The heart is one of my favorite subjects to write about! From my first science class with Ms. Shari Mosley in elementary school to advanced cardiology with Dr. Mike Cole in college, I have always been fascinated by how this muscle works.
Recently, I was asked to write a column on a common medical ailment that affects a great number of my friends and neighbors: Atrial Fibrillation, or A-Fib. A-fib. Chances are high that you have at least heard of the condition. With so many people who have the condition, medications that treat or keep the side effects of A-fib at bay have become household names. I bet that you have at least heard of Coumadin, Lopressor, or Pradaxa.
To understand the condition of A-fib and the problems that it can cause, let’s take a look at the electrical system of the heart. Unlike the rest of the body, the heart has the amazing ability to generate its own nerve impulses. We term this characteristic “automa
ticity.”
Just below the outside cells of the heart muscles, there is an underground highway of nerve-endings that generate and transmit electrical signals that tell the cells in your heart to beat at a certain rate and rhythm. This starts at the very top of the heart in the SA node. The SA node is the primary pacemaker for the entire heart and is what most healthy hearts rely on to tell it to begin a beat. The signal then travels down the electrical pathways to the AV node which is in the center of the heart.
The AV node is kind of like a hind-catcher in baseball that catches the pitches and holds it momentarily to allow the umpire to make the call. However, the AV node is holding the impulse to allow the top half our heart to beat.
From there, the electrical signal travels down the electric highway to the bottom half of the heart. Without the AV node holding onto the signal, our top half of heart and the bottom half would beat at the same time, which would be bad.
Atrial Fibrillation occurs when a certain group of cells in the atrium (top half of the heart) become irritated for some reason, generally due to low-oxygen reaching them. Remember, the entire heart has automaticity which means any group of cells can take over as the dominant pacemaker and “override” the SA node.
When this group of cells decides to tell the heart to beat, they generally send out hundreds of erratic and irregular signals to the AV node. The AV node can’t tell the difference between signals from the atrial cells and the SA node, thus, it lets through signals as it sees fit. When this occurs, we call it Atrial Fibrillation.
One of the great things about the delay in the AV node is that it doesn’t let through all the signals it receives at once. It usually does a good job of filtering out the hundreds of signals that the atrium is sending down to it and only lets 60-100 per minute pass. This is what makes the fact that the AV node holds the signal for a brief amount of time so important.
Occasionally, it will get bombarded with signals and begin to let them all through. I think about the “I Love Lucy” episode where she Lucy and Ethel can’t keep up with the conveyor belt at the chocolate factory so they just decide to eat them as they come down. In this condition, called “A-Fib Rapid Ventricular Response,” we can see the heart rate soar to rates around 200-250.
Next week, we will discuss the causes, treatments, and outlook for people living with A-fib. Until then, stay safe out there.
M. Eric Williams, MS, NR-P is a Mississippi native and Instructor of Emergency Medicine. He is a Doctoral Candidate in Higher Education and has 15 years’ experience in healthcare. If you have questions or comments, you may contact him at eric.williams@jcjc.edu.
