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Electrocardiography

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Electrocardiography

If you have ever watched a series about doctors in a hospital, you may have noticed a screen next to patients that shows waves and makes a beep sound. This is the machine that indicates a patient has passed away when the beep sound becomes continuous.

This machine is called an electrocardiography machine (or simply ECG), and it monitors the activity of the heart. Understanding the types of ECG and normal and abnormal ECG patterns is key to exploring the full medical potential of this technique.

The abbreviation ECG is used to designate both the electrocardiography technique and the diagrams they produce, which are called electrocardiograms.

How does the heart work?

First of all, we need to understand the basic functioning of the human heart to explore some of its behavioural characteristics. The heart is the organ that pumps blood throughout our bodies. Our blood carries oxygen (among other substances) to all our organs and body parts. The heart is a muscle with a soft structure that allows it to contract and expand continuously to pump blood through the veins and arteries. You can see its basic structure in the image below.

Electrocardiography Basic structure of the human heart StudySmarterThe basic structure of the human heart, ZooFari CC BY-SA 3.0

This image shows that the heart is divided into two sides, left and right, each with two chambers: the atrium (upper) and ventricle (lower). While each side performs a different role, they both act on two stages of pumping, which are controlled by valves and nodes.

The two-time pumping is produced by the contraction of the atria and the subsequent contraction of the ventricles. Each of these is controlled by the sinoatrial node or SA node (contraction of atria) and the atrioventricular node (contraction of ventricles). These movements are controlled by electric signals that can be measured and used to study and monitor the heart’s activity.

The study of the heart through these electric signals is called electrocardiography (ECG).

Upon measuring the electric signals, we can obtain an electrocardiograph or electrocardiogram.

An electrocardiogram is the graphical representation of the measured waves used in electrocardiography, which designates the general technique used to study the heart through electric signals.

The left part of the heart receives oxygenated blood from the lungs and distributes it throughout the body. The right part of the heart receives deoxygenated blood from the rest of the body and pumps it towards the lungs to be oxygenated.

The blood always flows from the (a) veins/arteries to the atria, (b) then to the ventricles, and (c) back to the veins/arteries. The function of valves is to regulate blood flow.

The ECG procedure

The movement of negative and positive ions inside and outside cells determines two things:

  1. The level of polarisation at cellular scales. If the net charge of a cell is zero, the cells are depolarised. When some ions leave the cell, there is a net electric force, and we say that the structure is polarised.
  2. By knowing the polarisation, we can know the state of cells. The flow of ions inside the cell causes the cells (and the whole muscular structure) to contract, and the flow of ions outwards causes cells (and the whole muscular structure) to relax.

Although it is out of the scope of this explanation to study how these processes work in-depth, we must mention that the flow of ions is a self-sustained process produced by the propagation of electric pulses generated in the nodes. The best way to measure the activity of these electric pulses is to measure the polarisation of the cells or, rather, the electric potential difference induced by this polarisation.

Upon measuring this potential difference, we can observe three distinct waves: the P wave, the QRS complex wave, and the T wave.

Wave
What it indicates
P waveIndicates the production and propagation of an electric signal by the sinoatrial node. It causes the contraction (depolarisation) of the atria.
QRS complex wave Indicates the electric signal leaving the atrioventricular node. This signal is responsible for the contraction (depolarisation) of the ventricles. At the same time, atria relax/repolarise.
T waveIndicates the relaxation (repolarisation) of ventricles.

The QRS wave is composed of the waves caused by the contraction of ventricles and the relaxation of the atria.

The presence of these waves can be seen in the image below.

Electrocardiography Normal sinus rhythm on ECG StudySmarterSchematic diagram of normal sinus rhythm in a human heart seen on an ECG, Wikimedia Commons

Types of ECGs

The types of electrocardiographic procedures depend on the scope of the test and the circumstance of the patient. In general, we can distinguish between four types of ECG tests.

A resting electrocardiogram

A resting electrocardiogram is carried out while the patient lies in a resting position. These ECGs are the most common since much of the noise that affects the signal can be avoided. This is crucial because the signals measured are very weak and need an amplification process. If the noise is not cancelled, it will be amplified too. This type of test is carried out for a preliminary evaluation of the state of the heart.

A stress or exercise electrocardiogram

A stress or exercise electrocardiogram is carried out while the patient is doing physical exercise, usually on a bike or a treadmill. These tests are less accurate since the prevention of noise is more difficult. They are used to investigate problems with the heart while a patient is doing physical activities.

Ambulatory ECGs/Holter monitors

Ambulatory ECGs/Holter monitors are devices strapped to the patient’s body. They are good for long-time monitoring of the heart (periods of one to two days) and detecting arrhythmias.

Loop recorders

Loop recorders are devices that are surgically implanted close to the heart. Medical professionals can record the data of the functioning of the heart for extended periods (weeks, months, or years) to detect specific kinds of problems like syncopes, seizures, or recurring palpitations. This method does not have a lot of noise issues since it is implanted under the skin. The device is usually only activated by the patient when experiencing specific symptoms.

Electrocardiography A Holter monitor StudySmarterA Holter monitor attached to a patient, Wikimedia Commons

Normal and abnormal ECG patterns

To have a visual reference, refer to the image above showing the intervals of the cardiac cycle. The peaks are labelled so you can quickly identify what each is associated with.

Normal ECG patterns

The typical time between the P wave and Q wave is about 0.12 to 0.2 seconds at rest. The same time is also typical between the R wave and the T wave. If the patient is doing exercise, we can expect a shortening of these times since the blood needs to be pumped quicker to meet the oxygen needs of the organs. This would be the normal form of an electrocardiogram.

Abnormal ECG patterns

When the heart is not functioning correctly, the general term for these symptoms is arrhythmia. An arrhythmia is the anomaly of times or intensity of the waves of an electrocardiogram. It is a general feature where the intensity of the waves is related to the potential difference generated in the phases of polarisation (relaxation of muscles), so we can find anomalies with a lower potential difference and anomalies with unusual periods of beating.

For instance, if the patient is not doing any exercise and the times between waves get shorter, it is a sign of a tachycardic episode. If one of the waves in the QRS phase can be distinguished from the main peak, we know there is a premature beat in one of the chambers.

By studying the specific waves in each case (their intensities and times), we can find many problems that are associated with specific parts of the heart (ventricles, atria, and nodes).

Electrocardiography (ECG) - Key takeaways

  • The heart is the organ that pumps blood throughout the body. It does so thanks to a complex pumping system based on chambers, valves, nodes, and electric impulses.
  • The study of the heart through these electric signals is called electrocardiography (ECG).
  • Different electrocardiographic procedures can be performed to identify different problems with a patient’s heart.
  • Electrocardiograms are made of three peaks that correspond to different phases of a heartbeat. The waves produced in these phases are the P wave, the QRS complex wave, and the T wave.
  • By understanding the usual features and anomalies of an electrocardiogram, medical professionals can accurately diagnose most heart problems.

Images

Heart diagram with labels in English. Blue components indicate deoxygenated blood pathways and red components indicate oxygenated blood pathways. https://commons.wikimedia.org/wiki/File:Heart_diagram-en.svg

Frequently Asked Questions about Electrocardiography

A Holter monitor is a portable device that measures the electric signals of the heart (over one or two days). It allows medical professionals to study the functioning of the heart and determine the presence of arrhythmias and other defects.

No, electrocardiographic techniques cannot detect defects in valves.

Electrocardiography is the study of the heart through the electric signals that cause the contractions that pump blood around the body. 

An electrocardiogram is the graphical representation of the measured waves used in electrocardiography, which designates the general technique used to study the heart through electric signals.

Final Electrocardiography Quiz

Question

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Answer

The heart has two atria and two ventricles.

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The blood passes first to the atria and then to the ventricles.

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Polarised cells indicate a relaxation of the heart and unpolarised cells indicate a contraction of the heart.

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Noise needs to be reduced in electrocardiography procedures.

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The order of the waves in an electrocardiogram is P wave, QRS wave, and T wave.

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Question

What can be observed in an electrocardiogram if the patient is doing exercise?

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Answer

A decrease in the time intervals between waves in an electrocardiogram. 

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Question

Which parts of the heart send the signals that contract the chambers?

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Answer

Nodes send the signals that contract the chambers. 

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Question

What is the function of the left part of the heart?

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Answer

The left side of the heart receives deoxygenated blood from veins and sends it to the lungs.

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What is the function of the right part of the heart?

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Answer

The right side of the heart receives oxygenated blood from the lungs and sends it to the rest of the body through arteries.

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Is the QRS wave a single wave?

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No, the QRS wave is composed of the waves caused by the contraction of ventricles and the relaxation of the atria.

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Question

How long can loop recorders be used for?

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Answer

Loop recorders can be used for years. 

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What does the intensity of the waves measure?

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Answer

The potential difference in the cells.

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Which particles flow in and out of cells causing contraction, relaxation, and potential differences?

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Answer

Ions flow in and out of cells causing contraction, relaxation, and potential differences.

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Question

What is the state of the chamber of a heart if its cells are polarised?

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Answer

The chamber of the heart is relaxed.

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Question

What is the state of the chamber of a heart if its cells are unpolarised?

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Answer

The chamber of the heart is contracted.

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