Electrocardiography Unit Health Article

Definition

The electrocardiography unit, also called an electrocardiograph, is an apparatus that indirectly measures the heart's electrical activity and records it as a graphic tracing.

Purpose

Electrocardiography is used to detect heart-function abnormalities. It indirectly detects the heart's electrical activity over time by measuring the electrical potential at the body's surface. If the heart's activity varies from normal, signs of this are seen in the surface electrical potentials. The electrocardiography unit produces a visual representation of the electrical potential, called an electrocardiogram (EKG or ECG), that is often recorded as a continuous line along a strip or special graph paper. When used to diagnose a heart ailment or check the effectiveness of a heart treatment, doctors, nurses, and other technicians read the strips, looking for telltale signs of various cardiac problems.

Because the electrical activity of the heart is the basis for its workings, many heart problems show up in an EKG tracing. The machine can detect coronary artery disease, where the blood vessels carrying blood to the heart have hardened and no longer work effectively; a heart attack, either current or previous; and arrhythmias, a heart beating at an abnormal speed or rhythm.

Description

The electrocardiography unit is a machine that transfers the very faint electrical signals of the heart into a visual representation of that activity. The unit commonly includes multiple electrodes and leads (often 12, but as few as three), a galvanometer to measure the electrical signal, an amplifier and filter to convert the faint electrical signal to one that can be seen, a computer screen or oscilloscope to display the output, and an ink-and-paper arrangement to produce hard copies of the signal.

In the simplest arrangement, three pairs of bipolar electrodes, where one is positively charged and the other negatively charged, are placed on particular areas of the patient's body. The electrodes are adhesive pads filled with conductive gel that are attached to the patient's skin. Wires called leads connect the electrodes to the unit. The electrical signal measured by each group of electrodes is also called a Lead. When used to identify the signal, the term is often capitalized.

Electrode placement

The first pair of electrodes has the positive electrode placed on the left arm and the negative on the right arm; this produces Lead I. The second pair has the positive electrode placed on the left leg and the negative on the right arm; this produces Lead II. Lead III comes from the third pair, where the positive electrode is placed on the left leg and the negative on the left arm. If an imaginary line is drawn between each pair of these electrodes it forms a triangle where the electrical difference (measured in volts) can be detected across each side of the figure. This arrangement is known as Einthoven's triangle, after the inventor of the electrocardiograph.

Finer measurements of electrical potential can be made if additional sets of electrodes are used. The first additional set of electrodes is known as the augmented connections because the signals are significantly weaker than Leads I–III and have to be additionally increased, or augmented, by the machine. These Leads are unipolar, meaning that they measure the electrical difference between that electrode and a group of others. Lead aV_R measures the signal between the right arm (the positive electrode) and the average of the signals from the left arm and the left leg (two negative electrodes). Leads aV_L and aV_F are similarly arranged, with the left arm and the left leg having the positive electrode, respectively. All six of these limb leads measure electrical activity in the frontal plane of the heart, through the middle from top to bottom.

A second additional set of electrodes are the unipolar chest Leads, also known collectively as the modified chest Lead (MCL). These Leads measure electrical potentials across the horizontal plane of the heart. They are unipolar, measuring the electrical difference between the positive electrode and the average of the collective signal from the right arm, left arm, and left leg. There are six leads placed across the chest, numbered V1–V6 from the patient's right to left. V1 and V2 are put on either side of the sternum, in the fourth intercostal space (the space between the fourth and fifth rib). V4 is placed in the fifth intercostal space (between the fifth and sixth rib) on the line that divides the clavicle in half. V3 is placed halfway between V2 and V4. Both V5 and V6 are placed horizontal to V4, with V5 on the line that runs down the body from the inner armpit and V6 on the line that divides the armpit.