Charlotte Lee, age 14, of Watertown, N.Y., for her question:

CAN YOU EXPLAIN ELECTROCARDIOGRAPHY?

Electrocardiography in medicine is the procedure by which a doctor obtains a tracing of the electrical activity of the heart. The rhythmic beating of the heart is maintained by an orderly series of discharges originating in the sinus node of the right atrium and proceeding through the antioventricular node of the bundle of His to the ventricles.

By attaching electrodes to various parts of the body, a record of this current can be obtained. This record is called an electrocardiogram, or ECG or EKG for short.

Prominent parts of the ECG are the P wave, a deflection caused by the current originating in the atrium; the QRS complex, showing the passage of the electrical activity into the ventricles; and the T wave, as the ventricles enter a quiet state again.

In a person whose heart is beating with an abnormal rhythm, the ECG is often helpful in showing the cause.

Recordings are made by an inked stylus on paper or by a heatea stylus on specially coated paper. Modern machines weigh 30 pounds or less.