cardiotonic drugs They are used to increase the strength of heart contractions. This is done in order to be able to pump more blood while requiring fewer heartbeats. In cardiology this feature is called positive inotropy.
In addition to increasing the strength of the heart, they also reduce their oxygen requirements. There are several groups of drugs that have these properties and are mainly used to treat heart failure:
- Digitalis and cardiotonic glycosides.
- ß-adrenergic agonists.
- dopamine and dobutamine.
- Phosphodiesterase 3 (PDE3) inhibitors.
Digitalis and cardiotonic glycosides
The various species of the plant digitalAS Purple foxglove OR foxglove lanata, allow us to extract several foxgloves with cardiotonic properties. Among them we find THE digoxinouabain or strophanthin, the former being the most used.
All members of this drug family exert similar actions and their differences are simply kinetic in nature. It is the most important cardiotonic group.
Mechanism of action of cardiotonics
positive inotropythat means, increase in muscle contractility characteristic of digitalis It is due to the competitive and selective inhibition capacity of the Na/K ATPase pump.
The Na/K ATPase pump is a mechanism whose function is to expel 3 sodium ions (Na) into the extracellular space and introduce 2 potassium ions (K) into the cell. With it, it manages to maintain the ionic gradient of the cell membrane.
The intracellular calcium (Ca) levels are regulated by the Na/K ATPase pumps and the Na/Ca bombs. As the former is inhibited, the concentration of Na in the cell will greatly increase and the latter will be activated.
Consequentially, the concentration of Ca inside the cell will greatly increase. Ca will bind contractile proteins thereby increasing myocardial contractility.
Although this heart tonic is one of the most used, you have to be very careful with it. It’s a drug with a narrow therapeutic indexi.e. the difference between therapeutic and toxic concentrations is very small.
Therefore, this drug It is for hospital use and requires monitoring. When the therapeutic dose is exceeded, the patient may experience symptoms characteristic of digitalis poisoning:
- Heart attack.
- Changes in the electrocardiogram.
Aside from heart problems, other effects also occur in the body AS:
- Vision blurred.
- Nervous changes.
- Gastrointestinal disorders.
How to treat cardiotonic poisoning?
- To treat poisoning from these cardiotonics, the cardiotonic must first be stopped.
- Then their blood levels are determined.
- Potassium is then given to displace the drug from its site of action.
- All this in the company of a specialist.
ß-Andrenergic agonists are drugs capable of stimulating cardiac contractility, the volume of blood pumped per minute and the heart rate if they bind to ß1 receptors.
They also have a vasodilatory action, which reduces blood pressure.in addition to peripheral resistance if they are ß2-receptor agonists, such as salbutamol or terbutaline.
Although they increase myocardial tone, they are not effective in treating long-term heart failure. This is due to in long treatments it favors the production of angiotensin, which is a hormone that promotes water retention, increasing blood pressure.
Even increasing the heart rate, Oxygen consumption increases, which can lead to ischemia.
dopamine and dobutamine
Dopamine and dobutamine are two drugs that they are used as cardiotonics when heart failure is severe. They are orally inactive drugs, so they are applied by intravenous infusion at the hospital level.
This measure is important as it can cause tachyphylaxis (reduced response to a drug) after 72 hours. However, dobutamide produces it less frequently. The characteristics of these two drugs are as follows:
- Dopamine is an agonist of D1 and D2 receptors, as well as agonergic β and α1 receptors.
- It is also capable of releasing norepinephrine as the dose increases.
- It is useful in severe heart failure associated with pulmonary congestion.
- We could include dobutamine in the above classification, as it preferentially stimulates ß1, and to a lesser extent ß2 and α1.
- It is used in severe heart failure associated with ischemic heart disease..
None of these cardiotonics are used for chronic treatment. Additionally, they can trigger adverse effects such as:
PDE3 inhibitors are a group that work by selectively inhibiting the phosphodiesterase III enzyme in the heart, such as enoximone or milrinone.
Phosphodiesterase III is responsible for the breakdown of cAMP. Therefore, if it is inhibited, cAMP levels will increase and consequently:
- Increases myocardial contractility.
- Increases the volume of blood pumped per minute.
- Reduces end-diastolic and pulmonary pressures.
Its chronic use increases arrhythmias and can cause death. Therefore, they are indicated in the acute phase of some types of severe heart failure unresponsive to other treatments.
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