ELECTROCARDIOGRAPHY BASIC
What is electrocardiography (ECG)
Electrocardiography is a medical test in which diagnose cardiac disorder i.e. MI (myocardial infarction) etc. by measuring the electrical activity of the heart via electrodes placed on the limbs and chest wall and are transcribed on to graph paper to produce an electrocardiogram and also known as ECG/EKG.
OR
Electrocardiogram
Electro means electrical activity
Cardio means heart
Gram means graphical representation
An electrocardiogram is a graphically representation of an electrical activity of the heart and used in the investigation of heart disease.
The device is used is known as electrocardiograph.
NOTE -- The term electrocardiogram was introduced by Willem Einthoven in 1893
Later in 1924 Einthoven received the Nobel prize for his life's work.
Indication of ECG (Why it's done)
- Due to cause of chest pain, palpitation, Dizziness, shortness of breath and weakness
- To determine heart rate (bradycardia and tachycardia)
- To diagnose abnormal heart rhythm (arrhythmias)
- Differentiate between cardiology disease and respiratory disease
- During cardiopulmonary resuscitation (CPR)
- Evaluation of suspected electrolyte imbalance
- Evaluation of rheumatic heart disease and congenital heart disease
- To detect myocardial infarction, ischemia and the presence of prior infarction as well
- How well certain heart disease treatments, such as a defibrillators and pacemaker are working.
- To detect the effects and side effects of cardiology drugs
Contraindication of ECG
- No absolute contraindications of ECG except patient refusal
ECG PAPER (Recording)
The electrocardiogram is recorded on to standard paper traveling at a rate of 25mm/s
The paper is divided into large squares
Each large square have 25 small squares
Width of 1 large square is 5mm and equivalent to 0.2sec or 200ms
Width of 1 small square is 1mm and equivalent to 0.04sec or 40ms
NOTE -- The electrical activity of the heart detected by the electrocardiogram machine is measured in Millivolts.
12 Leads ECG system
There are six chest leads (V1 to V6) and six limb leads (I, II, III, aVR, aVL, aVF)
Six chest leads V1, V2, V3, V4, V5, V6 that view the heart in the horizontal plane.
Six limb leads I, II, III, aVR, aVL, aVF that view the heart in the vertical plane
The information from these 12 Leads (6 chest and 6 limb) is combined to form a standard electrocardiogram.
Electrode placement of ECG
V1 -- Right sternal edge, 4th intercostal space (between 4th and 5th ribs)
V2 -- Left sternal edge, 4th intercostal space (between 4th and 5th ribs)
V3 -- Between V2 and V4
V4 -- Mid-clavicular line, 5th intercostal space (between 5th and 6th ribs)
V5 -- Left anterior axillary line, horizontally in line with V4
V6 -- Mid- axillary line, horizontally in line with V4 and V5
RA -- On the Right arm, avoiding thick muscles
LA -- On the Left arm, avoiding thick muscles
RL -- On the Right leg, except bony prominence
LL -- On the Left leg, except bony prominence
Diagram of Normal ECG waveform
Standard ECG Components
The ECG tracing is divided into the P wave, PR interval, QRS complex, QT interval, R-R interval, ST segment, PR segment, T wave and U wave.
P wave -- Depolarisation of atria
PR interval -- Time interval between onset of atrial Depolarisation and onset of ventricular Depolarisation
QRS Complex -- Depolarisation of ventricular consisting of the Q, R, and S wave
QT interval -- Time interval between onset of ventricular Depolarisation and onset of ventricular Repolarisation
R-R interval -- Time interval between two QRS Complexs
T wave -- Ventricular Repolarisation
ST Segment -- Show ST Segment Depression and ST Segment Elevation
ST Segment Depression -- Implies that the ST Segment is displayed such that it is below the level of the PR segment
ST Segment Elevation -- Implies that the ST Segment is displayed such that it is above the level of PR segment
PR segment -- The flat line between the end of the P wave and the onset of QRS Complex
U wave -- Representation of Purkinje fibres
Einthoven's Triangle
Einthoven's Triangle is an imaginary formation of three limb leads (I, II, III) in a Triangle used in electrocardiography, formed by the two shoulders and the pubis.
OR
Einthoven's Triangle is an imaginary formation of three limb leads (I, II, III) i.e the right arm (RA), left arm (LA), and the left leg (LA), were considered to form an equilateral triangle in the frontal plane with the heart in the center this is called einthoven's triangle.
Standard Limb Leads -- (Lead I, II, III) results from einthoven's views of the heart. Each lead represents the difference in voltage recorded at two of the limbs. They are bipolar recordings in the frontal plane.
Lead I = LA-RA
Lead II = LL- RA
Lead III = LL- LA
The axis of these leads defines the sides of an equilateral triangle. Each lead views the electrical activity of the heart (Dipole)
Augmented Limb Leads -- (aVR, aVL, aVF) record one limb minus a derived ground. They are unipolar recordings.
aVR = RA - 0
aVL = LA - 0
aVF = LL - 0
The axes of the augmented Limb leads are perpendicular to the axes of the standard Limb leads (i.e perpendicular to the sides of the equilateral triangle) and are in the frontal plane.
OR
NOTE- The Right Leg (RL) -- Electrode remove artifacts from the ECG and is not a directly participating factor in the visible trace.
To graphically represent the electrical forces in einthoven's triangle, we can draw them in such a way that they bisect each other passing through a common central point.
Einthoven's Triangle Law
This low state that the sum of the electrical currents recorded in lead I and Lead II equal to the sum of the electric current recorded in lead II.
Lead I + Lead III = Lead II
Labels: 12 Leads ECG System, Diagram of normal ECG waveform, ECG Paper, Einthoven's Triangle, Electrode placement and Standard Components of ECG, Indication and contraindication of ECG, what is electrocardiography
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