694
P A R T 8
Drugs acting on the cardiovascular system
■■
The sarcomere—the functioning unit of the heart
muscle—is made up of protein fibres: thin actin fibres
and thick myosin fibres, which react with each other
when calcium is present to inactivate troponin. The
fibres slide together, resulting in contraction. Failing
cardiac muscle cells lose the ability to effectively use
energy to move calcium into the cell, and contractions
become weak and ineffective.
■■
Cardiotonic (inotropic) agents are one class of drugs
used in the treatment of heart failure. These agents
directly stimulate the muscle to contract more
effectively.
■■
Cardiac glycosides increase the movement of calcium
into the heart muscle. This results in increased
force of contraction, which increases blood flow
to the kidneys (causing a diuretic effect), slows
the heart rate and slows conduction through the
atrioventricular node. All of these effects decrease
the heart’s workload. Digoxin is the cardiac glycoside
most commonly used to treat HF.
■■
Phosphodiesterase inhibitors block the breakdown
of cAMP in the cardiac muscle. This allows more
calcium to enter the cell (leading to more forceful
contraction) and enhances the effects of sympathetic
stimulation (which can lead to vasodilation but also
can increase pulse, blood pressure and workload on
the heart). Because these drugs are associated with
severe effects, they are reserved for use in extreme
situations.
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ONLINE RESOURCES
An extensive range of additional resources to enhance teaching
and learning and to facilitate understanding of this chapter may
be found online at the text’s accompanying website, located on
thePoint at
These include Watch and
Learn videos, Concepts in Action animations, journal articles,
review questions, case studies, discussion topics and quizzes.
WEB LINKS
Healthcare providers and students may want to consult
the following Internet sources:
Link to Australian Institute of Health and Welfare
publications on heart failure.
-
professionals/Clinical-Information/Pages/heart-failure.
aspx
Heart Foundation guidelines for prevention, detection,
management of chronic heart failure.
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