Exam 17: Drugs for Heart Failure

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Visualize the heart as having three fundamental characteristics:
1. It contracts with a specific force or strength (contractility).
2. It beats at a certain rate (beats per minute).
3. It conducts electrical impulses at a particular speed.
The ability to change the force of contraction, or contractility, is of particular interest to the pharmacotherapy of HF. Because the fundamental cause of HF is a weak myocardium, causing the muscle to beat more forcefully seems to be an ideal solution. The ability to increase the strength of contraction is called a positive inotropic effect and is a fundamental characteristic of the class of drugs known as the cardiac glycosides.
The ability of the heart to speed up or slowdown is a second characteristic important to pharmacology. A faster heart works harder but not necessarily more efficiently. A slower heart has a longer time to rest between beats, thus decreasing the workload on the heart.
A third fundamental characteristic of cardiac physiology is the electrical conduction through the heart. Some cardiovascular drugs influence the speed of this conduction. Slowing the conduction speed through the heart will cause the heart to beat slower, thus lessening cardiac workload.
These primary characteristics of cardiac function can be modified through pharmacotherapy to assist the heart in meeting the body's metabolic demands. The mechanisms by which HF medications accomplish this are;
Adrenergic blockers: Decrease cardiac workload by slowing the heart rate (b1) and decreasing blood pressure (a1) (example: carvedilol); Vasodilators: Decrease cardiac workload by dilating vessels and reducing preload (example: isosorbide dinitrate with hydralazine); ACE Inhibitors and ARBs: Increase cardiac output by lowering blood pressure and decreasing blood volume (example: Lisinopril; candesartan); Phosphodiesterase inhibitors: Increase cardiac output by increasing the force of myocardial contraction (example: milrinone); Diuretics: Increase cardiac output by reducing fluid volume and decreasing blood pressure (example: furosemide); Cardiac glycosides: Increase cardiac output by increasing the force of myocardial contraction (example: digoxin)