ELECTROCARDIOGRAM (ECG)

Sunday, November 28, 2010


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

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