Warning : Use the following information at your own risk. While accuracy is one my goals, there is always the possibility that some of the information could be wrong. There could be typos. I could also be severely mistaken in some of my knowledge. This site is meant to help clarify certain concepts of ECG and at no point should any life-or-death decision be made based upon the information contained within. Remember, this is just some page on the internet. (If you do find errors, please notify me by feedback.)
Without understanding the basic anatomy and physiology of the heart, your ECG experience will seem like a bunch of random facts and formulas. We are going to look very briefly at the anatomy and physiology of the heart. In fact, we will not even go into the names of the valves. Instead, we will focus on the most important aspects of the anatomy underlying the electrical conduction in the heart. This is not simply to satisfy your curiosity of what goes on behind the curtain; it is absolutely necessary to successfully grasp the topic of basic ECG interpretation.
Blood flow
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Your heart is an organ comprised of mostly muscle;
it is about the size of your
fist. It pumps blood. To do this, it possesses four chambers that
fill with
the blood needing to be pumped. When the chamber receives the order
(in the form of an electrical impulse), it contracts and pushes the blood
out of the chamber. Valves help the heart make sure that blood only
flows in one direction.
In figure 2-1, you see the four chambers. The R and L indicate right and left (relative to the person who owns the heart) and the A and V indicate atrium and ventricle. The yellow flaps represent the four valves found in the heart. These ensure that the blood flows in only one direction. |
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Figure 2-1 : A diagram representing the blood flow in the chambers of the heart (Left and right are always from the patient's perspective.) |
Later on, when we discuss the conduction system in the heart, I will say things like, "the electrical impulse travels through the atria." When I say something like this, I do not mean that this electrical current travels in the middle of the atria (where the blood is). What I mean is that the impulse travels along the wall of the atria.
The muscle of the heart (or more specifically, the heart's muscular middle layer) is called the myocardium. (Myo- refers to muscle.)
There are three main types of blood vessels :
- arteries : the vessels that lead blood AWAY from the heart (adjective form is arterial)
- veins : the vessels that lead blood back TOWARD the heart (adjective form is venous)
- capillaries : the microscopic vessels that connect arteries and veins; they serve as the place where gasses (oxygen, carbon dioxide, etc.) and nutrients are exchanged.
The aorta is the king of the arteries; it carries the blood pumped by the left ventricle and distributes it to lesser arteries. The vena cava is queen of the veins; it returns blood to the right atrium.
Imagine a muscle in one of your feet. It needs oxygen and nutrients; it receives these by capillaries. You cannot see these capillaries without a microscope, but they are everywhere. Starting at one of these capillaries, let's trace the general route that blood might take. After the capillaries, the deoxygenated blood travels into larger and larger veins and eventually empties into the vena cava. The vena cava takes the blood into the right atrium. It is pumped into the right ventricle, and subsequently pumped into the pulmonary artery. It follows through this system of smaller and smaller arteries until it eventually reaches capillaries in the lungs. The blood now releases some of its carbon dioxide and fills up on oxygen. Now it is back in a vein, heading for the pulmonary vein which will dump it into the left atrium. The left atrium pumps blood into the left ventricle. The massive left ventricle then pumps the blood into the aorta, which will lead it to a smaller artery and eventually back to a capillary bed in the tissue.
This giant loop can be divided up into two segments, or circulations :
- systemic circulation : starts with oxygenated blood in the aorta, ends with deoxygenated blood in the vena cava; the part of the loop that provides most of the body with its blood
- pulmonary circulation : the relatively short part of the loop that starts with deoxygenated blood in the pulmonary artery and ends with oxygenated blood in the pulmonary vein; the part of the loop responsible for "refilling" the blood with oxygen in the lungs
Apparently, there were some third grade teachers out there who taught students that the arteries carry only oxygenated blood and that veins carry deoxygenated blood. This, of course, is not true. That generalization only holds for systemic circulation. In pulmonary circulation, the opposite is true. Other exceptions to that "rule" are found in fetal circulation.
The pulse refers to the "wave" of blood that travels through the arteries whenever the ventricles contract. This can be felt by pressing one of your fingers against an artery. Pulses are generally only present in arteries; the movement of blood has steadied by the time it reaches the capillaries and veins. This is why puncturing capillaries and veins produces a steady flow of blood while punctured arteries tend to squirt blood at the same rate as the heart.
Be careful not to confuse some of the words I have mentioned. For example, the word atria looks a lot like aorta.
The Single Pump
The heart is essentially two single pumps : the left pump and the right
pump. Each pump follows the same
"v-to-a-to-v-to-a" pattern : vein to atrium to ventricle to
artery.
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Atrium The atrium (plural form is atria) is the first of the pump's two chambers. Think of the atrium as a sort of "pre-pump." Its main job is to help the ventricle fill. The atrium does so by beating a fraction of a second before the ventricle. In doing this, it pumps/pushes/packs/kicks the blood into the ventricle. If the atrium refused to pump, blood would still flow through it and into the ventricle. The atrium helps to pack blood into the ventricle. It this sense, it is similar to the Japanese subway workers who push people into the subway cars.
Ventricle The ventricle is the main chamber of the pump. It is the largest, strongest, and most important of the two chambers. Without the ventricle, you have no pump. While functional atria are a prerequisite for playing tennis, functional ventricles are a prerequisite for staying alive.
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Figure 2-2 : A single side of the heart (A=atrium, V=ventricle) |
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The Dual Pump
| The heart is essentially two of these pumps that share a common side. They also share parts of the electrical system that triggers them to contract. The right side of the heart is the smaller of the two pumps. It pumps deoxygenated blood into pulmonary circulation. The pulmonary artery carries the blood from the right ventricle to the lungs. The blood then travels through the pulmonary vein to the larger of the two pumps, the left atrium-ventricle. The left side pumps blood throughout the rest of the body (the systemic circulation). Because it has to push harder (i.e. higher pressure) than the right side, the left side is the strongest and largest half of the heart. (Notice in the diagram that the right side of the heart is actually on the left side of the image. Right and left are always relative to the person who owns the heart.) |
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Figure 2-3 : The two pumps of the heart |
The two atria usually contract together. The same holds true for the two ventricles. When you listen to a heart through a stethoscope, you will hear two thumps for every heart beat (these beats sound like "lub-dub" to some). The first corresponds to the closure of the valves that separate the atria and the ventricles. The second sound corresponds to the closure of the valves that separate the ventricles from the vessels they pump into.
Cells in the body need oxygen to live. Oxygen is carried in the blood. How do heart cells get blood? You might expect that an organ that sees all of the blood in the body pass through it on a regular basis would just use the blood it pumps. This, however, is not the case. Instead, the heart is "fed" from the outside. The vessels that carry the blood to feed the heart tissue are called coronary arteries and veins. These vessels are especially vulnerable to "clogs." When a vessel is blocked, the heart tissue downstream from the blockage can die. This is called a myocardial infarction (MI), or more commonly, a heart attack. Such events can hurt not only the contraction aspects of the heart muscle but the conduction aspects of it as well. These conduction effects may occur immediately or may not manifest themselves until years later.
Cardiac Output
When considering the heart's purpose, the bottom line measurement is cardiac output. Cardiac output (CO) is defined as the volume (liters) of blood pumped by the heart (left ventricle) per minute. Even though the pressure in the left side of the heart is greater than the right, the flow through both sides should be the same. The average CO for adult humans is 5 liters/minute. (If you are having trouble imagining what is being measured, just think of measuring the flow from a hose : how many 1 liter bottles can you fill in a minute.)
The cardiac output is calculated by multiplying stroke volume (liters / beat) by the heart rate (beats/min).
If your heart rate slows too much, the cardiac output will become inadequate to meet your body's demand. Increasing the heart rate will increase the CO. There is, however, a limit to the effectiveness of increasing the heart rate. At very fast heart rates, the stroke volume suffers in part because the ventricles have less time to fill. Thus, when the heart reaches a certain rate (usually greater than 150), the cardiac output will decrease as the heart rate increases. If there is little or no blood in the chamber, the heart will be "shooting blanks."
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