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How the Heart Works: Part 2

Updated on December 1, 2013

Electrical Paths through the Heart

When you think of the heart, I'm sure most people don't think of it as a series of electrical currents, but that's half the story when talking about the heart.

Electricity is what causes your heart to pump. A small current travels through, and wherever it runs, it causes a contraction (beat). That's why, for certain abnormal heart rhythms, you can "shock" somebody out of it.

-The pacemaker of the heart is the SA node. This is in the top corner of the heart.

-The top part of the heart is divided into 2 chambers, called the left and right atria.

-The middle of the heart that connects the electricity from the top to the bottom is called the AV node.

-The bottom of the heart is divided into 2 chambers, the left and right ventricles.

A Normal Heartbeat

In a normal heart rhythm (called normal sinus rhythm), the electrical pathways follow a very predictable course. For the purposes of this article, I'm going to skip the full terms for everything and use the shorthand.

Your heartbeat starts with a small electrical impulse in the top of your heart. It quickly travels from the top corner (the SA node), through the top portion of your heart (the atria), causing it to beat, pushing blood down into the bottom part of your heart (the ventricles). The electric impulse moves very quickly, too quickly for an efficient heartbeat. So after it causes the top part to beat, it slows down just a little while traveling to the bottom. This allows the blood to flow down from the top of the heart to the bottom. It then spreads through the bottom part of the heart, completing the heartbeat.

-Rhythm changes occur with any disruption of the electrical pathways.

-Afib has too many impulses from the top part of the heart.

-Blocks are caused by impulses not traveling through the middle of the heart appropriately.

How does an Abnormal Rhythm (arrhythmia) Happen?

OK, so we've covered the normal electrical pathways of the heart, but these can be interrupted. The most common disruption is a PVC. A PVC is just referring to the ventricles getting a little excited and beating early, and these typically don't affect a person.

However, more serious heart rhythms can occur. Another, rather common, arrhythmia is called afib. In afib the top part of the heart, for whatever reason, has become too excited. You have impulses starting from all over, so you don't have a true beat. Instead, with so many impulses from so many different places, the top half of the heart just kind of quivers.

There are also "blocks" that can occur. This simply means that the impulse coming from the atria are being blocked to some degree in the middle part of the heart, where the electrical impulse slows down. It could just be getting slowed down too much, or the impulse could be completely blocked and not travel to the ventricle. Some of these require a permanent pacemaker.

But, all isn't lost in these situations. Every portion of your heart has the ability to generate an impulse. If the regular pacemaker fails, the middle of your heart can generate the impulse.. but it's not quite as good as the top. A normal heart rhythm has 60-100 beats per minute (bpm). If your AV node is producing the impulse, it's normal is 40-60 bpm. And, should it fail, the bottom part can generate the impulse... at 20-40 bpm.

I hope this has helped you understand a little about the heart!


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