7.2 Basic Concepts
Before interpreting cardiac rhythms, it is vital to understand the anatomy and physiology of the heart. Let’s begin with a basic review of the cardiovascular system. The heart is a fist-sized organ that pumps blood throughout the body. It is the primary organ of the circulatory system. The heart contains four main chambers made of muscle and powered by electrical impulses. The brain and nervous system direct the heart’s function. The electrophysiology of the heart will determine the rate and rhythm. The blood pressure is maintained by the contractility of the heart muscle.[1] See Figure 7.1[2] for an illustration of the heart.
The parts of the heart are similar to the parts of a house. Both the heart and a house have the following components[3]:
- Walls
- Chambers (the rooms)
- Valves (the doors)
- Blood vessels (the plumbing)
- Electrical conduction system (the electricity)
Heart walls are muscles that contract (squeeze) and relax to send blood throughout the body. A layer of muscular tissue called the septum divides the heart walls into the left and right sides. Heart walls have three layers:
- Endocardium: The inner layer
- Myocardium: The muscular middle layer
- Epicardium: The protective outer layer
The epicardium is one layer of the pericardium. The pericardium is a protective sac that covers the entire heart. It produces fluid to lubricate the heart and keeps it from rubbing against other organs.[4]
The heart is divided into four chambers. There are two chambers on the top (called the left and right atria) and two chambers on the bottom (called the left and right ventricles). Blood flows through the chambers of the heart in the following order:
- Right atrium: Two large veins called the superior vena cava and the inferior vena cava deliver oxygen-poor blood to the upper right chamber of the heart called the right atrium. The superior vena cava carries deoxygenated blood from the upper body. The inferior vena cava carries deoxygenated blood from the lower body. The right atrium pumps this blood through the tricuspid valve into the right ventricle.
- Right ventricle: This lower right chamber of the heart pumps the oxygen-poor blood through the pulmonary valves and then through the pulmonary arteries to the lungs. (Note that arteries usually carry oxygenated blood, but pulmonary arteries carry deoxygenated blood to the lungs.) The lungs reload blood with oxygen while removing carbon dioxide, and the pulmonary veins carry oxygenated blood back to the left atrium. (Note that veins usually carry deoxygenated blood, but pulmonary veins are the only veins in an adult that carry oxygenated blood.)
- Left atrium: The upper left chamber of the heart receives the oxygenated blood and pumps it through the mitral valve into the left ventricle.
- Left ventricle: The lower left chamber of the heart is called the left ventricle. It is slightly larger than the right ventricle because it pumps oxygen-rich blood through the aortic valve to