An inotropic drug is a medicine that alters the force or strength of the heart's muscular contractions (heartbeats). There are two different types of inotropic drugs: negative and positive. Negative inotropic drugs make the heart beat less strongly, and positive inotropic drugs make the heart beat more strongly. Both kinds are used in the management of various conditions that affect the function of the heart.
One of the most important factors affecting the strength of the heart's muscular contractions is the level of calcium in the fluid inside the heart's muscle cells. Calcium is one of the most common electrolytes in the human body. Other common electrolytes are sodium, potassium, chloride and magnesium. We need the right balance of these electrolytes for proper functioning of the heart, nerves, muscles and kidneys.
Electrolytes are minerals that come from the foods we eat and fluids we drink. They're found in the fluids throughout the body -- not only in the blood and urine, but also inside the body's cells and in the space surrounding the cells.
Like their name suggests, electrolytes have an electrical charge, and they conduct electrical impulses (electricity). Electrical impulses generate muscle contractions. The level of each of the electrolytes must be kept within a narrow range in order for the body to function properly. In particular, it's important to maintain precise differences between the levels of electrolytes in the fluids inside the cells and outside the cells. It's these differences that allow our cells (especially nerve, heart and muscle cells) to maintain electrical voltages across their cell membranes, generate electrical impulses and produce muscle contractions.
Calcium plays an important role in the process of heart muscle contraction. In general, calcium levels are increased by positive inotropic drugs and decreased by negative inotropic drugs. But different types of inotropic drugs change calcium levels in different ways.
Next, we'll learn about the different kinds of inotropic drugs.
Positive Inotropic Drugs
Positive inotropic drugs improve heart function in cases of heart failure (when the heart is not pumping well enough), heart attacks and various types of heart disease. They can be divided into four types based on how they work:
Doctors very often prescribe one of the digitalis compounds, digoxin, for patients with heart failure. It increases the force of muscular contractions in the failing heart. Studies have shown that digoxin can improve the symptoms of heart failure in patients who are already taking diuretics (drugs that help rid the body of excess sodium salt and water) and ACE inhibitors, a type of vasodilator drug that helps widen blood vessels.
Apart from digoxin, other positive inotropic drugs should be used only for acute or end-stage heart failure because their prolonged use has been shown to worsen the disease outcome, and they may even increase the risk of death in some heart failure patients. These drugs can have a short-term benefit for some patients, but they may be harmful with long-term use.
Some positive inotropic drugs are given in pill form, and some are given intravenously (through a needle into a vein, usually in the arm). Patients who haven't responded to other treatments may be given intravenous positive inotropic drugs every so often on a regular basis. This is called intermittent therapy. It's a good idea for patients taking an intravenous positive inotropic drug to try stopping the drug every six months to see if they still need it.
Negative Inotropic Drugs
Negative inotropic drugs lessen the heart's workload by decreasing the rate and the strength of the heartbeat. These effects decrease the amount of blood that the heart pumps, the blood pressure in the vessels and the amount of oxygen that the heart uses. The action of these drugs also results in a decrease in the electrical activity in the heart.
The effects of these drugs make them very suitable for treating high blood pressure, angina (chest pain caused by heart disease) and heart attacks. Their effects on electrical activity make them useful for the treatment of some types of arrhythmia (irregular heart rhythm). Some of these drugs are used in the treatment of heart failure.
Although it seems contrary to what one would expect, negative inotropic drugs are sometimes used to treat heart failure, a condition in which the heart is weakened. Studies have shown that the negative inotropic drugs known as beta-blockers significantly improve heart function, symptoms, exercise performance and survival in certain types of heart failure.
Negative inotropic drugs can be divided into three classes based on how they work:
- Beta-blockers are usually used for treating high blood pressure, heart attack, chest pain, and irregular heart rhythm.
- Calcium-channel blockers are used for treating high blood pressure, chest pain, and irregular heart rhythm.
- Centrally acting sympatholytics are used for treating high blood pressure.
To learn more about inotropic drugs, take a look at the links on the next page.
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More Great Links
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- Fonarow GC. The treatment targets in acute decompensated heart failure. Rev Cardiovasc Med. 2001;2 Suppl 2:S7-S12.
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