McKenna's Pharmacology for Nursing, 2e

682 P A R T 8  Drugs acting on the cardiovascular system C ardiotonic agents are drugs used to increase the con­ tractility of the heart muscle of people experiencing heart failure. Heart failure (HF) is a condition in which the heart fails to pump blood around the body effect­ ively. Because the cardiac cycle normally involves a tight balance between the pumping of the right and left sides of the heart, any failure of the muscle to pump blood out of either side of the heart can result in a backup of blood. If this happens, the blood vessels become congested; eventually, the body’s cells are deprived of oxygen and nutrients, and waste products build up in the tissues. The primary treatment for HF involves helping the heart muscle to contract more efficiently to restore system balance. HEART FAILURE HF, a condition that was once called “dropsy”, or decompensation, is a syndrome that usually involves dysfunction of the cardiac muscle, of which the sar­ comere is the basic unit. The sarcomere contains two contractile proteins, actin and myosin, which are highly reactive with each other but at rest are kept apart by the chemical troponin. When a cardiac muscle cell is stimulated, calcium enters the cell and inactivates the troponin, allowing the actin and myosin to form actomyosin bridges. The formation of these bridges allows the muscle fibres to slide together or contract (Figure 44.1). The formation of these bridges and

subsequent contraction require a constant supply of oxygen, glucose and calcium. (See Chapter 42 for a review of heart muscle contraction processes.) Heart failure can occur with any of the disorders that damage or overwork the heart muscle: • Coronary artery disease (CAD) is the leading cause of HF, accounting for approximately 95% of the cases diagnosed (see Chapter 47 for a discussion of CAD). CAD results in an insufficient supply of blood to meet the oxygen demands of the myocardium. Consequently, the muscles become hypoxic and can no longer function efficiently. When CAD evolves into a myocardial infarction (MI), muscle cells die or are damaged, leading to an inefficient pumping effort. • Cardiomyopathy is a disease of the heart muscle that leads to an enlarged heart and eventually to complete muscle failure and death. It can occur as a result of a viral infection, alcoholism, anabolic steroid abuse or a collagen disorder. It causes muscle alterations and ineffective contraction and pumping. • Hypertension eventually leads to an enlarged cardiac muscle because the heart must work harder than normal to pump against the high pressure in the arteries. Hypertension puts constant, increased demands for oxygen on the system because the heart is pumping so forcibly. • Valvular heart disease leads to an overload of the ventricles because the valves do not close tightly, which allows blood to leak backwards into the ventricles. This overloading leads to muscle stretching

Troponin

Troponin-calcium

Myosin Actin

Calcium binds troponin

SARCOMERE Troponin keeps actin

Actinomyosin bridges form and sarcomere slides together— “contraction”

and myosin from reacting

Na pumped out of cell

1 2

Calcium enters cell

0

Na + rushes in

3

Na, calcium pumped out of cell

4

Troponin

FIGURE 44.1  The sliding filaments of myocardial muscles. Calcium entering the cell deactivates troponin and allows actin and myosin to react, causing contraction. Calcium pumped out of the cell frees troponin to separate actin and myosin; the sarcomere filament slides apart, and the cell relaxes.

Troponin released from Ca

Action potential—cell is stimulated

Actinomyosin bridges separate—sarcomere slides apart— “relaxation”