Shiken premium Upgrade Banner

If you've ever seen a TV show about hospitals, you might have noticed a screen with waves and a beeping sound next to the patient's bed. This is an electrocardiography machine, or ECG for short. It tracks the heart's activity, and helps doctors understand if the heart is working properly. By learning about the different types of ECGs and understanding normal and abnormal patterns, doctors can make better use of this technique. You might hear doctors or nurses refer to the diagrams created by ECGs as electrocardiograms, or ECGs for short.

How does the heart work?

To understand the heart's behavior, we need to know how it works. The heart is responsible for pumping blood throughout our bodies, which carries oxygen and other substances to our organs and body parts. It's a muscle that contracts and expands continuously to keep the blood flowing through veins and arteries. The heart has two sides, left and right, each with two chambers: the atrium (upper) and the ventricle (lower). Each side performs a different role, but they both pump blood in two stages, controlled by valves and nodes. These stages are controlled by signals that can be measured and used the heart's activity. This study of the heart using electric signals is called electrocardiography (ECG). An electrocardiogram is the graphical representation of the measured waves used in ECG. The left side of the heart receives oxygenated blood from the lungs and distributes it throughout the body, while the right side receives deoxygenated blood from the body and pumps it to the lungs to be oxygenated. Valves regulate blood flow.

The ECG procedure

The movement of negative and positive ions inside and outside cells determines two critical things: the level of polarisation at the cellular level and the contraction and relaxation of the muscular structure. 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. The flow of ions inside the cell causes the cells (and the whole muscular structure) to contract, while the flow of ions outwards causes cells (and the whole muscular structure) to relax ions is a self-sustained process generated by electric pulses produced in the nodes. Measuring the polarisation of cells or the electric potential difference induced by this polarisation is the best way to measure the activity of these electric pulses.

Three distinct waves can be observed upon measuring the potential difference: the P wave, the QRS complex wave, and the T wave. The QRS wave is caused by the contraction of ventricles and the relaxation of the atria. While we won't delve into the details of these processes, it's essential to note that they are self-sust and determine the state of cells and the muscular structure.

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

Schematic diagram of normal sinus rhythm in a human heart seen on an ECG
Schematic diagram of normal sinus rhythm in a human heart seen on an ECG

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

The resting electrocardiogram is a commonly used test that is carried out while the patient is in a resting position. This is because much of the noise that affects the signal can be avoided in this. The signals measured during this test are very weak and require an amplification process. If the noise is not cancelled, it will be amplified as well, which can lead to inaccurate results. The resting electrocardiogram is typically used for a preliminary evaluation of the state of the heart, and it can provide valuable information about the heart's health and function. By conducting this test, healthcare professionals can detect any abnormalities or irregularities in the heart's rhythm, which can help them diagnose and treat various cardiac conditions.

A stress or exercise electrocardiogram

A stress or exercise electrocardiogram is a test that is conducted while the patient is performing physical exercise, typically on a treadmill or a stationary bike. These tests are more challenging to conduct and have a higher likelihood of noise interference, which can make the results less accurate than those obtained during a resting electrocardiogram. However, they are still valuable diagnostic tools that are used to investigate problems with the heart while a patient is performing physical activities. By conducting a stress or exercise electrocardiogram, healthcare professionals can monitor the heart's response to physical stress and detect any abnormalities or irregularities in the heart's rhythm that may not be present during a resting state. This information is crucial for diagnosing and treating various cardiac conditions, including coronary artery disease, arrhythmias, and heart failure.

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 small devices that are surgically implanted near the heart to record data on the heart's functioning over extended periods of time, ranging from weeks to months or even years. These devices are used to detect specific kinds of problems, such as syncopes, seizures, or recurring palpitations. Unlike electrocardiograms, loop recorders do not have a lot of noise issues since they are implanted under the skin, close to the heart. This allows them to capture accurate and reliable data on the heart's rhythm and function over long periods.

The loop recorder is usually only activated by the patient when experiencing specific symptoms. When activated, the device records data on the heart's electrical activity and stores it for later analysis by healthcare professionals. This information can be used to diagnose and treat various cardiac conditions and heart failure. The use of loopers has revolutionized the way healthcare professionals monitor and diagnose cardiac conditions, allowing for more accurate and reliable detection of heart problems in patients.

A Holter monitor attached to a patient
A Holter monitor attached to a patient

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, as well as the R wave and T wave, is approximately 0.12 to 0.2 seconds at rest. These intervals represent the electrical activity of the heart as it contracts and relaxes to pump blood throughout the body. During physical exercise, the body's oxygen requirements increase, and the heart must work harder to meet this demand. As a result, the intervals between the waves on the electrocardiogram may shorten, indicating that the heart is beating faster to meet the body's oxygen requirements.

Shortening of the P-Q and R-T intervals is a normal response to physical exercise since the heart is pumping blood at a faster rate. However, if the intervals become too short or irregular, it can indicate an underlying cardiac condition. For example, a shortened P-Q interval may indicate an accessory pathway or an atrioventricular block in the electrical conduction system of the heart. Similarly, a shortened R-T interval may indicate an increased risk of ventricular arrhythmias, which can be life-threatening.

Therefore, it is essential to monitor the electrocardiogram during exercise to detect any irregularities that may indicate underlying cardiac problems. By doing so, healthcare professionals can provide timely and appropriate treatment to prevent potential complications and improve the patient's overall health.

Abnormal ECG patterns

Arrhythmia is a general term used to describe abnormal heart rhythms or irregularities in the timing or intensity of the waves on an electrocardiogram. The intensity of the waves on an electrocardiogram is related to the potential difference generated during the phases of muscle relaxation, called polarisation. Therefore, anomalies may be due to a lower potential difference or unusual periods of beating.

For example, if between waves on an electrocardiogram shorten when a patient is at rest, it may indicate a tachycardic. If a wave in the QRS phase is distinguishable from the main peak, it is a sign of a premature beat in one of the chambers of the heart. By analyzing the specific waves in each case, including their intensities and times, medical professionals can identify various problems associated with specific parts of the heart, such as the ventricles, atria, and nodes.

Electrocardiography (ECG) is the study of the heart's electrical signals, and it is a crucial tool for diagnosing heart problems. Different electrocardiographic procedures can be performed to identify various heart conditions accurately. Electrocardiograms are made up of three peaks that correspond to different phases of a heartbeat: the P wave, the QRS complex wave, and the T wave. By understanding the typical features and anomalies of an electrocardiogram, medical professionals can accurately diagnose most heart problems and provide appropriate treatment.


Heart diagram with labels in English. Blue components indicate deoxygenated blood pathways and red components indicate oxygenated blood pathways.

Heart diagram with labels in English. Blue components indicate deoxygenated blood pathways and red components indicate oxygenated blood pathways


What is a Holter monitor used for in 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.

Can a defective valve be detected by electrocardiography?

No, electrocardiographic techniques cannot detect defects in valves.

What is electrocardiography?

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

What is the difference between electrocardiogram and electrocardiography?

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.

Join Shiken For FREE

Gumbo Study Buddy

Explore More Subject Explanations

Try Shiken Premium
for Free

14-day free trial. Cancel anytime.
Get Started
Join 20,000+ learners worldwide.
The first 14 days are on us
96% of learners report x2 faster learning
Free hands-on onboarding & support
Cancel Anytime