What Makes a Normal Heart Beat?

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The heart is a large muscle with four chambers. There are two top chambers (left and right) called the atria and two bottom chambers (left and right) called the ventricles. These chambers fill with blood from the body and then contract to circulate or deliver poorly oxygenated blood to the lungs (right side) or well-oxygenated blood to the rest of the body's tissues (left side). The heart muscles contract after they are stimulated by an electrical impulse. A small area of special tissue in the right atrium called the sinoatrial (SA) node starts an electrical impulse (it’s like the heart’s spark plug) that travels out through the atrial muscle, stimulating that muscle, and, simultaneously down special electrical tracts (AV node, Bundle Branches, Purkinje Fibers – the heart’s “wiring”) to the ventricles and ultimately out into the ventricular muscle (see Figure 2 below).

The electrical impulses cause the heart to contract in a coordinated fashion: the atria contract first, because they get stimulated first. As they contract, they push blood into the ventricles. Then, as the electrical signal reaches the ventricles, they also contract and push blood to either the lungs or the rest of the body. This coordinated electrical activity can be recorded on an electrocardiogram (ECG or EKG). There are several components to the normal ECG, called waves or complexes. They are labeled P, QRS and T. The P waves represent the atrial stimulation, while the QRS waves represent the ventricular stimulation. The T waves represent the re-setting of the ventricular muscles, so that they can be stimulated by the next impulse. The short pause between the P waves and the QRS waves is the time that the impulse is traveling along the wiring system from the atria to the ventricles.

Figure 2. Graphic representation of a normal electrical impulse starting in the sinoatrial node (SA) and traveling through the right atrium (RA) and left atrium (LA) , through the AV node and right and left bundle branches to cause a causing the heart muscle to contract in an organized way. RV=right ventricle, LV=left ventricle.

 
Atrial fibrillation (sometimes called “A fib”) is an arrhythmia, an irregularity of the heart’s rhythm.  Instead of the SA node (the spark plug) starting and coordinating the electrical signal, many areas within the atria send out electrical impulses in a rapid, disorganized manner.  This whirlwind of electrical impulses or wavelets spreads over the atrial tissue causing the atrial muscle to quiver or fibrillate, instead of contracting in an organized, structured manner (Figure 3). Only a fraction of the electrical impulses are able to jump onto the “wiring system” and get across the AV node and down into the ventricles to make the ventricles contract.  The pattern with which all these abnormal atrial impulses are conducted through the AV node is irregular making the ventricles beat irregularly.  

Figure 3. Graphic representation of atrial fibrillation with whirlwind of electrical impulses forming in the right (RA) and left atria (LA) instead a single organized impulse from the sinoatrial (SA) node causing the atrial muscle to fibrillate (quiver) instead of an organized contraction to fill the right (RV) and left ventricles (LV).

What Causes Atrial Fibrillation?

In humans, atrial fibrillation can occur for no apparent reason (often in athletes or older people). This is because the size of our hearts is large enough to allow such abnormal rhythms to develop (veterinarians also see this type of atrial fibrillation in horses, which have large hearts). However, most pets have much smaller hearts than humans, so for atrial fibrillation to occur in your pet, the atrial chambers must usually be bigger than normal. This enlargement of the atrial chambers occurs with many types of heart disease, such as degenerative valve disease of the mitral valve or cardiomyopathy, dilated cardiomyopathy and hypertrophic cardiomyopathy. Generally, the smaller your pet, the more significant the heart disease must be to cause atrial fibrillation.  Thus, atrial fibrillation is much less common in cats than in dogs. And it’s less common in small breed dogs than large breed dogs.

Occasionally, giant and large-breed dogs develop atrial fibrillation without any other heart disease, similar to what we see in humans and horses (let’s face it, some Great Danes are almost as big as horses!). This is termed “lone” or primary atrial fibrillation.  In some cases, this is “lone” atrial fibrillation turns out to be the first indication of dilated cardiomyopathy; in other cases, the arrhythmia exists without underlying heart disease.

How Is Atrial Fibrillation Diagnosed?

Often, the first idea that a pet has atrial fibrillation is when your veterinarian detects an erratic heart rhythm while listening to the heart with a stethoscope. Many veterinarians describe the sound as bongo drums or shoes in a dryer. Your pet’s pulse quality may also be irregular. However, to confirm the diagnosis an electrocardiogram (ECG) needs to be performed (because other arrhythmias can sound like atrial fibrillation. There are specific ECG criteria that must be met prior to making the diagnosis.

At times, a pet with severe heart disease may suddenly become weaker, or have a relapse of heart failure upon development of atrial fibrillation, prompting you to visit your veterinarian. Again, the veterinarian will auscultate the abnormal rhythm and diagnose atrial fibrillation via an ECG examination.

In giant breeds with lone atrial fibrillation, the arrhythmia may be detected on routine examination – often the owners are unaware that any problem exists. Some dogs may develop mild exercise intolerance when they develop atrial fibrillation.

A normal ECG

What Happens When a Pet Develops Atrial Fibrillation?

People with lone atrial fibrillation typically require treatment to re-establish a normal heart rhythm because they usually have exercise intolerance or shortness of breath, symptoms of palpitations, and are at an increased risk of having a stroke. If a normal rhythm cannot be re-established, people are often placed on “blood thinners” to reduce the risk of stroke. However, dogs with lone atrial fibrillation rarely show exercise intolerance or shortness of breath and do not suffer from strokes, so treatment is rarely required. Their heart rate remains normal in most cases.

However, in a pet with serious heart disease, atrial fibrillation causes a worsening of clinical signs, and can further damage the heart. Cardiac output, or the amount of blood pumped out of the heart to the rest of the body, is reduced and often congestive heart failure develops or recurs. These pets usually have very fast heart rate (more than 200 beats per min) which, if not controlled, can lead to additional heart muscle damage and dysfunction.

How Is Atrial Fibrillation Treated?

Treatment in people is aimed at re-establishing and maintaining a normal rhythm, because this greatly reduces the risk of strokes. This is usually achieved by delivering a “shock” to the heart (termed electrocardioversion), or by using antiarrhythmic drugs. Recent studies in people suggest that re-establishing a normal rhythm is not essential and medical therapy might suffice if clinical signs are tolerable or absent.

In pets, cardioversion is generally not feasible. Several studies have examined electrocardioversion or chemical cardioversion in dogs with lone atrial fibrillation – none has shown consistent success. Newer studies are being conducted that may ultimately provide a technique that allows a normal rhythm to be established in these pets.

Pets with severe heart disease are generally not candidates for cardioversion. In these pets, a normal rhythm can rarely, if ever, be re-established. Instead, the veterinarian attempts to reduce the high heart rate with antiarrhythmic drugs. By reducing the heart rate (which is often more than 200 beats per minute), the veterinarian allows the heart to pump more efficiently, and prevents some of the damage that the high heart rate can produce. Additionally, the clinical signs associated with the development of atrial fibrillation (that is, congestive heart failure) resolve when the rate is controlled. The drugs that decrease the heart rate need to be given indefinitely to maintain rate control and will generally be added to drugs already being administered. Studies have shown that dogs in which the heart rate is reduced to 120 beats per minute (average over a day) do better than dogs in which the heart rate is not as well controlled. In other words, the closer to “normal” your veterinarian can bring the heart rate, the better the outcome.

People with atrial fibrillation are usually treated with anti-coagulants, such as warfarin, or other similar drugs (commonly, but incorrectly, termed blood thinners), to reduce the incidence of strokes, which can be devastating or fatal. However, dogs with atrial fibrillation do not appear to have increased risk of strokes and are therefore not treated with anti-coagulants. Cats with atrial fibrillation usually have severe heart disease, and are often treated with anti-coagulants whether they have atrial fibrillation or not.

What is the prognosis for a pet with atrial fibrillation?

Lone atrial fibrillation, like we see in large or giant-breed dogs, is well tolerated, even if we cannot convert it back to a normal sinus rhythm. In dogs with underlying severe heart disease, 50 percent of those with higher heart rates (more than 125 beats per minute) in one study had died from their heart disease within four months. On the other hand, 50 percent of those with lower heart rates (less than 125 beats per minute) lived to at least three years. Whether or not these numbers reflect what will happen with all dogs is hard to say, but we can say that bringing the heart rate close to normal improves survival.