The Heart: Anatomy and Function

The heart, to put it simply, is a pump. An incredibly amazing pump but a pump none the less. It propels blood through miles of blood vessels carrying nutrients and oxygen to every cell in our body. The heart begins beating about the 5th week of fetal life and continues nonstop until our death. For the average person with a resting heart rate of 70 beats per minute that’s over one hundred thousand times per day. During a lifetime of 70 years that’s well over 2.5 billion heartbeats. Imagine opening and closing your hand that may times, day and night, while you’re awake and while you’re asleep, without ever stopping to rest. And the heart does this tremendous amount of work, for the most part, without us even being aware it is there. During rest and normal daily activities we don’t have a conscious perception that our heart is doing anything at all.

The size of the average man’s heart is 700-800 ml, which is about the same as a three cup measuring cup or two 12 ounce cans of pop. The heart is divided into four chambers, two upper chambers or atria and two lower chambers or ventricles. The ventricles do the work of pumping blood to the body while the atria pump blood to the ventricles making them more efficient in their task.
There are four one-way valves in the heart, one at the outlet of each heart chamber. Remember, the heart is a pump and can’t think for itself. All it wants to do is pump blood but it doesn’t care where. The valves keep blood flowing in one, forward direction.

The heart is also divided into a right and left side each having one atria and one ventricle. The right side of the heart pumps blood to the lungs or pulmonary circulation where oxygen is added to the blood and carbon dioxide is removed. The normal pressure generated by the right ventricle is 25-30 mmHg. The left side of the heart pumps blood to the rest of the body or what’s called the systemic circulation. Under normal conditions the left ventricle generates a pressure equal to a person’s systolic blood pressure or about 120 mmHg. If you have high blood pressure or a stenotic aortic valve then the pressure the left ventricle needs to generate goes up. The higher the pressure the more work your heart has to do.

In order for each heart chamber to be effective as a pump it needs to contract as a unit. To make this possible some of the heart cells are designed to act as the electrical system or conduction system of the heart. This conduction system starts with a group of cells in the upper part of the right atrium called the sinoatrial node. This group of cells initiates the electrical impulse which triggers the heart muscle to contract. The sinoatrial node also sets the heart rate and is called the “pacemaker” of the heart.

Once the electrical impulse is generated in the sinoatrial node it races down the conduction system and signals all of the muscle cells of the atrium to contract as a coordinated unit. Both the right and left atria contract at the same time followed by the two ventricles which also contract at the same time. To allow time for the atria to finish contracting and deliver their blood to the ventricles the electrical impulse is delayed briefly by another group of cells located between the upper and lower chambers called the atrioventricular node. After this brief delay the impulse enters the lower chambers and again races down the conduction system triggering the ventricles to contract. By this time the sinoatrial node is ready to start the process all over again.

Abnormalities in the conduction system can affect how the electrical impulse is generated or conducted through the heart. Sometimes the impulse is generated too fast, sometimes too slow, sometimes it’s irregular or uncoordinated. Any of these abnormal heart rhythms can significantly interfere with the pumping function of the heart, disrupt its regular beating, or stop it all together.

In addition to cardiomyopathy Charity’s heart also had an irregularity in its conduction system called WPW syndrome. In this condition there is an extra conduction pathway which if triggered into firing could have caused Charity’s heart to race uncontrollably and therefore seriously affected its ability to pump blood. In fact most people who die suddenly from heart disease, either cardiomyopathy or a heart attack, probably die because of a problem with their heart’s conduction system.

Each heartbeat or cardiac cycle is separated into two phases. A resting phase, where the heart muscle is relaxing and the chamber fills with blood, called diastole and a work phase, where the heart muscle is contracting, generating pressure and pumping blood, called systole. Since the upper and lower chambers contract at separate times their valves open and close separately as well which accounts for the typical biphasic heartbeat, “lub-dub, lub-dub, lub-dub, ….”

Under resting conditions diastole takes up a little over half of the cardiac cycle and systole a little less than half. During exercise or stress when the heart rate increases diastole is shortened much more than systole. Conversely, when the heart rate slows down diastole is lengthened proportionately more as well. Another way to look at this is to compare someone who is active, physically fit and generally has a slower resting heart rate to someone who is more sedentary, less fit who generally has a higher resting heart rate. The heart of the more fit person spends a larger portion of its time resting as opposed to working than the heart of the less fit person. Imagine what that means over a lifetime of heartbeats.

While all of the heart chambers are important it is the left ventricle that pumps blood to most of the body and therefore is doing most of the work so let’s look at it a little closer. The amount of blood the left ventricle pumps is called the cardiac output (CO) and is reported in liters per minute. The cardiac output is determined by the heart rate (HR) and the amount of blood pumped with each contraction which is called the stroke volume (SV). Mathematically then, CO = HR x SV.

On average the stroke volume is 70 ml per square meter of body surface area. In the average person with a resting heart rate of 72 beats per minute the cardiac output is about 6.5 l/min. Since the average person has approximately 5 liters of blood in their body that means under resting conditions your heart pumps the equivalent of your entire blood volume in less than one minute.

To get an idea of how much work the heart is doing compared to the other organs in the body let’s look at how much oxygen it uses to accomplish its task. On average all of the tissues of the body extract and use about 25% of the oxygen available to them in the blood as it passes through the circulation. The heart however extracts and uses about 75% of the oxygen available to it; three times as much as the rest of the body. An impressive amount of work, isn’t it?

Also, our hearts have the ability to greatly increase this resting workload to accommodate periods of intense physical activity or stress such as occurs with illness or following injury. During exercise the cardiac output in men can reach a maximum of about 20 l/min and in women about 13 l/min. For a world-class, endurance athlete this maximum can be over 40 l/min.

The heart increases its cardiac output in two ways, first by increasing the heart rate and second by increasing the stroke volume. Remember, the equation CO = HR x SV. There is a maximum rate the heart can effectively achieve which varies with age. The highest maximum rate is reached during puberty and is about 220 beats per minute. This maximum rate decreases as you get older and is roughly equal to 235 minus your age in years. Higher heart rates can occur with certain disease conditions but they don’t allow enough time during diastole for the heart chambers to fill and actually result in a lower cardiac output.

Up to its maximum cardiac output the heart will pump out whatever amount of blood is returned to it. This principle is called the Frank- Starling Law of the Heart and is named after Otto Frank and Ernst Starling, the two researchers who first discovered this unique ability of heart muscle. As more blood is returned to the heart more blood enters the ventricle during diastole, the ventricle muscle fibers stretch more and respond with a more forceful contraction during systole which in turn pumps out more blood resulting in a larger stroke volume. During exercise the stroke volume can increase by 30-50%.

Even though the left ventricle’s only job is to pump blood it doesn’t empty itself of blood, no matter how hard it contracts. The maximum amount of blood in the left ventricle during the cardiac cycle occurs at the end of the resting phase and is called the end-diastolic volume (EDV). As mentioned before the amount pumped during the contraction phase is the stroke volume. A way to measure how well the heart is doing its job of pumping blood is to compare the stroke volume to the end-diastolic volume and report this as a percentage called the ejection fraction (EF). Mathematically EF = SV/EDV x 100%. On average the ejection fraction of a normal, healthy heart is 67%. The range of normal is between 60-75%. When the heart becomes weakened, such as in cardiomyopathy, it becomes less effective and can’t pump as much blood with each contraction and the ejection fraction goes down. At its worst point Charity’s heart had an ejection fraction of 13%.

Cardiomyopathy

The word cardiomyopathy is made up of three parts: cardio- referring to heart; myo- referring to muscle; and pathy- referring to disease. Therefore cardiomyopathy means literally, heart-muscle-disease. Cardiomyopathy can be subdivided into one of three basic types each having a number of possible causes. The types of cardiomyopathy include hypertrophic – where the heart muscle is thickened and stiff as a consequence of a specific abnormality; restrictive – where the pumping function of the heart is restricted by the disease process; and dilated – where the chambers of the heart are stretched out as a result of the heart muscle being weakened and its function being impaired. Dilated cardiomyopathy is the type which affected Charity and is the focus of the remainder of this discussion.

Even though this disease is not completely understood there are certain concepts which are important to keep in mind. One concept is that everyone is not equally susceptible to getting cardiomyopathy. A second concept is that there may be many factors which contribute to a person becoming sick such as genetic, environmental and their own immune system. The third concept is that there is an initial illness followed by a partial or complete recovery which is followed by a period of apparent good health after which the person becomes sick again and develops signs of heart-muscle-disease. The severity of the initial illness can vary greatly and may be different from the subsequent cardiomyopathy.

There are over 75 known causes of dilated cardiomyopathy, the most common being coronary artery disease which leads to multiple heart attacks and progressive worsening of the heart function. When the specific cause of cardiomyopathy is not known the term idiopathic is used. This was the situation in Charity’s case.

Heart-muscle-disease has been recognized since 1891 and the term cardiomyopathy was first used by Brigden in 1957. A population-based study from Olmstead County, MN showed an incidence of 6 cases of cardiomyopathy per 100,000 population per year and three times as many men as women were affected. It is estimated that idiopathic dilated cardiomyopathy (IDC) occurs one tenth to one fourth as often as the cardiomyopathy that is secondary to coronary artery disease.

As mentioned the exact cause of IDC is not known but several factors may play a role. When family members were carefully screened one study found that about one-fifth of the cases were familial. Other studies have looked at the possibility that a viral illness may initially affect the heart and then after a period of apparent recovery, which can vary greatly in length, signs of cardiomyopathy develop. Not all viruses affect the heart though and not all people who get infected with a virus which can affect the heart develop cardiomyopathy. Still other studies have suggested that an autoimmune mechanism is involved. Here the body’s own immune system attacks and weakens the heart muscle.

In people who get IDC the first indication that they are sick may be the finding of an enlarged heart, an irregular heart rhythm, heart failure, or even sudden death. The onset and progression of symptoms may be gradual over a period of years or rapid over a period of only a few days. Sometimes the signs of advanced cardiomyopathy develop shortly after what otherwise seemed to be a simple respiratory infection or flu-like illness.

As IDC progresses the cardiac output goes down and symptoms of fatigue and weakness develop. As the function of the heart gets worse it can’t pump out blood at the rate it is being returned and the venous circulation becomes congested or backed-up. This venous congestion causes people to feel short of breath with minimal exertion or when they lay flat at night. It may also cause swelling in their legs or pain in their abdomen. About 10% of people with IDC have chest pains similar to angina. Some people, however, even with a very low ejection fraction may have very few of these symptoms of decreased heart function. Outwardly, Charity had very few symptoms and the ones she did have she tried to hide as best she could. She always tried to do the same things all of the other kids were doing.

In evaluating someone with IDC several tests can be done with specific abnormalities being expected. A chest x-ray typically will show an enlarged heart and there may be fluid in or around the lungs. An electrocardiogram is usually abnormal. Irregular rhythms and conduction problems are common. An echocardiogram which is an ultrasound of the heart generally shows dilation of the heart chambers and impaired pumping function of the heart muscle, particularly of the left ventricle. A radionuclide scan which is a movie of the heart made after injecting a radioactive solution into the person’s vein will give results similar to the echocardiogram. Cardiac catheterization usually confirms the findings from the ultrasound or radionuclide scan but the coronary arteries are generally found to be normal. Cardiac catheterization is an invasive test where a long catheter or plastic tube is inserted in an artery in the groin or arm and then maneuvered until it is directly in the heart chambers or coronary arteries. A contrast solution is then injected and an x-ray video taken. A biopsy can also be done through this catheter where a small piece of tissue from the inner lining of the heart chamber is removed and looked at under a microscope. Sometimes a specific cause of the heart muscle damage can be determined this way. It is important to differentiate IDC from other causes of cardiomyopathy. If the specific cause can be found sometimes the disease process can actually be halted or reversed.

There may be a number of different causes for IDC and therefore the natural history of this disease is variable. A latent phase may exist for several years before the person develops symptoms or before the diagnosis is made. In a long-term study of symptomatic patients seen at the Mayo Clinic during the 1960’s and early 1970’s, 23% of the patients had a normal survival but the other 77% died from their heart disease. Two-thirds of the deaths were within two years of diagnosis. In a study of 24 young patients with IDC, also treated at the Mayo Clinic, 37% died within the first year and only 34% were alive five years after diagnosis. More recent studies of patients treated during the 1980’s however, have shown an improved survival.

Certain characteristics in IDC are able to predict a poor outcome. People who have a low ejection fraction, increased pressures in the lung circulation or the right side of the heart, episodes of rapid ventricular rhythms, advanced age, or a poor tolerance for exercise generally have a low chance at long-term survival. The most common cause of death is progressive worsening of the heart function but an irregular heart rhythm can cause the person to die suddenly up to 45% of the time. Problems from blood clots is the cause of death in only a small number of people.

Treatment of IDC consists of a combination of lifestyle modification, medications and sometimes surgery. Losing excess body weight; limiting dietary sodium; abstinence from tobacco and alcohol; immunization against influenza; and a supervised exercise program to maintain general fitness are all important aspects of lifestyle modification. Medications used in IDC include vasodilators, to decrease the workload on the left ventricle; diuretics, to eliminate excess water from the body; digoxin, to improve the heart muscle’s contraction and prevent too rapid a heart rate; blood-thinners, to prevent clots from forming; antiarrhythmics, to prevent irregular heartbeats; and beta-blockers, to also help control the heart rate and improve the heart’s function. Mechanical devices which augment the function of the left ventricle can be implanted on a short-term basis when someone is critically ill or waiting for a heart transplant. Finally, heart transplantation can significantly improve a person’s life expectancy and is used in the most severe cases when all other treatments haven’t been enough. In one study 85% of adults receiving a heart transplant were alive one year later. After five years that number had dropped to 70%.

One interesting anecdotal case was recently reported from the University of Minnesota. A woman with advanced cardiomyopathy had a small pump implanted which basically took over the function of her own diseased heart. She couldn’t receive a heart transplant because of a high antibody count. After nearly two years of living with the heart-assist pump the woman’s doctors discovered that she no longer needed the device and simply turned it off. During the period her heart was resting it had healed itself. More research is needed before this can become a routine method of treatment but prior to this it wasn’t even known that the heart had the ability to repair itself.

In summary, our heart is a pump; delivering blood continuously to every cell in our body. A simple mechanical job being done by an amazingly sophisticated organ. It starts working a few weeks after conception and doesn’t stop until our death. It responds to accommodate our most rigorous activity and with proper training can greatly increase its already abundant reserve capacity. Unfortunately, our heart is also susceptible to disease; sometimes caused by our own neglect, sometimes by factors inherent in us or outside ones beyond our control. In one respect our heart seems almost unbreakable yet in another so fragile. It can work for a lifetime without rest under every conceivable condition yet be stopped forever, in literally a heartbeat, by a single electrical disturbance.

References

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