Page 719 - The Veterinary Care of the Horse

The aortic and pulmonary valves are together known as the semilunar valves.

VetBooks.ir The control of heart rate and rhythm

The heart is made up of specialized cardiac muscle cells. The stimulus for the heart to
contract is an electrical impulse that is received by the specialized cells of the sinoatrial node

(SA node) in the wall of the right atrium. This causes a wave of contraction to spread across
the atria, pushing the blood into the ventricles. The electrical stimulus then reaches the

atrioventricular node (AV node) at the bottom of the septum between the two atria. The signal
is briefly delayed here so atria empty and the ventricles fill with blood. Then the signal is
conducted down specialized fibres which cause the ventricles to contract in a regular and co-

ordinated fashion. The regular beating of the heart is known as sinus rhythm. Any deviation
from the normal rhythm of the heart is known as an arrhythmia.

The heart rate is controlled by the autonomic nervous system which makes adjustments
all the time without the horse being aware of it. The sympathetic branch of the autonomic

nervous system is responsible for speeding up the heart rate to meet the increased demands
for oxygen, e.g. when the horse is exercising. The parasympathetic nerves restore the heart

rate to the resting level.

The normal heart sounds

We tend to think of each heartbeat as having two sounds: ‘lub dup’. However, in most resting

horses it is possible to hear four different sounds when listening carefully with a stethoscope:
‘le lub dup dup’ making up each heart beat or cardiac cycle. Each of these sounds

corresponds to a particular event, e.g. the closing of valves or acceleration or deceleration of
blood that occurs as the blood flows through the heart. By positioning the stethoscope at

different sites on the chest wall, the vet is able to listen to the sounds coming from each of
the four valves individually. The mitral, the aortic and pulmonary valves are heard best on the

left side of the chest. Sounds from the tricuspid valve are listened to from the right side.
Each cardiac cycle can also be divided into:

• systole which is the part of the cycle where the ventricles contract

• diastole during which the ventricles relax and refill with blood.

Many heart abnormalities are detected at a routine veterinary examination for another reason,

e.g. during a prepurchase examination. The significance of detected heart abnormalities can
be difficult to evaluate and the horse may need more than one examination and/or referral to
a cardiologist.