Auscultation | Echocardiography | ECG | MRI | Pedigree

Electrocardiography (ECG)

You are probably familiar with this scene from a typical hospital television show: A patient is hooked up to a monitoring machine that shows voltage traces on a screen and makes the sound "...pip...pip...pip.....peeeeeeeeeeeeeeeeeeeeee" as the patient goes into cardiac arrest. This type of machine is routinely used to obtain an electrocardiogram (ECG), which is a graphic representation of the heart's electrical activity over time. To obtain a standard ECG (as shown in the figure below), a patient is connected to the machine with 3 electrical leads (one to each wrist and to the left ankle) that continuously monitor heart activity. However, ECG can look a lot different, depending on the number of leads used. For example, for a detailed evaluation of the heart's function, multiple leads are attached to the chest region. Here, we will talk only about the representative three leads, a standard ECG.

Each peak in the ECG is identified with a letter from P to U that corresponds to a specific electrical activity of the heart:

The P-wave represents the electrical excitation (or depolarization) of the atria, which leads to the contraction of both atria.

The QRS complex represents the depolarization of the ventricles, which initiates the ventricular contraction. The contraction starts shortly after Q and marks the beginning of the systole.

The T-wave represents the return of the ventricles from excited to normal state (repolarization). The end of the T-wave marks the end of systole.

The U-wave is usually very small and represents the repolarization of a collection of specialized muscle fibers that make up the pacemaker system, which is responsible for spreading the electrical signal throughout the ventricle.

Obviously, by counting the number of QRS complexes that occur in a given time period, one can determine the heart rate of an individual, but an ECG can give a lot more information. For example, since the ECGs obtained from different individuals have roughly the same shape for a given lead configuration, any deviation from this shape indicates a possible abnormality or disease.