This test is used routinely in the evaluation of people with high blood pressure and is the traditional way of seeing if the hypertension has had any affect on the heart.
When heart muscle contracts, it creates an electrical impulse, which can be detected on the surface of the body. The electrocardiogram (ECG or EKG) is performed by applying six electrodes to the skin on different parts of the chest. Another four are applied to each wrist and ankle. These pick up the electrical signal coming from the heart, which is amplified and written out on a paper strip. Each electrode looks at the heart from a different viewpoint, and the electrical waveform is slightly different in each one. The pattern of waveforms in the different leads provides a lot of information about the structure and function of the heart.
For the evaluation of high blood pressure, the two most important things are whether the heart is enlarged, and whether there's any sign of damage from a heart attack.