The next time the radio is on, tune to a Country/Western music station and listen to the lyrics. Some people say that all country music songs are about working hard, your mother (or father), the railroad, your dog, drinking, breaking up, or just hanging out. I think most country music songs are about love. Some of these songs talk of “a pounding in my chest,” “heart beating like a drum,” and even of a “pulse racing like a locomotive” (there’s that railroad thing again!). All of these feelings are wonderfully normal for someone in love. However, when these sensations occur unprovoked, there may be something wrong with the heart rhythm. This publication was written for people who may have a heart rhythm disorder, also known as an “arrhythmia” (pronounced ah-RITH-me-uh).
The heart is a fascinating organ! Some people even say that it is the center of the soul, but that debate is best left for the theological web sites to engage in. Your heart can usually perform a terrific job of pumping blood for many years with a minimum of trouble. In fact, if you figure that the normal heart beats approximately 72 times every minute, that adds up to nearly 38 MILLION beats every year! So, it’s not surprising that after 30 or 40 years “the engine don’t quite run too smooth.”
As mentioned above, the heart is a pump. The walls of the heart are actually made of muscle tissue. When the muscular walls of the heart contract, the blood inside the heart gets squirted out into the arteries of the body. Remember from grammar school biology that the arteries bring blood away from the heart and the veins bring the blood back to the heart. The heart has four chambers, two on the left side and two on the right side. Each side (left and right) is further divided into a receiving chamber (atrium) and a pumping chamber (ventricle). The atria and ventricles are separated by special one-way valves that keep the blood flowing in the proper direction. The right side of the heart pumps blood to the lungs (via the pulmonary artery) and the left side of the heart pumps blood to the rest of the organs (via the aorta, which is the main artery of the body). These organs include the brain, the kidneys, the stomach, the intestines, and so on, as well as the skin, bones, and muscles of the entire body. The lungs, of course, serve to transfer oxygen from the air we breathe to the red blood cells, which carry the oxygen on a substance called hemoglobin. The lungs also serve to eliminate excessive carbon dioxide, a by-product of our metabolism. All of our organs require a constant supply of oxygen and nutrients to function properly. Since these substances are carried by the blood, the continuous circulation of blood is vital to maintain all of our bodily functions. The amount of blood the heart pumps every minute is known as the “cardiac output,” which is generally measured in liters per minute. If the cardiac output decreases for any reason, the organs may fail to function correctly. In the case of the brain, for example, if the cardiac output drops significantly the brain will be deprived of oxygen, and this can cause lightheadedness, weakness, and even loss of consciousness (fainting).
The heart also needs a constant blood supply. As mentioned above, the left side of the heart ejects blood into the aorta, the main artery of the body. The first branches of the aorta are the small arteries that supply blood to the heart itself. These are the “coronary arteries.” One interesting bit of trivia is that of all the organs of the body, the heart is unique in that the arteries that feed this organ run on the outside and branch inward, diving into the muscle to supply the deeper tissues. All other organs’ main blood supply is carried to the center of the organ, from where the blood vessels branch outward. The three main branches of the coronary arteries surround the heart. In fact, the coronary arteries got their name from the word “corona.” Coronary artery disease refers to the formation of fatty plaques on the inside of the arteries resulting in a narrowing of the opening (called the arterial lumen), similar to a blocked pipe. No one knows what causes these “blockages” to build up. The process of narrowing of the arteries is generally referred to as “arteriosclerosis,” or more correctly “atherosclerosis.” Certain risk factors increase one’s chance of developing blockages. These risk factors include hypertension, diabetes, smoking, an elevated cholesterol level, and a family history of coronary artery disease. If the blockages are severe enough to restrict the blood flow to the heart, then the heart muscle suffers from a lack of oxygen (a condition known as ischemia, pronounced iss-KEY-mee-ah). This is often perceived as a painful sensation or pressure in the chest, called angina (ann-JIY-nah), which often occurs with exercise and is relieved by rest. If the artery gets occluded completely, blood flow to the muscle fed by that artery stops entirely and the muscle dies, forming an area of scar tissue on the heart. This process is known as myocardial infarction or “MI” for short and is known by it’s more common name, a Heart Attack. A heart attack is usually associated with crushing chest pain that can radiate down the arms or up to the jaw, but 10-15% of heart attacks are “silent” since they go unrecognized as such until months or years later when the scar is picked up on a routine test. Once a heart attack occurs, that part of the heart is never able to pump again. This can reduce the cardiac output and in severe cases cause congestive heart failure, a condition where fluid builds up in the lungs because the heart cannot circulate the blood adequately. Another cause of congestive heart failure is known as cardiomyopathy. Cardiomyopathy simply means a disease of the heart muscle and can be due to a number of reasons, including ischemia, valvular heart disease, excessive alcohol use, or what’s referred to as “idiopathic cardiomyopathy.” The word idiopathic means that we just don’t know what causes it. In some cases, idiopathic cardiomyopathy was due to a severe viral infection that left the muscle weak. However, recent data suggests that there may be a genetic basis to many forms of cardiomyopathy. Heart muscle problems like myocardial infarction and cardiomyopathy can permanently reduce the cardiac output. On the other hand, a temporary decrease in the cardiac output can result from disturbances in the heart rhythm.
The Cardiac Cycle
Let’s take a deeper, more detailed look at how the heart beats, and what controls the rhythm. The various and complex set of events that occur with each heart beat are referred to as the “cardiac cycle.” As mentioned in the previous section, the top chambers of the heart are called the atria and the bottom chambers (that do most of the work of pumping) are called the ventricles. The job of the atria is to receive blood from the veins and store it prior to each heart beat. The left atrium receives blood from the lungs via four separate pulmonary veins, and the right atrium receives blood from the main body veins called the “vena cavae” (the superior vena cava carries blood from the head, upper chest and arms, while the inferior vena cava carries blood from the lower chest, abdomen and legs). The normal heart beat begins in the top chambers. Both atria contract and push the stored blood across a pair of one-way valves into the ventricles to help fill them. The valves are known as atrio-ventricular (AV) valves and include the mitral valve, located between the left atrium and the left ventricle, and the tricuspid valve, which separates the right atrium from the right ventricle. A split second after the atria contract, the ventricles begin to contract. As the pressure in the ventricles increases, the AV valves close (to prevent blood from flowing backwards). The sound made by the AV valves closing is known as the first heart sound (the lub of “lub-dub”). With continued contraction of the ventricles the blood is ejected out the arteries though another pair of valves – the aortic and the pulmonic valves (on the left and right sides of the heart respectively). At the end of the cardiac cycle, all the chambers relax, the aortic and pulmonic valves close (making the dub sound) and keeps blood from flowing backwards into the heart, and the atria begin to fill up with blood again to start the next cycle.
The coordinated contraction of the various chambers of the heart is actually controlled by the electrical system of the heart (most people don’t even realize that the heart has an electrical system). Disturbances within the electrical system are the cause of all arrhythmias, and the medical specialty dealing specifically with heart rhythm disorders is known as Cardiac Electrophysiology (known as “EP” for short). In fact, Cardiac Electrophysiology was the first sub-subspecialty of cardiology that was officially recognized by the American Board of Internal Medicine – the ABIM has offered a “Board Certification” examination in Clinical Cardiac Electrophysiology since 1992.
The electrical signal that initiates each normal heart beat arises from a small structure located at the top of the right atrium. This structure is called the “Sinus Node” or the “Sinoatrial Node” (pronounced sign-o-AAY-tree-uhl Node). The sinus node is known as the “natural pacemaker” of the heart because it is normally responsible for controlling the heart rate (it sets the pace). Thus, when someone performs some physical activity or gets frightened or angry and the organs of the body need more blood, the sinus node speeds up to make the heart beat faster. When that person relaxes, the sinus node slows down again to a normal rate (the normal heart rate is between 60 and 80 beats per minute at rest). From the sinus node, the electrical signal rapidly spreads across the right atrium and the left atrium, like waves spreading across a pond. This electrical signal causes the atria to contract. The atria are electrically insulated from the ventricles by the AV groove that runs around the outside of the heart. The AV groove is part of the fibrous skeleton of the heart that also includes the valvular structures. There is one area of the heart where the atria and the ventricles are electrically connected, and this is deep in the center of the heart. This connection comprises the second electrical structure of the heart called the Atrioventricular Node or AV Node. The AV Node acts like the “main wire” of the heart, since all the electrical signals from the atria must pass through the AV node in order to get to the ventricles. The AV Node is connected to a bundle of special cells in the heart designed to rapidly conduct the electrical signal through the ventricles. These fibers are referred to as the Bundle of His. The Bundle of His branches downstream into a right bundle branch (to the right ventricle) and a left bundle branch (to the left ventricle). The fibers eventually branch out to the distant ventricular tissues and are at that point referred to as Purkinje Fibers. It is the rapid conduction of the electrical signal through these special fibers that stimulates and coordinates the contraction of the heart chambers, giving rise to a very efficient “wringing” action as the ventricular muscle vigorously contracts.