Pocket Reference for ECGs Made Easy – Book and Pocket Reference Package
Pocket Reference for ECGs Made Easy – Fifth edition is a best one for ECG beginners which comes with entirly new reference of books and learnings to make it more effective.
LOCATION AND SURFACESOF THE HEART
The heart is a hollow muscular organ that lies in the space between the lungs (i.e., the mediastinum) in the middle of the chest. It sits behind the sternum and just above the diaphragm (Figure 1-1). Approximately two thirds of the heart lie to the left of the midline of the sternum. The remaining third lies to the right of the sternum.
The base, or posterior surface, of the heart is formed by the left atrium, a small portion of the right atrium, and proximal portions of the superior and inferior venae cavae and the pulmonary veins. The front (anterior) surface of the heart lies behind the sternum and costal cartilages. It is formed by portions of the right atrium and the left and right ventricles (Figure 1-2). The heart’s apex, or lower portion, is formed by the tip of the left ventricle. The apex lies just above the diaphragm at approximately the level of the fifth intercostal space, in the midclavicular line.
STRUCTURE OF THE HEART
Layers of the Heart Wall The walls of the heart are made up of three tissue layers: the endocardium, myocardium, and epicardium. The heart’s innermost layer, the endocardium, is made up of a thin, smooth layer of epithelium and connective tissue and lines the heart’s inner chambers, valves, chordae tendineae (tendinous cords), and papillary muscles. The endocardium is continuous with the innermost layer of the arteries, veins, and capillaries of the body, thereby creating a continuous, closed circulatory system.
The myocardium (middle layer) is a thick, muscular layer that consists of cardiac muscle fibers (cells) responsible for the pumping action of the heart. The heart’s outermost layer is called the epicardium. The epicardium contains blood capillaries, lymph capillaries, nerve fibers, and fat.
The heart has four chambers (Figure 1-3). The two upper chambers are the right and left atria. The purpose of the atria is to receive blood. The right atrium receives blood low in oxygen from the superior vena cava (which carries blood from the head and upper extremities), the inferior vena cava (which carries blood from the lower body), and the coronary sinus, which is the largest vein that drains the heart. The left atrium receives freshly oxygenated blood from the lungs via the right and left pulmonary veins.
The heart’s two lower chambers are the right and left ventricles. Their purpose is to pump blood. The right ventricle pumps blood to the lungs. The left ventricle pumps blood out to the body
There are four one-way valves in the heart: two sets of atrioventricular (AV) valves and two sets of semilunar (SL) valves (Table 1-1). The valves open and close in a specific sequence and assist in producing the pressure gradient needed between the chambers to ensure a smooth flow of blood through the heart and prevent the backflow of blood. AV valves separate the atria from the ventricles. The tricuspid valve is the AV valve that lies between the right atrium and right ventricle. It consists of three separate cusps or flaps. It is larger in diameter and thinner than the mitral valve. The mitral (or bicuspid) valve has only two cusps. It lies between the left atrium and left ventricle.
The pulmonic and aortic valves are SL valves. The SL valves prevent backflow of blood from the aorta and pulmonary arteries into the ventricles. The SL valves have three cusps shaped like half-moons. The openings of the SL valves are smaller than the openings of the AV valves and the flaps of the SL valves are smaller and thicker than the AV valves. Unlike the AV valves, the SL valves are not attached to chordae tendineae.