Overview
The equine heart (cor) is conical, with a blunt apex formed by the left ventricle. Wrapped in the pericardium, it fills the middle mediastinum, roughly between the 3rd and 6th ribs, mostly left of the median plane. The base, where the great vessels leave, lies halfway up the thorax; the apex points ventrally, close to the sternum and the diaphragm. The cardiac notch of the left lung leaves an area where the heart touches the thoracic wall under the thoracic limb.
Parts
- Right atrium and ventricle: receive blood from the venae cavae and the coronary sinus and pump it into the pulmonary trunk.
- Left atrium and ventricle: the left atrium receives the pulmonary veins; the thick-walled left ventricle forms the apex and ejects blood into the aorta.
- Valves: tricuspid (right atrioventricular), mitral (left atrioventricular), pulmonary and aortic.
- Fibrous skeleton: the horse has cardiac cartilages (cartilagines cordis) in the aortic ring, which may ossify in old animals; it lacks the constant cardiac bones of cattle.
- Coronary arteries: the circulation is balanced; the left coronary artery gives the paraconal interventricular branch and the right one the subsinuosal branch.
Clinical relevance
- Auscultation: the mitral area is at the left 5th intercostal space, the aortic at the 4th and the pulmonary at the 3rd, under the triceps; the tricuspid is heard on the right, at the 3rd or 4th space. The resting heart rate of the adult is around 28–44 bpm.
- Physiological arrhythmias: second-degree atrioventricular block and sinus arrhythmia are common at rest because of high vagal tone and disappear with exercise or excitement.
- Atrial fibrillation: the most common clinically important arrhythmia in the horse. It may go unnoticed at rest but reduces performance at high-intensity exercise; it is treated with quinidine or transvenous electrical cardioversion.
- Valvular regurgitation: murmurs from aortic and mitral regurgitation are common in older horses; echocardiography grades their severity.
- Toxic cardiomyopathy: ionophores (monensin and others), found in cattle or poultry feed, are highly toxic to horses and cause myocardial necrosis.