It is the record or graphical registration of electrical activities of the heart
Hans Berger is considered as the father of the modern electroencephalography
EEG represents the summated electrical activity of the brain recorded from the surface of the scalp
The electrical activity recorded directly from the surface of the brain is called electrocorticogram
Usually EEG is recorded by a set of locations for electrodes placed on the skull called montage
Montages may consists of unipolar or bipolar system of electrodes
Unipolar method
In this one electrode is active and another is indifferent electrode
The indifferent electrode is applied on some part of the body at a definite distance from the cortex
Bipolar method
In this both electrodes are active and the potential difference between these two electrodes is amplified and recorded
Normal EEG
The different waves have been identified based on their frequency and amplitude
Usually a normal EEG in a wakeful person with or without closed eyes consists of two types of waves alpha and beta waves
ELECTROCARDIOGRAH
The instrument (ECG machine) by which the electrical activities of the heart are recorded is called electrocardiograph
ELECTROCARDIOGRAH
ECG machine amplifies the elect cal signals produced from the heart and records these signals on a moving strip of paper
The markings (lines) on this paper is called ECG grid
The ECG paper has horizontal and vertical lines at regular intervals of 1 mm
Every 5 th line ( 5 mm) is thickened
Duration
The duration of different waves of ECG is donated by the vertical lines
Interval between two thick lines ( 5 mm) = 0.2 sec
Interval between two thin lines (1 mm) = 0.04 sec
AMPLITUDE
The amplitude of ECG waves is denoted by horizontal lines
Interval between two thick lines ( 5 mm) = 0.5 m V
Interval between two thin lines (1 mm) =0. 1 Mv
Speed of the paper
The movement of paper can be adjusted in two speeds, 25 mm/ sec and 50 mm/sec
The speed paper during recording is fixed at 25 mm/ sec
If the heart rate is very high, the speed of the paper is changed to 50 mm/sec
Waves of normal Electrocardiogram
The waves of ECG recorded by limb lead II are considered as the typical waves
Normal electrocardiogram has the following waves - P, Q, R, S and T
The P wave represents depolarization of the atria, that is the transmission of electrical impulses
from the SA node throughout the atrial myocardium.
The QRS complex represents depolarization of the ventricles as the electrical impulses spread throughout the ventricular myocardium.
The T wave represents repolarization of the ventricles (atrial repolarization does not appear as a separate wave because it is masked by the QRS complex).
P Wave
It is a positive wave and the first wave in ECG
It is also called atrial complex
Cause
It is produced due to the depolarization of atrial musculature
Duration --0. 1 sec
Amplitude
0.1 to 0. 12 Mv
QRS Complex
It is also called the initial ventricular complex
Q wave is a small negative wave
It is continued as the tail R wave, which is positive wave
R wave is followed by a small negative wave. The S wave
Cause
QRS complex is obtained because of the depolarization of ventricular musculature
Duration -- 0.08 – 0. 10 sec
Amplitude
Q wave – 0.1 to 0.2 mV
R wave – 1 Mv
S wave - 0.4 m V
T Wave--------It is the final ventricular complex and is a positive wave
Cause
T wave is due to the repolarization of ventricular musculature
Duration ----0.2sec
Amplitude----0.2mV
Atrial replarization is not recorded as a separate wave in ECG because it is merged with QRS complex
Nerve cells show changes of electrical potential during their activity
The current generated by neurons in the brain is conducted by the surrounding fluids , to the scalp and can be recorded
The graphic record if such activity is called electroencephalogram (EEG)
Several types of waves have been identified which differ in frequency and potential (voltage)
Four major types of waves common in human subjects are Alpha, Beta, Theta and Delta waves
Brain Waves
Electrical recordings from the surface of the brain or even from the outer surface of the head demonstrate that there is continuous electrical activity in the brain.
Both the intensity and the patterns of this electrical activity are determined by the level of excitation of different parts of the brain resulting from sleep, wakefulness, or brain diseases such as epilepsy or even psychoses and the entire record is called an EEG (
The intensities of brain waves recorded from the surface of the scalp range from 0 to 200 microvolt, and their frequencies range from once every few seconds to 50 or more per second
.
The character of the waves is dependent on the degree of activity in respective parts of the cerebral cortex, and the waves change markedly between the states of wakefulness and sleep and coma
Much of the time, the brain waves are irregular, and no specific pattern can be discerned in the EEG..
EEG machine
It may have 8 or 16 or 32 channels for recording EEG from different areas of the scalp
It contains the following divisions
1. Electrode selector switch
This helps in selecting different electrode placements( montages) unipolar or bipolar montages are utilized for recording EEG
2. Calibrator
This is meant for calibrating the sensitivity of the instrument
Normal calibration is 7 uV/MM OR 50 uV/7 mm
3. Writing system
This consists of a pen connected to a galvanometer and ink flows into the pen from an ink reservoir. Pin writes the different waves on the chart
4. Paper moving system
This helps in movement of the paper at a constant speed
The speed of paper can be varied depending on the requirement. Normal speed is 30mm/s
5. EEG Paper
It is a graph paper with vertical lines at 3 cm intervals
It is a folded and stacked in a storage bin and is allowed to move under the writing pens
6. EEG jelly
This consists of bentonite powder mixed with saline and glycerin
This paste is applied to the electrodes to reduce the resistance between the scalp and the electrodes
7. Electrode set up
Electrodes are silver cup electrodes applied over the scalp in definite pattern
The standard set of electrodes for adults consists of 22 electrodes
The electrodes are named with a letter and a subscript
The letter denotes the underlying region front polar (Fp), frontal (F), central© , parietal(P), occipital(O)
The subscript Z represent midline or zero and a number indicates lateral placement
Procedure
Fix the electrodes over the subjects scalp on a clean shaven head
Connect the electrode system to the instrument
Calibrate the instrument for a normal sensitivity of 7 mm/50 Uv
Record EEG in the following states
Resting closed eyes and open eyes
Effect of hyperventilation for 2 or 3 minutes
Effect of photic stimulation at different frequencies with the eyes open
During sleeping or in drowsy state
Analyze the EEG based on the frequency amplitude and distribution of the waves in various leads
Sources of EEG
Deep structures like hippocampus, thalamus or brain stem do not contribute directly to the surface EEG
Pyramidal neurons are the major projection neurons in the cortex so the synaptic activity in the pyramidal cells is the principle source of EEG activity
The potential changes in the EEG are due to current flow in the fluctuating dipoles formed between the dendrites and the cell bodies of the cortical cells
The shifting dipole between the dendrites and the cell body when conducted through a volume conductor produces a wave pattern
| EEG rhythm | Frequency Hz | Amplitude voltage | Location & condition in which waves are prominent |
| Alpha | 8-13 | 50 | Present in parieto-occipetal areas Waking relaxing with the eyes closed persons(synchronized waves) |
| Beta | 14-30 | 5-10 | Commonly seen in infants present in frontal region (desynchronized waves) seen in alert adult persons with the eyes opened |
| Theta | 4-7 | 10 | This may be present in parietal and temporal regions in children and also in certain brain disorders |
| Delta | 1-4 | Up to 200 | Recordable during sleep and certain brain disorders |
1. Alpha- rhythm
It is also referred as synchronized EEG
It is seen in wakeful but relaxed persons with the closed eyes
When the subject opens his eyes the alpha rhythm is replaced by low amplitude waves (beta waves)
This is referred as alpha-block
This effect is called desynchronization and can be brought abort by increasing mental activity or by applying different types of sensory stimuli
2. Beta- rhythm
It is the high frequency low amplitude EEG seen in wakeful alert active persons with the eyes open
It is also seen in infants
It is also called desynchronized EEG
These are found in parietal find frontal regions of the scalp
They have a frequency of 13-32 /sec and are of 5-10 microvolt
3. Theta- rhythm
This waves are often formed during disappointment and frustration in young children and adolescents
It is seen in parietal and temporal areas rarely but often in brain disorders
They occur at the rate of 4-7/sec and their potential is 10 microvolt
4. Delta- rhythm
These are produced during the state of unconsciousness and deep sleep
They occur at the rate of about 0.5-3.5 per second with a potential of 20-
200 microvolt
They are usually found over the parietal and temporal regions of the brain
This is the slowest wave with maximum amplitude It is seen during sleep and certain brain disorders
Significance of EEG
1. It is utilized as a diagnostic tool in epilepsies
E.g.
Grand mal epilepsy, petit mal epilepsy
In petit mal epilepsy dime and spike pattern appears for few
seconds
In grand mal epilepsy rapid waves with spikes appear for few
minutes
2. Localization of certain brain lesions
Useful to localize haematomas, brain tumors, necrosed areas etc
3. It is useful in the diagnosis and prognosis of brain injuries,
vascular lesions
4. Useful in the diagnosis of meningitis, encephalitis congenital
brain defects
5. Useful in neurophysiological investigation for data collection
6. Increased intracranial pressure
During this condition delta- waves with an amplitude up to 100
microvolt and frequency of 3 cycles per second may be observed
7. Cerebral tumors
Tumors cause progressive destruction of cortical tissues and as a
result abnormally large slow delta-waves arise from the
damaged cerebral cortex
0 comments:
Post a Comment