Charlie goes to Medical School

12-Lead ECG Readout - Courtesty of MoodyGroove

The EKG…well first off, it really should be E-C-G…The ECG is a ubiquitous symbol frequently used to represent health care in a variety of settings.  On medical TV shows, you will frequently hear the blip of an ECG in the background.  On things from billboards to stationary, an ECG wave is used in forms from clip art to an iconic logo.  Since the ECG is such a common symbol, it should be interesting for us to take a look at it.  So what is an ECG and what does it tell us?

Electrocardiography is a tool medical professionals use to map the electrical activity in the heart.  A common misconception is that it directly represents the beating of the heart when in actuality it is just mapping the electrical signals which trigger the mechanical actions of cardiac muscle.  So you are able to see information about the timing of cardiac events (e.g. atrial fibrillation), but not necessarily the quality of those events (e.g. valve prolapse or regurgitation).

Understanding the Signals in the Heart

Action Potential - Courtesy of Chris 73

What are these electrical events that we are looking for?  Well, first you are going to have to understand a little about what kind of electrical signals we are talking about.  Nerves and muscles cells maintain an electrical imbalance across their cellular membrane.  The inside of the cell is slightly negative to the outside.  During a signal (Action Potential), the cell depolarizes…or actually, the polarity reverses somewhat.  When one cell gets a stimulus to fire, it then passes that signal on or performs some action.  The end result of this process for our purposes here is a muscle contraction.

However, cardiac muscles have two interesting characteristics…which are not entirely important for understanding the ECG, but are worth discussing.  One, differing from nerves and other muscle cells which have quick electrical signals, cardiac muscle cells have slow electrical signals caused by a delayed repolarization.  This regulates the heart and prevents it from seizing (tetanus).  Second, cardiac muscle and pacemaker cells will continuously depolarize and self-trigger following a contraction.  This means that a heart will continue beating without any input from an outside source and that heart cells will continually contract without any input.

The Signaling Pathway

Cardiac Pacemaker Signal (Slow Repolarization) - Courtesy of Ksheka

Heart cells will self-trigger and regulate themselves but at different frequencies from each other, so a series of pacemaking cells is used to coordinate cardiac muscle tissues into a series of effective contractions.  I won’t go into names here, but the pathway is somewhat important for this discussion.  To start, a set of pacemaking cells near the top of the heart signal the right and left atria (top chambers) to trigger their contraction.  They also signal a second set of pacemaking cells which then trigger the next series of signals.  This second set of pacemaking cells send signals down the heart’s septum (middle wall).  These signals will eventually trigger the ventricles (bottom chambers).  However, since the ventricles are much larger, the signals are routed through a series of conducting fibers which disperse the signal evenly so that the ventricle muscle cells are all triggered at approximately the same time.  This means that the atria get signaled first, causing them to depolarize and contract, followed by the much larger ventricles through a longer pathway, causing them to depolarize and contract.  The chambers are coordinated, but offset so that the atria push blood into the ventricles followed by the ventricles contracting to send blood out into the aorta and pulmonary arteries.

Reading the EK…er..C…G

The Ps and Qs of an ECG, The Cardiac Cycle - Courtesy of Anthony Atkielski

An electrocardiogram uses several leads to determine a baseline for electrical activity in the body as well as any deviations from it.  The depolarizations of the cardiac muscle tissue of the atria and ventricles will show up as brief electrical potential (or voltage) differences between leads that go across the heart.  These potential differences will then get recorded on a readout…a piece of paper or a vitals monitor or a new iPhone.

The P wave is the depolarization of the atria muscle.  This event is triggered by the first set of pacemaking cells and propogated through the muscle cells causing them to fire as the send the message.  This is why the P wave is round.

The QRS complex is a large spike representing the depolarization of the ventricular muscle.  This event is triggered by the second set of pacemaking cells and propogated through conducting fibers which coordinate the the depolarization of the ventricular muscle.  This is why the QRS complex is sharp.  The ventricles also contain more muscle mass, so the QRS complex is larger than the P wave.

The T wave is the repolarization of the ventricular muscle tissue.  This event is important because repolarization of a muscle cell must occur before it can once again depolarize.  This event naturally follows depolarization, but can be affected by electrolyte levels.  This event is not coordinated like the depolarization of the muscle, so the wave is round.

Atrial muscle tissue also repolarizes, but this event typically occurs during ventricular depolarization (the QRS complex) which overpowers any reading that the atrial repolarization might give off.

These events together represent the cardiac cycle on an ECG.  A normal cardiac cycle is referred to as sinus rhythm.  In analyzing the ECG, the size and shape of the waves on an ECG are considered as well as the duration of events and the intervals in between them.

This is a very, very simplified description of an electrocardiogram.  There is a lot of diagnostic information that can be garnered from analysis of electrocardiography, much of which I have yet to learn.  Although, at the same time, a few months ago Wife went under the knife to help correct a little issue with an old cooking injury.  It was somewhat of a relief to be able to see normal sinus rhythm on the monitor when they brought me into recovery to be with her.