How does an EKG work?

I read a recent guest post on KevinMD where Dr. Edward Pullen talks about how an EKG works, which made me want to review its function for my blog as well. The electrocardiogram, or ECG (abbreviated EKG for the German, electrokardiogram), is one of the most common tools in Cardiology. The technology was invented somewhere around the turn of the 20th century, primarily credited to a researcher named Einthoven. He was later honored with the Nobel prize for the invention in 1924 (Lots more info at Wikipedia).

The basic idea is that you put a series of electrodes (most commonly 12) across the chest, arms, and legs. A computer then uses each of the stickers to generate a series of circuits which correspond certain vectors across the chest. Einthoven, on the other hand, had to use buckets of salt water to achieve the signal conduction. With each heartbeat, the electrical impulses of the heart generate a measurable voltage along the 12 vectors and you can determine certain aspects of how well the heart is functioning.

The ECG is now used in several ways. There is the standard 10 second strip that you might get in a doctor's office. They can also do longer "rhythm strips" which uses the same machine but just gathers data for a longer time. There are Holter monitors, which gather every heartbeat for 24-48 hours and come with a button that a patient can push when they feel a symptom. The doctor can then correlate the button push to the ECG and see if anything was happening in the heart at that precise moment. There are extended "event monitors" which you wear for about 30 days, but only record when the button is pushed, because they would otherwise be gathering an unmanageable amount of data. Some monitors also permit 24 hour, real-time monitoring where you wear the monitor and someone at a remote data center continuously looks for anything abnormal. Last, there are stress tests, where you get hooked up to a ECG machine while you walk on a treadmill or ride a bike (see my post on nevernotrunning.com when I went for a run on a stress test).

There are two primary functions of the ECG, to determine the heart's rhythm and to determine if the heart is getting enough oxygen. The Holter and event monitors I mentioned above are prime applications for looking into what is going on for patients who feel skipped beats (palpitations) or a racing heart. The ECG changes in a very specific way (the ST segment in the image below) when the heart is not getting enough oxygen, and that is the idea behind a stress test. When you walk/run and "stress" your heart, if you have blockages in the heart arteries (coronaries) then the ECG will change and your doctor gets important information about how to help. The exercise ECG stress test is only moderately accurate though, which is why we often have to use nuclear stress tests, cardiac CTs, and cardiac catheterization.