McKenna's Pharmacology for Nursing, 2e

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P A R T 8  Drugs acting on the cardiovascular system

These two branches of the autonomic nervous system work together to help the heart meet the body’s demands. Drugs that influence either branch can exert autonomic effects on the heart. Myocardial contraction The end result of the electrical stimulation of the heart cells is the unified contraction of the atria and ventricles, which moves the blood throughout the vascular system. The basic unit of the cardiac muscle is the sarcomere (Figure 42.4). A sarcomere is made up of two contrac­ tile proteins: actin , a thin filament, and myosin , a thick filament with small projections on it. These proteins are anchored at the Z bands, the outer edges of each sarco­ mere. These proteins readily react with each other, but at rest they are kept apart by the protein troponin . When a cardiac muscle cell is stimulated, calcium enters the cell though channels in the cell membrane and also from storage sites within the cell. This occurs during phase 3 of the action potential, when the cell is starting to repolarise. The calcium reacts with the troponin and inactivates it. This action allows the actin and myosin proteins to react with each other, forming actomyosin bridges. These bridges then break quickly, and the myosin slides along to form new bridges. As long as calcium is present, the actomyosin bridges continue to form. This action slides the proteins together, shortening or contracting the sarcomere. Cardiac muscle cells are linked together: When one cell is stimulated to contract, they are all stimulated to contract. The shortening of numerous sarcomeres causes the contraction and pumping action of the heart muscle. As the cell reaches its repolarised state, calcium is removed from the cell by a sodium–calcium pump, and calcium released from storage sites within the cell returns to the storage sites. The contraction process requires energy and oxygen for the chemical reaction that allows the formation of the actomyosin bridges, and calcium to allow the bridge formation to occur. The degree of shortening (the strength of con­ traction) is determined by the amount of calcium present—the more calcium present, the more bridges will be formed—and by the stretch of the sarcomere

before contraction begins. The further apart the actin and myosin proteins are before the cell is stimulated, the more bridges will be formed and the stronger the con­ traction will be. This correlates with Starling’s law of the heart. The more the cardiac muscle is stretched, the greater is the contraction. The more blood that enters the heart, the greater is the contraction that is needed to empty the heart, up to a point; however, if the actin and myosin molecules are stretched too far apart, they will not be able to reach each other to form the actomyosin bridges, and no contraction will occur. ■■ The heart, a hollow muscle with four chambers comprising two upper atria and two lower ventricles, pumps oxygenated blood to the body’s cells and also collects waste products from the tissues. ■■ The two-step process known as the cardiac cycle includes diastole (resting period when the veins carry blood back to the heart) and systole (contraction period when the heart pumps blood out to the arteries for distribution to the body). ■■ Impulses generated in the heart—not the brain— stimulate contraction of the heart muscle. ■■ The heart’s conduction (or stimulatory) system consists of the sinoatrial (SA) node, the atrial bundles, the AV node, the bundle of His, the bundle branches and the Purkinje fibres. ELECTROCARDIOGRAPHY Electrocardiography is a process of recording the patterns of electrical impulses as they move through the heart. It is an important diagnostic tool in the care of the person experiencing cardiac problems. The electro­ cardiography machine detects the patterns of electrical impulse generation and conduction through the heart and translates that information into a recorded pattern, which is displayed as a waveform on a cardiac monitor or printout on calibrated paper. An electrocardiogram (ECG) is a measure of electrical activity; it provides no information about the mechanical activity of the heart. The important aspect of cardiac output—the degree to which the heart is doing its job of pumping blood out to all of the tissues—needs to be carefully assessed by looking at and evaluating the person. The normal ECG waveform is made up of five main waves: the P wave, which is formed as impulses originat­ ing in the SA node or pacemaker pass through the atrial tissues; the QRS complex, which represents depolarisa­ tion of the bundle of His (Q) and the ventricles (RS); and the T wave, which represents repolarisation of the ventri­ cles (Figure 42.5). KEY POINTS

Ca 2+

Ca 2+

Ca 2+

Actin filaments Myosin filaments

Ca 2+ channels

Z bands

Troponin

Z bands

FIGURE 42.4  A sarcomere, the functioning unit of cardiac muscle.