lec7-sem2-CVSWK3-20140920 (1).pdf
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Transcript of lec7-sem2-CVSWK3-20140920 (1).pdf
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ELECTRO CARDIOGRAM ECG
BASIC PRINCIPLE
LEARNING OBJECTIVES
At the end of the lecture the student should be able to know
About ECG
Basic principle in generation of ECG waves
About ECG paper and its calibration
About different leads and their arrangement
About different components of ECG
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ELECTROCARDIOGRAM ECG
Graph of electrical activity of heart
Gold standard for diagnosis of cardiac arrhythmias
Helps detect electrolyte disturbances (hyper- &
hypokalemia)
Allows for detection of conduction abnormalities
Screening tool for ischemic heart disease during stress
tests
Helpful with non-cardiac diseases (e.g. pulmonary embolism or
hypothermia
PROPAGATING ACTIVATION WAVEFRONT
At rest, cells have a negative transmembrane
voltage surrounding media is positive
When cells depolarize, they switch to a positive
transmembrane voltage surrounding media
becomes negative
This leads to a propagating electric vector
(pointing from negative to positive)
PROPAGATING ACTIVATION WAVEFRONT
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BASIC PRINCIPLE OF ECG RECORDING
Wave of depolarization traveling towards a positive
electrode causes an upward deflection on the ECG
Wave of depolarization traveling away from a
positive electrode causes a downward deflection on
the ECG.
COMPONENT OF ECG
WAVES
P atrial depolarization
QRS complex ventricular depolarization
T ventricular repolarization
SEGMENTstraight line between waves
S-T segment
end of ventricular depolarization to start of vent.
Repolariztion
INTERVAL---wave+segment
P-R interval
Q-T interval
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RECORDING ECG
ECG PAPER
Recorded on a calibrated graph paper
Smallest divisions are 1mm square in both X and Y axis
Runs at a paper speed of 25 mm/sec
X axis represent time
Y axis represents voltage
On Y axis 1 mm=0.1 mV
On X axis 1mm =0.04 sec
VERTICAL AXIS
1 small square = 0.1 mV
10 small square =10 mV
1 big square = 5 small square = 0.5 mV
2 big square = 10 smallsquare= 1 mV
ECG PAPER HORIZONTAL AXIS
1 inch = 1 second
Each inch is divided by dark black lines into 5 big square
Each big square= 1/5=0.2 sec
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Each big square is further divided into 5 small square
1 small square = 0.2/5= 0.04 sec
1smallsquare = 0.04 sec =1mm
1 second =25 smallsquare
60second =1min =25 X 60=1500 small square =300 big square
ECG PAPER CALIBRATION
ECG LEADS SYSTEM
Lead
two electrodes which are placed on body surface and connected to
ECG machine for measuring the potential fluctuations between two
points
Standard ECG has 12 lead system
6 limb leads
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6 chest leads
Another classification is
Bipolar leads---- standard limb leads
Unipolar leads---- augmented limb leads, chest leads
BIPOLAR LIMB LEADS
1 positive 1 neagtive elecrode
Right arm always negative
Left leg always positive
L1 between LA(+) and RA(-)
L2 between LF(+) and RA(-)
L3 between LF(+) and LA(-)
UNIPOLAR LEADS
1 positive electrode
1negative reference point
Summation of 2 negative leads
Augmented unipolar limb leads
aVL,aVF, aVR
Verticalplane
Precordialchest leads
V1-V6
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horizontalplane
AUGMENTED LIMB LEADS
aVR
from Right Arm Positive
Other 2limbs negative
aVL
from Left Arm Positive
RA and LF negative
aVF
from Left Foot Positive
RA and LA negative
PRECORDIAL (CHEST) LEAD
POSITION
V1 Fourth ICS, right sternal border
V2 Fourth ICS, left sternal border
V3 Equidistant between V2 and V4
V4 Fifth ICS, left Mid clavicular Line
V5 Fifth ICS Left anterior axillary line
V6 Fifth ICS Left mid axillary line
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ECG INFORMATIONIN DIFFERENT LEADS
The 12 leads allow tracing of electric vector in all three
planes of interest
SUMMARY OF LEADS
Limb Leads Precordial Leads
Bipolar I, II, III
(standard limb leads)
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Unipolar aVR, aVL, aVF (augmented limb leads) V1-V6
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ANATOMIC GROUPS
P WAVE
Depolarization of both atria;
First upward deflection
Duration less than 0.1 sec
Followed by QRS complex
Shape and duration of P may indicate atrial
enlargement
PR INTERVAL
From onset of P wave to onset of QRS
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Normal duration = 0.12-2.0 sec (120-200 ms) (3-4 horizontal boxes)
Represents atria to ventricular conduction time (through His bundle)
Prolonged PR interval may indicate a 1st degree heart block
QRS COMPLEX
Represents ventricular depolarization
Larger than P wave because of greater muscle mass
of ventricles
Normal duration = 0.08-0.12 seconds
Composed of Q, R ,and S wave
Q wave
negative deflection after P wave
Depolarization of septum
Not always seen
R wave
First positive wave after Q or P
S wave
Negative
J POINT
Point where QRS complex returns to isoelectric line
Beginning of ST segment
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ST SEGMENT
Early repolarization of ventricles
From Jpoint to onset of T wave
T WAVE:
Represents repolarization or recovery of ventricles
Interval from beginning of QRS to apex of T is referred to
as the absolute refractory period
QT INTERVAL
Measured from beginning of QRS to the end of the T
wave
Normal QT is usually about 0.40 sec
QT interval varies based on heart rate
EVENTS IN FORMATION OF ECG WAVES
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THANK YOU