Heart Sounds
The beauty of the larger livestock heart is that it beats more slowly, allowing the listener to appreciate each sound and the rhythm. Some sounds heard in large animals are not heard in smaller species. Murmurs, the sounds between the beats, occur in all animals and can be stable for years or indicate heart disease.
What am I hearing?
When it comes to cardiology in any animal or species, we start the examination with a sound and rhythm check. Function and physiology assessments are scrutinized further when the initial exam identifies something unusual because regurgitant jets or changes in heart appearance may not always be clinically relevant.
So, what are you hearing when you listen to the heart?
The loudest heart sounds are the “lub” and the “dub”, or S1 and S2 sounds that are associated with the closing of the atrioventricular (AV) and semi-lunar valves. Notice that we wrote “associated with” and not “caused by” the closure of the valves? You will still find publications stating that the sound we hear is the closure of the valve itself, like the slamming of a door. But the valves are thin, slightly elastic tissues therefore cannot make noise! Instead, we hear the turbulence in the blood flow (think “washing machine”), caused by a sudden direction change (acceleration or deceleration) as the pressure in the outflow space increases and pushes the upstream valves closed.
When you think of it based on turbulence, then the S1 sound is not the AV valve closure, but actually early associated with ventricular contraction that suddenly changes the blood flow direction, increases ventricular pressure enough to push the AV valves closed, and opens the semi-lunar valves. The S2 sound is then not the semi-lunar valves closing, but the slowing and backflow of blood towards the semilunar valves as the velocity of the blood slows, then causing closure of the semi-lunar valves.
What about S3 and S4?
These are normal in horses, cattle, and other large animals because the heart is very big, and the heart rate is slow. S3 is caused by rapid ventricular filling – a “whoosh” of blood falling in as the ventricle relaxes, and S4 is caused by ventricular filling due to atrial contraction – an brief push of blood.
Murmurs
Now that we have a better understanding of the sounds that we are supposed to hear, what about “other” sounds? Any additional sound that occurs between the lub and the dub, or between the dub and the lub, is called a “murmur”. A murmur is described based on location, intensity (grade), timing, duration, shape, quality, and radiation.
With just the location, intensity and timing, the source of the murmur can be often identified.
Grades:
Unfortunately, the severity or importance of the murmur does not always relate to the severity or “loudness” heard. Low grade murmurs are more likely associated with small jets but this does not prevent them from being clinically significant or impacting on heart function.
In contrast, louder murmur are associated with larger jets, and although they are more likely to impact on heart function, many remain stable whereas others suddenly decompensate.
For this reason, all murmurs are worth monitoring and possibly investigating further.
What to do next?
The identification of a cardiac abnormality is by far the most difficult step. Once a concern has been raised, the detailed description may either provide a likely underlying mechanical or physiological condition, or enough evidence to search the literature for recommendations. The ACVIM consensus statement for equine cardiology is a great place to start, but admitted is due for being updated, according to the authors.
In general, a murmur should be investigated at minimum with ultrasound, and ideally with echocardiography. Other diagnostics that may be of benefit include blood analysis: a general complete blood cell count (CBC) and serum biochemistry (with inflammatory SAA or fibrinogen), and/or the cardiac-specific troponin I (cTnI) test. The cTnI is becoming more easily available through commercial labs, but these labs will employ various testing methods creating inconsistency in the sensitivity of the result, especially at lower levels.
References
1) ACVIM Consensus Statement: Recommendations for Management of Equine Athletes with Cardiovascular Abnormalities, J Vet Intern Med 2014;28:749–761, DOI: 10.1111/jvim.12340
2)Hövener et al. 2021, Association between cardiac auscultation and echocardiographic findings in Warmblood horses. Animals, 11:3463. DOI: 10.3390/ani11123463
3) Colin C. Schwarzwald. 2019, Equine Echocardiography. Vet Clin North Am Equine Pract. 35(1):43-64. DOI: 10.1016/j.cveq.2018.12.008
4) Tanya M. Rossi,1 Peter A. Kavsak, M. Grant Maxie, David L. Pearl, W. Glen Pyle, Peter W. Physick-Sheard, Analytical validation of cardiac troponin I assays in horses Journal of Veterinary Diagnostic Investigation 2018, Vol. 30(2) 226–232, https://www.uoguelph.ca/CardiovascularResearch/PDF/2018/Rossi2018.pdf
5) Malin Eriksson, 2014, Initial Evaluation of the Meritas Troponin I test for Measurement of Equine Cardiac Troponin I, https://stud.epsilon.slu.se/7705/11/Eriksson_M_150316.pdf