Amiodarone, antiarrhythmic agent

Hello friend!

Let’s get to the heart of  ….. the human body! It is obviously an essential organ – it pumps the blood that flows in the body, which is responsible for carrying oxygen and nutrients to all the cells of the body and transporting the waste of the cells back to the excretory processes.

It makes sense to keep the heart in working condition, in proper rhythm. Sometimes, the rhythm is affected. How? And what is the solution?

What is responsible for the rhythm of the heart? Pulses of electricity in a section of the heart initiate the process. This is something that happens at a frequency of 60 to 100 times a minute.

Alright, what’s the electricity deal? Like nerve cells, cardiac cells send messages electrochemically. Natural chemicals cause an electrical signal. There are chemical ions in the body that are electrically charged. The important ions are sodium and potassium that have one positive charge each, calcium that has two positive charges and chloride that has one negative charge. Each cell is surrounded by a cell membrane which is semi-permeable – it allows certain ions to pass through and blocks others.

In between sending electrical signals, the cell is at rest. In this state, the inside of the cell is negatively charged compared to the outside of the cell. The resting membrane potential is about -90 mV (millivolts). This means the inside of the cell is 90 mV less than the outside of the cell.

When the cell gets excited, an action potential occurs. In a nerve cell, there has to be a stimulus – a thought or a motor movement or some kind of mental or physical initiation. In a cardiac cell, the specialized conducting nature of the tissue causes it to have an action potential without any external stimulus.  The action potential is caused by a burst of electrical activity caused by a depolarizing current. This means the resting potential of -90 mV moves toward zero mV. When the depolarization of the cell reaches a threshold of about -70 mV, the cell fires an action potential and changes from negative to positive, then plateaus at around zero mV and repolarizes back to -90 mV.

The heart is a muscle with four chambers. The top two smaller chambers are the atria – the left atrium and the right atrium. The lower two chambers are the thick-walled, more muscular ones called the ventricles, left and right. How does the heart work? Two major veins called the superior vena cava and the inferior vena cava carry deoxygenated, impure blood from the upper and lower parts of the body respectively to the right atrium. The right atrium pumps this blood into the right ventricle. The blood is pumped by the right ventricle through the pulmonary artery into the lungs, where it is oxygenated. The purified, oxygenated blood is brought back via the pulmonary vein to the left atrium of the heart, which pumps it into the left ventricle. The left ventricle pumps the blood out into the rest of the body.

So, what is the sequence of events in a heartbeat? A group of cells called the sinoatrial node (SA node) on the wall of the right atrium is the natural pacemaker of the heart. An electrical impulse arises at a specific frequency from the SA node and is conducted through the heart, with a series of precisely timed depolarizations of heart cells in a four-part cycle. The atria contract, while the ventricles remain relaxed and passively full of blood. More and more blood is forced by the atria into the ventricles which keep on expanding to their muscular stretching limit. The valves between the atria and the ventricles close, following which there is a brief period of rest.  Then the ventricles contract – the deoxygenated blood from the left ventricle goes to the lungs as explained above, and the oxygenated blood from the right ventricle goes through the systemic circulation to the rest of the body. Next, the ventricles go back to resting state, and passively fill with blood.

Why would the heartbeat become irregular? It could be one of several reasons. It could be congenital. Or, it could be because of rheumatic heart disease, blood pressure or hyperthyroidism. Other reasons could be an excess of caffeine, alcohol, stress or even some cough and cold medicines sold over the counter.

Antiarrhythmics act on the heart in a variety of ways to slow the conduction so that the heart has the leisure to develop a more regular rhythm. There are different classes of antiarrhythmics based on the mechanism of action. Amiodarone is a Class III antiarrhythmic that blocks the heart’s potassium channels and thereby slows conduction.

Therapy with amiodarone should be started in a hospital under medical supervision. Severe lung problems, sometimes fatal, have arisen as a side-effect. Sometimes, the drug could worsen the heart’s arrhythmia. It may take 2 weeks or even longer for its effect to be seen.

Amiodarone is an iodine-containing medication, so the patient should not be on simultaneous radioactive iodine therapy. This medicine is not to be used during pregnancy or during breast-feeding. The drug can increase the effect of blood-thinner warfarin. It can increase the sensitivity of the skin to the sun. It can even turn the skin blue-gray in color, a condition that can persist for several months after the medication is stopped.

Other side-effects can be tiredness, constipation, loss of appetite, smell or taste changes, nausea and vomiting, dizziness, insomnia, headache, flushing and decreased sexual appetite.

The severe lung problem that amiodarone can cause, that was mentioned above, is interstitial lung disease. In this condition, a dramatically decreased diffusion capacity of the lung is seen on testing – this means that the amount of oxygen transferred from the air in the lungs to the blood is greatly reduced. This can be a deadly condition.

Amiodarone increases the effect of blood thinner warfarin and another type of anti-arrhythmic agent digoxin. Amiodarone is absorbed extensively by the fat tissue in the body, hence remains in the body for a long time. The half-life of excretion of this medicine from the body is a couple of months on an average – that means only about 50 per cent of the medicine is excreted from the body in a couple of months, then in another couple of months about 50 per cent of the remaining amount is excreted, and so on.

Yes, there are all these adverse reactions and side-effects. Yet, amiodarone is very useful in several types of arrhythmia. It is considered a “broad-spectrum” antiarrhythmic, with several effects:

  • It increases the time for the repolarization of the heart after each contraction.
  • It increases the time period in which the heart muscle cells are electrically stimulated in its action potential.
  • It decreases the speed with which the electrical impulses move through the heart’s electrical system.
  • It reduces the speed of the generation of electrical activity in the pacemaker.
  • It reduces the speed of electricity through accessory pathways.
  • It dilates blood vessels, resulting in decrease in blood pressure – this effect is useful in congestive cardiac failure, in which there is a weakening of the heart muscle.

As always, the medical professionals seek to balance out the benefits and the adverse effects of the drug and use it to the patient’s best advantage.

Until next week, do take care of yourself and your health ….

Dr. Ajit Damodaran

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One Response to “Amiodarone, antiarrhythmic agent”

  1. fluicectert Says:

    Super website / Hope to come back again,,

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