Electrocardiogram (ECG): Procedure, Types & Clinical Correlation (2023)

  • Written By Shilpa Madhukar Pachapurkar
  • Last Modified 24-01-2023

Electrocardiogram (ECG): Procedure, Types & Clinical Correlation (1)

Electrocardiogram (ECG): Have you ever observed in movies or series, when a patient is in critical condition a graph is being shown near his/her bed? What is this graph and what does it signify? This graph is commonly called an electrocardiogram, commonly abbreviated as EKG or ECG. Let us learn more in detail about ECG in this article.

What is ECG?

An electrocardiogram, abbreviated as ECG or EKG, is a measurement of the electrical activity of the heart during the cardiac cycle and can be used to identify if there are any issues with the normal functioning of the heart. Heart muscles are self-excitable and generate electrical impulses. It has become the most commonly used test in patients’ evaluation and an essential part of cardiac care plans.

Electrocardiogram (ECG): Procedure, Types & Clinical Correlation (2)
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Development of ECG: A Brief History

The development of electrocardiography machines \(120\) years ago revolutionized the study of heart diseases. Initial attempts to record the heart impulses were done using a mercury capillary electrometer by Augustus Waller in \(1887.\) Willem Einthoven is considered to be the founder and father of cardiac electrocardiography.

He started his studies with the mercury capillary electrometer and improved its distortion mathematically to record the good representation of ECG. He further improvised the ECG recordings with the introduction of a string galvanometer that he designed.

Relation between Electrocardiogram (ECG) and Electrical Activity during Cardiac Cycle

A special bunch of cells, the sino-atrial node (SA node), also commonly referred to as a natural or primary pacemaker, is located on the upper right-hand side of the right atrium. It is this part of the heart that generates the electrical impulse which then passes down the other chambers of the heart. The path and duration of this electrical activity of the heart are traced using a special machine and such tracing is called an electrocardiogram. It is a painless method commonly used to monitor heart health. A typical cardiac cycle has the following steps:

  1. SA node sends the electrical impulse, it travels first through atrial muscles causing their depolarization.
  2. This depolarization leads to atrial contraction, pumping the blood to the ventricles.
  3. Then the electrical impulse travels to the atrioventricular node, the bundle of His and Purkinje fibers causing the depolarization of ventricular muscles.
  4. Depolarization of ventricular muscles takes place causing the ventricles to contract and pump the blood out of the heart.
  5. At the same time, atrial repolarization takes place and atria relax.
  6. This is followed by ventricular relaxation owing to the repolarization of ventricular muscles.
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Electrocardiogram or ECG Machine

To trace the electrical activity of the heart, special kinds of machines are used and they are known as ECG recording machines or electrocardiograms. The process of developing ECG is called electrocardiography. A typical machine uses few electrodes (sensors) and a central unit. Today’s modern-day ECG machines have analog to digital converters which convert the analog inputs to digital inputs. Such digital signals are either displayed on the screen or printed on paper.

Parts of Electrocardiogram or ECG Machines

All modern-day ECG machines come with a keyboard, screen, and printers attached to the screen. Apart from the central unit, the important part of the electrocardiogram machine is the ‘lead’. Lead is a pair of electrodes attached to two different body parts. One pair of electrodes makes one lead. Commonly 10 electrodes are attached to the body parts to measure electrical potential differences.

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These electrodes have conductive patches that can carry and amplify electrical signals generated during cardiac activity. The leads are attached to the arms, legs, and chest.

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Types of Electrocardiogram or ECG Machines

Many different models of ECG machines are available in the market. The smaller, portable device may use only a single pair of electrodes while \(6\) paired portable machines are also used.

  1. Resting ECG machines are immovable and comparatively larger with screens and printers.
  2. Some devices like the Holter monitor, which is a wearable device, can measure continuous ECG for \(24\) to \(48\) hrs.
  3. An event monitor is a portable device and measures ECG only at certain times and for a few minutes. Some wearable devices automatically record the ECG when an abnormality is detected.

ECG Procedure

The ECG process is non-invasive and does not take long. There are no restrictions on food and drink before the test. However, it is good to let the doctor or technician know about the allergies to sticky solutions or tapes if you have any. The steps carried out to develop ECG are as follows:

  • 1. Remove upper clothing so that electrodes can be attached to the chest and limbs.
  • 2. Electrodes are attached to arms, legs and chest with a sticky gel. Electrodes work best with clean, dry and oil or lotion-free skin.
  • 3. In resting type ECG, no movement is allowed during resting ECG as electrical impulses from other muscles may interfere with that of heart muscles.
  • 4. The recording of resting ECG takes about \(5\) to \(10\) minutes.
  • 5. With wearable Holter devices, one is free to move. For an exercise stress test (EST), one is supposed to walk on a treadmill or ride a bicycle while taking an ECG and this procedure takes about \(15-30\) minutes.

ECG Waves

A typical ECG is a wave pattern and consists of \(3\) waves as \({\rm{P}}\) wave, \({\rm{QRS}}\) complex, \({\rm{T}}\) wave and the intervals between them. These waves are representations of the contraction and relaxation of the atria and ventricles of the heart during a cardiac cycle. Details of each of these waves and intervals are as follows:

  1. \({\rm{P}}\)wave: It represents the wave of depolarization of atria due to the signals received from the SA node. It lasts for \(0.08\) to \({\rm{0}}{\rm{.1sec}}\) (\(80- 100\) milliseconds) in duration.
  2. PR interval: the time period between the onset of \({\rm{P}}\) wave to the beginning of \({\rm{QRS}}\) complex is known as the \({\rm{PR}}\) interval. The duration of the \({\rm{PR}}\) interval is typically \(0.12\) to \({\rm{0}}{\rm{.20sec}}\) (\(120\) to \(200\) milliseconds). This time represents the gap between the onset of atrial depolarization and the onset of ventricular depolarization. It is due to the delay caused by AV nodes in sending signals.
  3. QRS complex: The \({\rm{QRS}}\) complex represents ventricular depolarization. Owing to the large muscle mass of ventricles compared to atria, the amplitude of the \({\rm{QRS}}\) complex is much more than the \({\rm{P}}\) wave. This is a rapid phase of ECG and the duration of the \({\rm{QRS}}\) complex is \(0.06\) to \(0.10\sec \) (\(60\) to \(100\) milliseconds). Counting the number of \({\rm{QRS}}\) complexes is used for the interpretation of the rate of heartbeats.
  4. ST-segment:It is part of the ECG cycle following the \({\rm{QRS}}\)) complex and ending at the onset of the \({\rm{T}}\) wave. This segment represents the period where both the ventricles are completely depolarized.
  5. T wave: This wave of electrocardiogram represents ventricular repolarization. It is longer in duration than the \({\rm{QRS}}\)) complex because the wave of repolarization is slower than that of depolarization. Sometimes a small U wave may be seen after the \({\rm{T}}\) wave and it represents the remnants of ventricular repolarization.
  6. QT interval: It is measured from the beginning of the \({\rm{QRS}}\)) complex to the end of the \({\rm{T}}\) wave. It represents the time for both, ventricular depolarization and repolarization. This interval can range from \(0.2\) to \({\rm{0}}{\rm{.4sec}}\)) depending on the heart rate
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Graphical Representation of ECG

ECG basically is a technique used to note the electrical changes taking place in the heart chambers during each cardiac cycle. The curves of ECG and the electrical change can be described as shown below:

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When is ECG Recommended?

ECG is a painless and non-invasive diagnostic test for identifying the functioning of the heart and heart-related problems in people of all age groups. A doctor uses ECG when a patient suffers from any of the following symptoms:

  1. Dizziness
  2. Chest pain
  3. Light-headedness
  4. Rapid pulse
  5. Shortness of breath
  6. Heart palpitations
  7. Reduced ability to exercise, fatigue and weakness

Some of these symptoms indicate heart problems such as heart attack, angina, coronary artery disease, ischemia (poor blood flow to heart muscles), atherosclerosis (blockage of major arteries), etc.

Electrocardiogram tells two major kinds of information.

  1. First, the doctor can determine how long it takes for an electrical impulse to pass through the heart. Thus, one can know if the electrical activity is slow, medium or fast.
  2. The second information a doctor can get is if the amount of electrical activity is normal or not. If the heart is overworked or weak, the amount of electrical activity is not normal.

Significance of ECG

Some of the heart problems that can be diagnosed with ECG are:

  1. Congenital heart defects in conducting system of the heart
  2. Abnormal rate and rhythms (arrhythmia) of heart
  3. Damage to heart muscles
  4. Enlargement of heart
  5. Damage to heart muscles when one of the arteries is blocked
  6. Poor blood supply to the heart
  7. Abnormal position of the heart
  8. Heart inflammations
  9. Disturbance in conducting system of the heart
  10. Previous heart attacks
  11. ECG is also used to assess the functioning of artificial cardiac pacemakers and the effect of medications on heart activities.

Clinical Correlation with Variation in ECG

The variation in normal ECG signifies cardiac-related issues. The following table shows the clinical correlation of variation in ECG with the cardiac issues:

Cardiac ProblemVariation in ECG
Atrial fibrillation: Irregular and rapid depolarisation of atria1. Length of the \({\rm{P – }}\)wave significantly decreases.
2. Can occur if the heartbeat is more than \(175-180\) beats per minute.
Atrial enlargement: Increase in the size of atriaAn elevated \({\rm{P – }}\)wave is observed
AV conduction block: Increases the period of PR intervalA \({\rm{PR}}\) interval of more than \({\rm{0}}{\rm{.2sec}}\) is an indication of a medical condition known as \({\rm{AV}}\) conduction block
Intraventricular conduction delay: Arises due to non-specific or incomplete blockage of bundlesAn interval of \(0.10\) to \(0.11\) seconds in QRS complex shows an incomplete bundle branch block
Tachycardia: It is the medical term for a heart rate over \(100\) beats per minute.1. Increase in number of QRS complexes and decrease in width of QRS complex.
2. Decrease in \({\rm{R – R}}\) interval.
Bradycardia: It is the medical term for a heart rate less than \(60\) beats per minute.1. Decrease in number of QRS complexes and increase in width of QRS complex.
2. Increase in \({\rm{R – R}}\) interval
1. Hyperkalemia (High magnesium levels in blood)
2. Early repolarization
3. Myocardial infarction and ischemia
4. Left ventricular hypertrophy
5. Left ventricular aneurysm
6. Pulmonary embolism
7. Pericarditis
8. Hypothermia
Elevation in ST segment
1. Subendocardial ischemia
2. Digoxin

3. Hypokalemia (Low potassium levels in blood)
4. Reciprocal changes in acute myocardial infarction
Depression in ST segment
Cardiac arrest: The heart has stopped beatingFlat ECG
Electrocardiogram (ECG): Procedure, Types & Clinical Correlation (9)
1. Ischemia
2. Hypokalemia
3. Hypomagnesemia
(Low magnesium levels in blood)
Appearance of U-wave
1. Hypokalemia
2. Hypomagnesemia
Low amplitude of T-wave
1. Hyperkalemia
2. Hypocalcemia
(Low calcium levels in the blood)
3. Left ventricular hypertrophy
High amplitude of T-wave


An electrocardiogram is the most used diagnostic test these days where lifestyle diseases prevail. It traces the electrical signals of heart muscles and thus indicates any possible deviations from the normal. Doctors recommend this test first for diagnosis in case of any symptoms related to heart trouble. ECG is the representation of one cardiac cycle and intervals between wave patterns to two consecutive ECGs is also interpreted for some medical conditions.

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Frequently Asked Questions (FAQs) on ECG

Q.What is normal ECG reading?
Ans: The total time duration of one cycle of ECG curves is about /(120/) to /(200/) milliseconds. The different waves and intervals of ECG have different durations.

Q. Can ECG detect a heart attack?
Ans: Yes, and ECG is a diagnostic test for detecting heart attack. During a heart attack, the electrical activity of the heartbeat/cardiac cycle is changed which can be detected by ECG.

Q. When ECG is abnormal?
Ans: Basically, when the heart shows changed electrical activity, it gets reflected in ECG and thus, ECG becomes abnormal. The altered electrical activity of the heart may be due to various medical conditions.

Q.Does being nervous affect ECG?
Ans: No, being nervous does not affect ECG, but it may produce minor artifacts.

Q. What does a standard or normal ECG look like?
Ans: The normal ECG curve is shown below:

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Practice Questions on ECG (Electrocardiogram) Concept

Now that you are provided with all the necessary information about ECG and we hope this detailed article is helpful to you. If you have any queries on this article or in general about Electrocardiogram, ping us through the comment box below and we will get back to you as soon as possible.

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