Making Sense of the ECG A Hands-On Guide by Andrew Houghton
Making Sense of the ECG is an best edition for those who want to learn the ECG in a good manner with essential concept that we really suppose to know in diagnosing and mananging cardiac cases we hope this will help you.
Chapter 1 – Anatomy and physiology
The heart is a hollow muscular organ that pumps blood around the body. With each beat, it pumps, at rest, about 70 millilitres of blood and considerably more during exercise. Over a 70-year life span and at a rate of around 70 beats per minute, the heart will beat over 2.5 billion times.
The heart consists of four main chambers (left and right atria, and left and right ventricles) and four valves (aortic, mitral, pulmonary and tricuspid). Venous blood returns to the right atrium via the superior and inferior vena cavae, and leaves the right ventricle for the lungs via the pulmonary artery. Oxygenated blood from the lungs returns to the left atrium via the four pulmonary veins, and leaves the left ventricle via the aorta (Figure 1.1).
The heart is made up of highly specialized cardiac muscle comprising myocardial cells (myocytes), which differs markedly from skeletal muscle because heart muscle:
- Is under the control of the autonomic nervous system
- Contracts in a repetitive and rhythmic manner
- Has a large number of mitochondria which make the myocytes resistant to fatigue
- Cannot function adequately in anaerobic (ischaemic) conditions
Myocytes are essentially contractile but are capable of generating and transmitting electrical activity. Myocytes are interconnected by cytoplasmic bridges or syncytia, so once one myocyte cell membrane is activated (depolarized), a wave of depolarization spreads rapidly to adjacent cells.
Myocardial cells are capable of being:
- Pacemaker cells: These are found primarily in the sinoatrial (SA) node and produce a spontaneous
- electrical discharge.
- Conducting cells: These are found in:
- The atrioventricular (AV) node
- The bundle of His and bundle branches
- The Purkinje fibres
- Contractile cells: These form the main cell type in the atria and ventricles.
All myocytes are self-excitable with their own intrinsic contractile rhythm. Cardiac cells in the SA node located high up in the right atrium generate action potentials or impulses at a rate of about 60–100 per minute, a slightly faster rate than cells elsewhere such as the AV node (typically 40–60 per minute) or the ventricular conducting system (30–40 per minute), so the SA node becomes the heart pacemaker, dictating the rate and timing of action potentials that trigger cardiac contraction, overriding the potential of other cells to generate impulses. However, should the SA node fail or an impulse not reach the ventricles, cardiac contraction may be initiated by these secondary sites (‘escape rhythms’).