"The anger of health and social care workers is right"

ECG monitors the electrical activity of the heart and is used in many clinical settings. This article explores how the technology works and how it is implemented

The electrocardiogram assesses the electrical activity of the heart; it is commonly used as a non-invasive monitoring device in many different healthcare settings. This article is the first in a three-part series of articles, discussing cardiac electrophysiology, ECG indications, monitoring, and troubleshooting.

Citation: Jarvis S (2021) ECG 1: Purpose, Physiology, and Practicality. Nursing time [online]; 117: 6, 22-26.

Author: Selina Jarvis is a research nurse, Guy and St Thomas’ NHS Foundation Trust.

An electrocardiogram (ECG) is a quick bedside test that assesses the electrical activity of the heart. This is a non-invasive, inexpensive technique that provides key information about heart rate and rhythm, and helps in the assessment of heart disease. ECG monitoring is often used in many different healthcare settings, including emergency care, cardiac care, and preoperative evaluation.

This article is the first in a three-part series, discussing cardiac electrophysiology, ECG indications, monitoring, and troubleshooting. Part 2 of the series will use a methodical approach to explain, focusing on cardiac ischemia; Part 3 will explore heart rhythm and conduction abnormalities.

The heart is an organ that acts as a mechanical pump; it is composed of four chambers (left and right atria and left and right ventricles), which contract in sequence during the cardiac cycle and are regulated by a conductive system. To understand the basics of ECG, it is important to consider the normal electrophysiology of the heart, where electrical pulses of the heart are generated and transmitted to the heart muscle, causing contractions (heartbeats).

There are two main cell types in the heart: 

Cardiomyocytes contract and relax in response to electrical stimulation. In the resting state, the potassium ion (K+) level inside the cell is higher than that outside the cell; as well as the negatively charged protein, this creates a chemical gradient. Compared with the inside of the cell, there are more sodium ions (Na+) and calcium ions (Ca2+) outside of the cardiomyocytes. In general, this means that there is a voltage difference across the cell membrane, called the transmembrane potential (TMP). When the net movement of Na+ and Ca2+ enters the cell, TMP becomes more positive; when the net movement of positive ions comes out, TMP becomes more negative.

In response to electrical stimulation, the cardiomyocytes depolarize, and the fast Na+ channel on the cell membrane opens, allowing Na+ to enter the cell; because it is positively charged, the TMP becomes corrected and increases to -70 millivolts (mV) (resting potential is -90mV) ). This is the point at which sufficient Na+ fast channels are opened to generate an inward Na+ current, and is called the threshold potential. When the charge is greater than -40mV, the L-type calcium channel opens and allows Ca2+ to flow inward. This leads to excitation-contraction coupling, which leads to contraction of the heart muscle. After this, repolarization occurs; the cardiac membrane potential returns to a resting state without muscle contraction.

The electrical conduction system of the heart (Figure 1) regulates its overall electrical activity and includes the following components:

Each heartbeat is initiated by an electrical impulse generated by the SAN; this impulse travels through the atria to the AVN, and then through the left and right ventricles, the His bundle, the subsequent bundle branches and Purkinje fibers. As a result, when the impulse is conducted through different areas of the heart, the atria and ventricles contract in sequence. Under normal conditions, the SAN is the pacemaker of the heart; however, if there is a problem with the SAN, another conduction area center (such as AVN, His bundle, or bundle branch) can assume the role of a pacemaker in an event called escape rhythm (Jarvis and Saman, 2018; Newby and Grubb, 2018).

In healthy individuals, the ventricles contract and relax in a coordinated manner, which are called systolic and diastolic periods, respectively. The left and right atria are synchronized during atrial contraction and diastole, while the left and right ventricles are synchronized during ventricular contraction and diastole. A complete cycle of these events is called the cardiac cycle, during which the pressure in the heart chambers rises and falls, causing the heart valves that regulate blood flow between the heart chambers to open and close.

The pressure on the left side of the heart is about five times that on the right side, but the same amount of blood is pumped out in each heartbeat. During the cardiac cycle, blood flows from the high-pressure zone to the low-pressure zone (Marieb and Keller, 2018).

The origin of the electrocardiogram can be traced back to the discovery of myocardial electrical activity. In 1901, Willem Einthoven made a breakthrough, which promoted the first step of electrocardiography, and he subsequently won the Nobel Prize in 1924 (Yang et al., 2015).

The electrocardiogram is used as a technique for diagnosing heart disease and detecting abnormal heart rhythms. They can also be used as general health assessments for certain occupations, including aviation, diving, and military (Chamley et al., 2019). According to professional associations, adequate education of medical staff is essential for ECG monitoring and developing the skills to interpret waveforms and ECG data (Sandau et al., 2017).

In routine clinical practice, there are four main heart rhythm monitoring methods:

The 12-lead ECG is a non-invasive method for monitoring the electrical activity of the heart. This bedside test can provide important diagnostic information or be used as part of a baseline assessment; Box 1 outlines some instructions for using it.

If you are concerned that the patient’s acute symptoms may be related to the heart, continuous cardiac monitoring can be used in a hospital setting. This may help:

Continuous cardiac monitoring is also an important part of non-invasive vital signs monitoring and has clinical benefits in a medical ward setting (Sun et al., 2020).

ECG is a graphical representation of the electrical activity of the heart. Its voltage is plotted on the vertical axis and time is plotted on the horizontal axis. It is recorded on an electrocardiogram that runs at a speed of 25 millimeters per second. The standard pink ECG paper consists of 5 x 5 mm squares, and each square contains 25 smaller 1 x 1 mm squares. The width of 1mm of each small square represents 40 milliseconds. On the vertical axis, the height of the ECG wave or deflection represents its amplitude (Prutkin, 2020). Figure 2 shows the appearance of a normal ECG and its relationship with the phases of the cardiac cycle.

In a normal cardiac cycle, the atrial contractions that occur are related to P waves (atrial depolarization), and because the muscles in the atria are relatively thin, the amplitude is low. This is in contrast to QRS complexes, which represent the propagation of electrical impulses through the ventricles (ventricular depolarization). The first deflection of the QRS complex is the Q wave, which is a negative wave that starts depolarization of the ventricular septum. The R wave represents the depolarization of the left ventricular myocardium, and the next negative deflection is the S wave, which represents the terminal depolarization. After that, a T wave appears, representing the repolarization of the ventricle.

The electrocardiogram also records many other parameters:

It is important to understand the normal range of various ECG parameters (Table 1): if any measurement is outside the normal range, you need to think and investigate to determine the cause and decide to take action. Part 2 and Part 3 of this series will discuss this in more detail.

It is important to remember that electrical "leads" actually represent the potential difference measured at two points in space. The electrical impulse conduction between these two points in space can be detected by electrodes located at various points of the body; then it is displayed as a waveform on the electrocardiograph/monitor.

There are multiple configurations for electrode positioning; continuous ECG monitoring uses a 3-lead configuration, but the standard 12-lead ECG includes:

To accurately position the thoracic electrodes, first determine the sternal angle (Louis angle); this is done by feeling the bony protrusion on the top of the sternum, which is connected to the second rib above the second intercostal space. By moving your finger down, you can feel the fourth intercostal space: here, the electrodes of V1 and V2 should be placed on the right and left sides of the sternum, respectively. By feeling the fifth intercostal space, move your finger to the midclavicular line, V4 can be placed on the midclavicular line. Then V3 should be placed between V2 and V4. V5 is placed at the fifth intercostal space outside the anterior axillary line, and V6 is placed at the fifth intercostal space at the mid-axillary line.

To record the limb leads (Figure 3b), four electrodes are placed on the body. In the upper extremity, one electrode pad is placed under the right clavicle (arm), and the next electrode pad is placed under the left clavicle (arm); in the lower extremity, the cable is connected to the left hip/ankle (LL) and the right hip/ Electrode pads on the ankle (RL).

It is important to comply with local policies. All limb electrodes are placed on the bone area, not on the muscles, to avoid movement artifacts caused by muscle oscillations. Positioning electrodes in this formation allows the heart to be electrically mapped in three dimensions.

When performing any cardiac monitoring, the first step is to briefly explain to the patient the purpose of the test and what they should expect, and obtain their informed consent. It is important to ensure that they are not allergic to the gel used on the ECG electrodes by asking them if they have had any reactions before.

It is important for health professionals to be able to place the electrodes accurately—this will help avoid inaccurate diagnosis and treatment—and good contact between the electrodes and the skin is important, and the skin should be clean and dry. It may be necessary to shave excess hair and clean oily skin with alcohol or gauze. Then connect the electrodes to the patient according to the instructions of the machine. The ECG is displayed on the monitor of the machine, and its clarity, waveform size and any interference should be checked.

Insufficient ECG monitoring can be dangerous; for example, misinterpreting artifacts (ECG pulses that are not related to the electrical activity of the heart) during ECG monitoring can be costly and cause delays in care. Other potential problems and their solutions are listed in Table 2.

Excellent ECG tracking must be obtained to help with proper interpretation and provide the best care. The ECG Guidelines of the Association for Cardiology Science and Technology (2020) provides more information about the reporting standards used by professional associations.

ECG monitoring is a standard configuration for patients in various environments. Understanding the basic physiology that underpins the electrical and mechanical events of the heart is essential for the interpretation of the electrocardiogram. Part 2 of this series will focus on this and introduce important ischemic pathologies, while Part 3 will cover heart rhythm disturbances and conduction defects.

Selina Jarvis is the recipient of the Mary Seacole Development Award, focusing on improving the care of patients with heart disease.

Chamley RR et al. (2019) ECG interpretation. European Heart Journal; 40: 32, 2663-2666.

Jarvis S, Saman S (2018) Heart System 1: Anatomy and Physiology. Nursing time [online]; 114: 2, 34-37.

Marieb EN, Keller S (2018) Essentials of Human Anatomy and Physiology. Pearson.

Newby DE, Grubb NR (2018) Cardiovascular disease. In: Ralston SH et al. (Editor) Davidson's Principles and Practice of Medicine. Elsevier.

Prutkin JM (2020) ECG tutorial: electrical components of ECG. Latest website

Sandau KE et al. (2017) updated the practice standard of electrogram monitoring in the hospital environment center: the scientific statement of the American Heart Association. Circulation; 136:19, e273-e344.

The Society of Cardiology Science and Technology (2020) Consensus Clinical Guidelines: ECG Reporting Standards and Guidelines. Science and Technology Committee.

Sun L et al. (2020) The clinical impact of multi-parameter continuous non-invasive monitoring in hospital wards: a systematic review and meta-analysis. Journal of the Royal Society of Medicine; 113: 6, 217-224.

Yang XL et al (2015) The history, hot spots and trends of electrocardiogram. Journal of Geriatric Cardiology; 12: 4, 448-456.

This is very educational and helps to deepen my understanding of ECG evaluation.

Log in or register for a new account to join the discussion.

The Nursing Times hosted a free webinar to discuss the role of technology in improving wound assessment and management.

Cornwall Cooperative NHS Foundation Trust

Jersey Department of Health and Social Services