Your Heart Rhythm

A Primer On Cardiac Arrhythmias


The Electrocardiogram

The electrical signals that flow through the heart can be picked up from the skin, amplified, and printed on paper for your doctor to examine. This recording is called an "electrocardiogram" [pronounced EE-LECK-TRO-CAR-DEE-OH-GRAM], The electrocardiogram is sometimes referred to as an "ECG," "EKG," or "Cardiogram." By examining the ECG, your doctor can determine if an arrhythmia is present. The ECG can also give your doctor information about the overall health of your heart ­ whether it may be enlarged or whether it may be damaged from an old heart attack.

The ECG is generally recorded using 10 wires or electrodes that a technician attaches to your arms, legs, and across your chest. The electrodes are attached with either rubber straps, suction cups, or sticky pads (to hold them onto the skin). Good electrical contact is necessary to record the tiny signals generated by the heart, so a special cream is sometimes used to help maintain this contact. When it's finished, the ECG looks like a bunch of squiggles, but there is a tremendous amount of information on that paper. The ECG machine automatically records the signals from a number of different combinations of the electrodes. Each of these combinations is called a "lead" [pronounced LEED], and you may hear physicians referring to a "12-lead," which is a standard ECG containing recordings from 12 different combinations of electrodes. For example, Lead I records the electrical signals between your left arm and your right arm, and lead II records signals between your right arm and your left leg. The "precordial" or chest leads (referred to as V1 to V6) are applied across your chest. Having twelve leads allows the doctor to examine different anatomical areas of the heart ­ almost like a three-dimensional picture of the electrical activity.

The figure above is an example of a "squiggle" from one of these leads. The "squiggle" is actually the electrical signal that is produced during one single heart beat. Remember, however, it's the electrical signal that causes the heart beat, so there is actually a split-second delay between the production of the signal and the contraction of the heart muscle. There is a small hump in the beginning of the signal. This hump, called the "P-Wave," is the signal produced by the right and left atria. There is a flat area after the P-Wave which is part of what is called the PR Interval. During the PR interval the electrical signal is traveling through the AV node. The next large "squiggle" is called the "QRS Complex." The QRS Complex is tall, spikey signal produced by the ventricles. Following the QRS is another hump called the "T-Wave," which represents the electrical resetting of the ventricles in preparation for the next signal. When the heart beats continuously, the P-QRS-T waves repeat over and over as seen below.

Normal Sinus Rhythm

You might ask just what causes the electrical signals ­ what could actually be producing electrical currents large enough to be detected from the outside? Actually, each individual muscle cell (and nerve cell, for that matter) is capable of producing a tiny electrical signal through a complicated bio-electrical process known as "depolarization." It turns out that the membrane that surrounds each muscle cell has a number of special protein pores that control the flow of charged atomic particles in and out of the cell. The particles are known as "ions" and examples of these are individual atoms of the metals sodium, potassium, and calcium. Ions are charged with a tiny bit of positive or negative electrical energy, and it's the movement of these ions in and out of the cell that creates electrical currents that we can measure. Most tissues in the heart are "sodium-dependent," meaning that depolarization occurs because sodium rushes into the cell. Some cells in the heart, particularly in and around the sinus node and within the AV node are "calcium-dependent," meaning that it's movement of calcium ions that is primarily responsible for the cells firing. You may have heard of medications known as "Calcium-channel Blockers." These drugs are designed to block the movement of calcium into these cells, and so they primarily affect the sinus node and the AV node. In fact, understanding the physiologic mechanisms at work in the heart is of paramount importance in prescribing the proper medications to treat cardiac arrhythmias, which is why EP docs make good use of the advanced scientific data uncovered by dedicated researchers in the field.