How Does an Electrocardiogram Work?
What is Electrocardiogram:
An electrocardiogram is a kind of test which looks at your heart beat, rhythm and electrical impulse of heart. They monitor electrical activities of the heart and show it as a graphical representation which is known as electrocardiograph. By monitoring they provide us information if the heat has enlarged due to hypertension i.e high blood pressure or any other kind of myocardial infarction. This test is carried out by a trained healthcare specialist at clinics and hospitals. This test mainly involves attaching a small number of small and sticky sensors i.e known as electrodes to your arms, legs and chest from where these electrodes monitor electrical signalling of the heart.
Why it is Performed:
An Electrocardiogram is often used alongside other tests to help diagnose and monitor conditions affecting the heart. It can detect so many abnormalities of missfunction happening inside our heart. Some of the abnormalities which are detected by electrocardiogram are listed below:
Abnormal heart rhythm (arrhythmias)
If there are any blocked or narrowed arteries in your heart (coronary artery disease) are causing chest pain or a heart attack.
To find weather you have had a previous heart attack or not.
To monitor if the medicines are causing any side-effects to our body system.
Different Types of Electrocardiogram:
There are mainly three different types of ECG is performed:
Resting ECG: when your body is in resting state.under this type machine examine your heart beat during resting condition.
Ambulatory ECG: This type of ECG is conducted for 24 hours. The heart’s electrical impulses are measured by a device called the Holter Monitor.
Cardiac stress test: This test is used to measure ECG when you are on an exercise bike.
Types of Electrocardiogram Waves Produced During Checkup:
1. The P-wave:
It represents the electrical excitation (or depolarisation) of the atria, which leads to the contraction of both the atria.
Features of P- wave:
P duration < 0.12 sec
P amplitude < 2.5 mm
Frontal plane P wave axis: 0° to +75°
May see notched P waves in frontal plane
2. The QRS Complex:
It represents the depolarisation of the ventricles, which initiates the ventricular contraction.
Features of QRS complex:
QRS duration ≤ 0.10 sec
QRS amplitude varies from lead to lead and also from person to person. Two determinants of QRS voltages are:
Size of the ventricular chambers (i.e., the larger the chamber, the larger the voltage)
nearness of chest electrodes to ventricular chamber (the closer, the larger the voltage)
3. T-wave:
It represents the return of the ventricles from excited to normal state (repolarisation) and the end of the T-wave marks the end of systole.
The normal peak of the T wave is usually in the same direction as the QRS wave except in the right precordial leads. In the normal ECG the T wave is always upright.
By counting the number of QRS complexes that occur in a given time period, a person can determine the heart beat rate of an individual.
How to Perform Electrocardiogram:
They have small sticky electrodes attached to the arms, chest, legs.
These electrodes are connected to the ECG machine with the help of wires that help in detecting the electrical impulses occurring at each heartbeat.
These electrodes can detect every minute form of changes happening in heart muscles and draw every depolarising pattern of heartbeat on a graph.
Feature of Normal Electrocardiogram :
Heart beat should be in between 60-90 beats per minute (bpm).
PR Interval: 0.12 - 0.20 sec
QRS Duration: 0.06 - 0.10 sec
QT Interval (QTc ≤ 0.40 sec)
Medical Use of ECG:
The main function of ECG is to obtain information regarding the heart impulse. There is a great medical use of this information regarding your health issues like:
chest pain
shortness of breath
Lightheadedness
Dizziness
fainting spells
It is also required prior to any type of heart surgery, including surgery for pacemaker placement.
Why is an Electrocardiogram Performed:
ECG is done to determine or detect:
Abnormal heart rhythm (arrhythmias)
If there are any blocked or narrowed arteries in your heart (coronary artery disease) are causing chest pain or a heart attack.
To find weather you have had a previous heart attack or not.
To monitor if the medicines are causing any side-effects to our body system.
MCQ Questions on ECG:
1. ECG (Electrocardiogram) Was Developed First By
(a) Wilhelm His
(b) Steward
(c) Hubert Mann
(d) Willem Einthoven
Answer: (d)
2. This is the Classic ECG Change in MI (Myocardial Infarction)
(a) ST-segment elevation
(b) T-wave inversion
(c) Development of an abnormal Q wave
(d) All of these
Answer: (d)
3. In Which of these Conditions can Widened QRS and Tall-tented T Waves be Observed
(a) Hyponatremia
(b) Hyperkalemia
(c) Hyperglycemia
(d) Hyperphosphatemia
Answer: (b)
4. A Particular ECG Change Observed in Hypokalemia is
(a) ST segment elevation
(b) U wave(a position deflection after the T wave)
(c) Tall peaked T waves
(d) Widening of the QRS complex and increased amplitude
Answer: (b)
5. ECG Report Must Consist of the Following Information
(a) Rhythm, cardiac axis
(b) Conduction intervals
(c) Description of the ST segments, QRS complexes, T-waves
(d) All of these
Answer: (d)
6. For the Normal Heartbeat, Depolarization Stimulus Originates in
(a) His-bundle areas
(b) Epicardium
(c) Sinoatrial (SA)node
(d) Atrioventricular (AV) node
Answer: (c)
7. The Characteristics – slurring of the Initial QRS Deflection, Shortened PR Interval, and Prolonged QRS Duration are of this Condition
(a) Atrial tachycardia
(b) Left bundle branch block
(c) WPW (Wolff-Parkinson-White) syndrome
(d) Myocardial ischemia
Answer: (c)
8. P Wave Indicates
(a) Depolarization of right ventricle
(b) Depolarization of left ventricle
(c) Depolarization of both atria
(d) Atrial to ventricular conduction time
Answer: (c)
9. Ventricular Muscle Depolarization is Indicated By
(a) PR interval
(b) P wave
(c) U wave
(d) The QRS complex
Answer: (d)
10. ECG Identified By the PR Interval Tends to Become Longer with Every Succeeding ECG Complex Until There is a P Wave Not Followed by a QRS is Observed in
(a) Third-Degree Atrioventricular Block
(B) Second-Degree Atrioventricular Block, Type II
(C) Second-Degree Atrioventricular Block, Type I
(D) First-Degree Atrioventricular Block, Type II
Answer: (c)
FAQs on Electrocardiogram: Definition, Types & Uses
1. What is an electrocardiogram, commonly known as an ECG?
An electrocardiogram (ECG) is a medical test that produces a graphical representation of the heart's electrical activity during a cardiac cycle. As per the CBSE syllabus, it is a non-invasive procedure used to assess the heart's rhythm and diagnose various cardiovascular conditions. The machine used to record this data is called an electrocardiograph.
2. What are the main waves in a standard ECG and what do they represent?
A standard ECG trace consists of three primary electrical events, or waves, that show how the heart functions:
P-wave: Represents the electrical excitation, or depolarisation, of the atria. This leads to the contraction of both upper chambers of the heart.
QRS complex: Represents the depolarisation of the ventricles, which initiates ventricular contraction or systole. It marks the start of the main pumping action of the heart.
T-wave: Represents the return of the ventricles to their normal resting state, a process known as repolarisation. The end of the T-wave signifies the end of systole.
3. What is the clinical importance of an ECG test for a student to know?
An ECG is a crucial diagnostic tool with several important applications. It is used to:
Detect abnormalities in the heart's rhythm, such as arrhythmias (irregular heartbeats).
Diagnose if a person has experienced a myocardial infarction (heart attack) by identifying damaged heart muscle.
Determine if the heart chambers are enlarged, which can be a sign of conditions like high blood pressure (hypertension).
Assess the overall health of the heart before surgeries or to monitor ongoing heart conditions.
4. What are the different types of ECG procedures a patient might undergo?
There are three main types of ECG tests, each used in different situations:
Resting ECG: The standard test performed while a person is lying down and at rest. It provides a baseline snapshot of the heart's electrical function.
Ambulatory ECG (Holter Monitor): A portable device records the ECG continuously for 24 hours or more while the person goes about their daily routine. This is useful for detecting intermittent arrhythmias.
Cardiac Stress Test (Exercise ECG): The ECG is recorded while the patient exercises on a treadmill or stationary bike. This shows how the heart performs under physical stress.
5. What is the key difference between an 'electrocardiograph' and an 'electrocardiogram'?
Although the terms sound similar, they refer to two different things. The electrocardiograph is the physical machine, including the electrodes and recording device, that detects and amplifies the heart's electrical signals. The electrocardiogram (ECG) is the final output—the actual graph or tracing on paper or a screen that a doctor interprets.
6. Why is the QRS complex in an ECG much larger in amplitude than the P-wave?
The amplitude of an ECG wave is directly related to the mass of the heart muscle being activated. The QRS complex represents the electrical depolarisation of the ventricles, which have a significantly larger and thicker muscle mass than the atria. Consequently, they generate a much stronger electrical signal. The P-wave, representing atrial depolarisation, is smaller because the atria have thinner walls and less muscle mass.
7. How can a doctor identify a heart abnormality by analysing an ECG graph?
Doctors analyse the shape, size, and duration of the different waves and intervals in an ECG. Deviations from the normal pattern can indicate specific problems. For example, an abnormal S-T segment can suggest a heart attack, while a prolonged P-R interval might indicate a heart block. By counting the QRS complexes over a minute, doctors can accurately determine the heart rate and diagnose conditions like bradycardia (slow heart rate) or tachycardia (fast heart rate).
8. Can a normal ECG reading guarantee that a person is not at risk of a future heart attack?
No, a normal ECG does not predict future heart attacks. An ECG provides a snapshot of the heart's electrical function at the exact moment of the test. It is excellent for detecting current issues like an ongoing heart attack or an existing arrhythmia. However, it cannot predict the future development of coronary artery disease or a blockage that could lead to a heart attack later.
