Explaining how an EKG works

What is an EKG?

EKG stands for electrocardiogram.  It should probably and sometimes is abreviated ECG, but EKG seems to have stuck as more popular.

Scientists have known for over 120 years that the heart gives off electrical currents when it beats, but it was the Dutch scientist Willem Einthoven in the early 20th century who discovered the nature of this phenomenon and who developed the electrocardiogram as a tool to look at the electrical conduction of the heart.  It has been a staple of diagnostic cardiovascular medicine since then.  The basic tracing used has changed little in decades, although the sophistication of the recording devices and the computer algorithms for automated interpretation have made dramatic changes in the last 20 years.

Each heartbeat begins with one tiny area of the heart muscle depolarizing.  When this happens the electrical depolarization, that triggers the muscle cell to contract quickly spreads to the next cell, and so on until the entire heart muscle contracts.  When the process proceeds normally, a small area in the right atrium called the sinoatrial node acts as the pacemaker for the heart, and triggers each heartbeat.  Specialized heart muscle cells called the purkinje fibers rapidly carry this stimulus to throughout the atrium and to a tiny group of cells that connect the atria to the ventricles, called the AV node, also called the Bundle of His.  This causes a very brief delay, and then the stimulus races off through the conduction bundles of specialized fibers in the ventricles to the muscle cells of the ventricles and then they contract to pump blood to the lungs and body.

The EKG is a measurement over time of the net electrical activity of the heart muscle.  It is measured in several directions simultaneously, and by interpreting the electrical currents of the heart throughout a series of heartbeats, from several angles, we can determine much about the function of the heart.  A piece of graph paper is dragged past a marker hooked to the measurement device of the electrical current at a fixed rate, and  this leaves a graph of the net electrical current between two electrodes.  This is the EKG.

Each heartbeat can be looked at in its various functions on the EKG.  The depolarization of the atrium is a relatively small current, that happens first, and is called the “P” wave.  After the P wave is the delay as the AV node does its job to stall long enough for the atrial contraction to pump blood into the ventricles, and then the depolarization of the ventricles takes place in a predictable pattern in a predictable period of time.  If it takes longer than usual for the ventricles to depolarize, we deduce that the specialized conduction fibers are not functioning normally, and we call this “heart block.”  If the electrical activity occurs in an unusual order, we can suspect injury to some parts of the heart muscle, and possibly a myocardial infarction, commonly called a heart attack.

The EKG can easily tell us the heart rate, the heart rhythm, and much more.  Newer variations include the ability to record 24 hour or longer ambulatory EKGs to look for rhythm problems, and electrophysiologists look at EKG like measurements of specific areas of the heart during cardiac catheterization to find the areas causing rhythm problems.  EKGs can be done during exercise as a stress test, can be synched with images of the heart during nuclear medicine scans to give even more information, and are used to time images during CT angiograms to improve images of coronary arteries.

Many thanks to Willem Einthoven.  His 1923 Nobel Prize in Medicine was well deserved, and has withstood the test of time.

Edward Pullen is a family physician who blogs at DrPullen.com.

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