The heart is the most important muscle in the body – without the heart, none of the other muscles would get the fuel that they need – oxygen. The heart is also a complex muscle that must operate constantly throughout the life of an animal. It must contract and relax in a rhythmical and predictable fashion to pump blood through the body. It does this by receiving electrical signals from two nodes within the heart: the sinoatrial (SA) node and the atrioventricular (AV) node.
The SA node is a collection of muscle fibers and nerve endings in the wall of the right atrium, the holding chamber for blood, where the rhythm of cardiac contraction is established. It is programmed to discharge or start the electrical circuit, which causes the heart to beat in the horse 28 to 44 times per minute at rest. During exercise, of course, the SA node discharges far more frequently, up to 240 times per minute. The interval between each discharge is very predictable in the normal horse – it is frequently likened to a metronome, which lets musicians keep a reliable beat to their music. The SA node establishes a rhythm and is known as the heart's pacemaker.
The AV node is a mass of muscle fibers located in the wall between the atria and the ventricles, the pumping chambers, that transmit the cardiac impulse from the SA node to the muscular walls of the ventricles. The electrical impulse travels through nerves called the Purkinje fibers throughout the ventricles in an explosive fashion, and causes the ventricles to contract. This final event is what causes blood to course through the body in its rhythmical, predictable way.
We hear the results of this cardiac activity as the heart sounds – primarily S1 and S2, which sound like 'lub-dub'. S1 is the sound that marks the approximate beginning of the contraction, and S2 is heard at the end of the contraction when the ventricles begin to relax.
The Importance of a Steady Heartbeat
The job of the heart is to pump blood filled with oxygen to the entire body – and certain areas of the body, especially the brain, cannot survive without oxygen for more than a few minutes. So if the heart were to stop completely, the brain and consequently the individual would die. If the heart develops an arrhythmia, that is it does not beat rhythmically, the result is a decreased blood flow and consequently lower oxygen levels delivered to the tissues.
As the heart beats, it performs two functions. In the period between S1 and S2 (from lub to dub), known as systole, the heart pumps blood from the ventricles into the body. We feel this pumping of blood as the pulse. In the period between S2 and S1 (from dub to lub), known as diastole, the heart relaxes – and this relaxation period is vital to the proper function of the heart as a pump. It is during diastole that the heart fills with blood returning from the body – without this relaxation period and return of blood to the heart, there would be nothing for the heart to pump out. If the electrical impulses to the heart become erratic, then the heart, in essence, becomes disorganized and doesn't know when to relax and when to contract. The heart pump loses its direction.
If your horse develops an arrhythmia, your veterinarian will want to evaluate your horse's cardiac system. As usual, the first step is to perform a careful physical examination. Your veterinarian can look for outright signs of cardiac failure that may occur when an arrhythmia is severe. The veterinarian will also listen carefully to evaluate the rhythm of the heart. As she listens, she will palpate the pulse, to see if every audible beat results in a spurt of blood coursing through the vessels. If there are skipped pulses, then it is likely that the electrical impulse to the heart may have been a useless one.
The definitive way to evaluate the rhythm of the heart is with an electrocardiogram, known as an ECG or EKG. The ECG takes advantage of the fact that the heart beats according to an electrical impulse that can be measured using a set of electrodes. The ECG produces a series of recognizable waveforms – labeled p, q, r, s, and t.