’) or some other malformation of the heart and/or major blood vessels. These animals typically begin to develop other signs of heart problems soon after birth. Indications that there may be a more serious issue include blue-tinged gums, laboured breathing, a weak and/or irregular pulse, weakness, lethargy, weight loss and ill thrift.

Where concerns exist about a foal’s heart or circulation, an ultrasound examination is typically carried out. This is a non-invasive way to visualise the function of the heart valves and evaluate the cardiac chambers and the blood flow through them. Horses have been bred for athletic performance for many generations, so we typically don’t see many cases of birth defects associated with the heart. They can arise spontaneously from time to time and the prognosis is generally poor, as treatment options are limited. Happily, the vast majority of heart murmurs detectable after birth rapidly resolve as the foal’s system matures and adjusts to life outside the uterus.