Electrocardiogram (ECG) interpretation

An electrocardiogram (ECG, EKG) is a graphical record of the electrical currents produced by the heart during its action, which is useful for assessing heart rate and rhythm, as well as diagnosing conditions such as myocardial ischemia and arrhythmias (Hampton, 2019).

Every contraction of the heart is associated with changes in its electrical potential, occurring due to the exchange of positive and negatively charged ions (Fleming, 1980). At rest, the inside of the cardiac cell is negatively charged in comparison to the outside (a difference of –90 millivolts). When the cell is stimulated, ion exchange occurs, briefly altering polarity, so the inside of the cell becomes positively charged compared to the outside, referred to as depolarisation. When two or more electrodes are placed on the skin, on opposite sides of the heart, a circuit is formed, enabling any electrical activity between them to be recorded.

The aim of this article

The heart contains a very specialised system of neuro-muscular tissues, these are responsible for the development and transmission of the depolarisation wave through the myocardium (Figure 1). The system can be broken down into component parts:

1. The sino-atrial (SA) node - this is situated in the right atrium of the heart close to the inlet of the inferior vena cava. It has the highest discharge rate (up to 100 beats per minute) but is usually slowed by the influence of the sympathetic nervous system. It is the normal site of impulse formation in the heart.

2. The atrio-ventricular (AV) node - this lies in the septum, close to the junction of the atria and ventricles, above the fibrous ring. Its discharge rate is slower than the SA node, but it can act as a secondary pacemaker should the SA node fail. In normal conduction, it receives impulses from the SA node

Recording an ECG

1. Explain the procedure to the patient and obtain consent before commencing
2. Ensure that the patient is as comfortable as possible, and is physically and mentally relaxed. The environment should be warm, and no other electrical apparatus should be attached, if at all possible, as this could affect the validity of the reading
3. It may be possible to obtain a clearer EGC recording by reducing the skins natural resistance, by abrading the skin with a skin prep, or alcohol swab prior to applying the electrodes, ensuring that the skin is dry prior to attaching the electrodes. Additional electrode cream or gel may be used
4. Ten electrodes are attached to the chest and limb (Figures 2 and 3). Ensure that the electrodes are connected correctly and that the wires are laid out in the correct fashion
5. Once the electrodes have been applied, wait for a brief period in order

Nursing and Midwifery Council: standards of proficiency for registered nurses

Part 1: Procedures for assessing people’s needs for person-centred care

2.1 take, record and interpret vital signs manually and via technological devices

2.3 set up and manage routine electrocardiogram (ECG) investigations and interpret normal and commonly encountered abnormal traces

2.5 manage and interpret cardiac monitors, infusion pumps, blood glucose monitors and other monitoring devices

Fleming JS. Interpreting the electrocardiogram. London: Kluwer Academic Publishers Group; 1980

Hampton J. The ECG made easy. 9th edn. London: Elsevier Health Sciences; 2019