Cardiac Rhythm and Arrhythmias

A heart ECG with segments marked

Note: We will cover HIV vaccines soon; these are some notes from my pharma class.

Cardiac Rhythm and Arrhythmias

Most days we don’t think about it, but our heart is pretty important.  Beating constantly, it provides a flow of blood that keeps our organs fed and keeps us alive.  To get a sense for how important it is, if your heart stopped for just 10 seconds, you’d faint.

The heart is a combination of great strength and great weakness.  On the one hand, it beats roughly 60 times a minute for the entire duration of your life without fail.  On the other, it almost never recovers from damage.  If a part of it gets cut off from oxygen and dies, it will never recover, as contrasted to our skin, which is constantly replenishing itself.

How does the heart work?

The heart has 4 major chambers: 2 atria and 2 ventricles.  The atria are the receiving rooms: they are where blood flows in from the body.  After the atria are filled properly, the blood moves into the ventricles, which are thick and muscular.  From there, the ventricles pump the blood either to the lungs – where it receives vital oxygen – and to the rest of the body.

The electrical system and failures

Electrically, the heart works by chemical gradients.  There is a dynamic equilibrium between three key ions: potassium, sodium and calcium.  There is a lot of potassium inside the heart’s cells and a lot of sodium and calcium outside of them.

When a heart muscle needs to contract, and in response to the appropriate stimulation, it opens its sodium channels.  This allows an inflow of positive ions, which makes the cell become depolarized, or gain a positive charge.  Becoming positive makes the cell contract.

Then the sodium channels close and potassium channels open.  Because there is so much potassium inside, it tends to leave the cell and, because its positively charged, this reduces the cell’s charge.  However, for a short time, the outflow of potassium is moderated by an inflow of calcium.  Eventually the calcium channels close, allowing the cell to return to its initial charge.


An arrhythmia is a disturbance of normal heart rate, whether to beat too fast or too slow.  Arrhythmias can be extremely dangerous and a cause of sudden death.

There are many different ways arrhythmias can arise.  One of the most important is reentry.  Reentry is when your heart, so to speak, short circuits itself, and electrical impulses start going in a loop, out of control.

Other causes include disruptions of the electrical pathways of the heart.  There are many, many potential issues that can cause that, such as high levels of ions, genetic disorders, or sudden heart failure.

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