This tutorial is dedicated to getting back to basics. I am going to give a pretty thorough review of electrocardiogram (ECG) interpretation. For some of you this will seem elementary and could just be a good refresher. For others this could be the first time you have been taught this information. I am not sure how many parts there will end up being, quite a few I presume. After I finish I will create another blog with the tutorial and nothing else, for quick reference.
So lets get started.
I don't feel it is pertinent for me to show you a single ECG strip or try and teach you your first rhythm until I give a simple cardiology review.
THE FUNCTION OF THE HEART:
When talking about the function of the heart, it is required for you to think of the two different types of functioning; electrical and mechanical. The electrical functioning of the heart has to do with impulses sent through its electrical pathways. These impulses are regulated by the brain and cellular electrolyte exchange. I will get further into this. The mechanical functioning of the heart is initiated by the electrical functioning and has to do with the actual pumping of blood.
One way I like to explain this is by establishing two separate values with commonly interchanged terminology. When using the term heart rate (HR) you should be referring to the electrical impulses, and when using the term pulse (P) you should be referring to mechanical output. You see a HR and you feel a P. They can be different, and this will be explained in this tutorial. The value the monitor (not pulse oximeter) gives you is a HR. The P is what you feel when conducting vital signs. It will behoove you to look at the HR while taking a P, this can indicate malignant ectopy.
Note: During this tutorial I will provide definitions for certain terms, they will be in italics.
Ectopy - when used in electrocardiography, an unnatural presentation. Possibly a premature beat (ectopic beat).
Pathological - altered or caused by disease.
Heart location - The heart sits in the middle of the thoracic (chest) cavity. It is slightly tilted to the left which causes most of the mass to sit just left of the mediastinum. The heart is protected by the sternum and ribs. The apex of the heart is located behind the fifth left intercostal space, slightly medial to the midclavicular line. There are pathological conditions that alter the location of the heart, dextrocardia, COPD (chronic obstructive pulomonary disease/disorder), and CHF (congestive heart failure) are the most common. I may briefly touch on these conditions later on.
Heart chambers - The heart consists of four chambers. Two atria and two ventricles. Blood enters the atria and is pumped into the ventricles, during atrial contraction. The ventricles then pump the blood out of the heart, during ventricular contraction. There are pathological conditions that may effect each chamber, hypertrophy being the most common.
Heart layers - The four chambers are lined by the endocardium which is surrounded by the myocardium (heart muscle). The pericardium then surrounds the entire heart. The pericardium is a protective sac that consists of two layers. The visceral pericardium (epicardium) is the inner layer and lines the heart (visceral tissue always lines the organ). The parietal pericardium which is a fibrous outer layer. In the pericardial cavity is about 25 ml of fluid between the two layers of the pericardium; this reduces the friction caused by the beating and moving of the heart. There are pathological conditions directly involved with each layer of the heart endocarditis, myocardial infarction, pericarditis, and pericardial tamponade. I will explain these conditions as they become pertinent.
Heart valves - In total, there are four heart valves. There are two heart valves which separate the atria from the ventricles, collectively called the AV (atrioventricular) valves. These are named by how many chordae tendineae are attached to them. Between the right atrium and ventricle is the tricuspid valve (three chordae tendineae). Between the left atrium and ventricle is the bicuspid valve (two chordae tenineae) which is sometimes called the mitral valve. An easy way to remember which valve is which is that tri has the letters R and I and right starts with R and I. The chordae tendineae keep the valves from folding back into the atria.
There are two valves that blood must flow through as it leaves the heart. These are collectively referred to as the semilunar valves. These are named due to there shape in relation to the moon. The pulmonary semilunar valve connects the right ventricle to the pulmonary artery, and the aortic semilunar valve connects the left ventricle to the aorta.
The heart sounds that can be auscultated, commonly referred to as "lub-dub" are actually the sounds of the valves flapping closed. The AV valves close first and then the semiunar valves.
Below is a cross section as if you were looking down at the valves.
Heart size - The normal heart is about the size of your fist. The left ventricle predominates in size and has a much thicker wall of myocardium. This is due to the higher pressure needed from the left side to perfuse the entire body. Certain pathologies may alter the size of the heart, and hypertrophy is almost always the result.
Hypertrophy - The opposite of atrophy. In cardiology, abnormal growth of heart muscle due to added stress on the heart.
Above is a good example of the blood flow of the heart.
Frank Starling's Mechanism/Law - The more blood that enters the ventricle during diastole, the greater the contraction during systole.
Systole - Cardiac contraction, when the heart contracts
Diastole - Cardiac relaxation, when the heart fills
1 - Starting with the superior & inferior vena cavas blood flows into the right atrium. The force put on the atria to allow optimal atrial filling is referred to as preload.
2 - After the right atrium is full of blood the tricuspid valve opens, allowing blood to flow freely into the right ventricle. The right atrium then contracts filling the ventricle even more; this is part of the Frank Starlings mechanism.
3 - The right ventricle then contracts while the pulmonary semilunar valve is open and blood enters the pulmonary artery (the only non-oxygenated artery).
4 - Blood flows from the pulmonary artery into the lungs. Blood fills the pulmonary capillaries which surround the alveoli of the lungs. This is where CO2 is exchanged for O2. After the blood becomes oxygenated it leaves the heart through the pulmonary veins (the only oxygenated veins).
5 - The pulmonary vein dumps it's volume into the left atrium using the residual pressure put on it by the right ventricle and pulmonary system (preload).
6 - The left atrium sends blood through an open bicuspid/mitral valve in a manor similar to the right side of the heart.
7 - The oxygenated blood enters the left ventricle, ready to be sent to the rest of the body. The left ventricle then contracts and the blood passes through the aortic semilunar valve into the aorta. The force that the ventricles have to contract against is known as afterload.
8 - After ventricular systole the aortic semilunar valve flaps closed and the blood passes over the aortic arch. Blood that doesn't make it over the arch falls back onto the closed aortic semilunar valve and into the coronary arteries. The right and left coronary arteries are the first exits attached to the ascending aorta. They are used to perfuse the heart.
9 - Most of the blood ejected from the left ventricle passes over the aortic arch and enters into the arteries of the body. From the arteries the blood enters into arterioles and then the systemic capillaries. O2 is exchanged for the bodies waste CO2. The blood then needs to head back towards the lungs. The blood enters into venules and then veins. All the systemic veins eventually lead back to the vena cavas.
Basic Cardiology Part II
In part II I will discuss the electrical conduction system.