Heart Beat 101

by Dr. Stephen Sinatra
Filed Under: Diagnostic Tools, Heart Health
Last Reviewed 02/06/2014

I did a lot of cardiac education “back-in-the day” when I worked in the hospital, and learned that many people really wanted to understand about their heart’s electrical system. I hope explaining these electrophysiology basics, which I find so fascinating, helps you better understand how your heart works. 

Forgive me if this content is “technical”, but there’s just no other way to explain it. It’s mathematical…

The basic electrocardiographic (ECG, or EKG from the German for “kardio”) representation of the heart beat is an important diagnostic tool in cardiology. It’s made up of several parts: the P wave, the PR interval, the Q wave, the QRS complex, the T wave, and the QT interval.

And ECG can give us vital information about how the heart’s electrical system is “conducting” business, if you will. heart beat 101
   
This ECG picture is of a normal heartbeat. Each starts with a small, convex waveform, followed by a talk spiked complex, and ending with a somewhat larger domed wave. Each of these components represents electrical energy moving through each pulsation of the heart, and each has a “normal range” that we can actually measure with calipers. Pure science.

Without getting too complicated, ECG paper is just like graph paper and the light colored vertical lines represent 0.05 milliseconds (msec). Each dark colored vertical line is made up of five smaller ones, or 0.20 msec  and we use calipers to measure the exact timing…pure math.  

The first dome is the P wave, and we use calipers just like you did in geometry class to measure time intervals. The P wave represents the sinus node’s electrical stimulation as it shoots across the atria—the two upper pumping chambers of the heart—and that discharge causes them to contract and eject blood into the two lower chambers. The normal time it takes from the initiation of this electrical discharge until it hits the next upstroke is 0.18 to 0.20 msec, and is called the PR interval.

The tall complex in the middle is referred to as the QRS complex; the Q wave is not always visible in each of the 12 leads or perspectives when looking at the EGG. The R wave is the first upward wave in the tall complex, followed by the down stroke or S wave. The QRS interval is considered within normal of it is 0.10 to 0.12 msec, starting at the end of the PR interval, and reflects the electrical stimulation of the two ventricles (lower pumping chambers).

Lastly, the T wave represents a purely electrical phenomena: the repolarization of the heart’s electrical conduction system and the resting phase between contractions.  The QT interval, from the end of the PR interval to the end of the T wave is approximately 0.40 msec.

The ECG represents the electrical event that triggers the muscular contraction component of each heart beat. It’s amazing!

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