Download Electrical signals from the heart

1
University of Al-Qadisiyah -College of Medicine
Dept. of Physiology- Medical physics
Lecturer: Dr. Khalid Ibrahim
Date:
Lec. No.:
Chapter: 9 Part II
Electrical signals from the heart
The electrocardiogram (ECG)
The impulse that starts the electrical activity of the heart begins high in
right atrium in a tiny area called sinus node or Sino-atrial node (SA-node), is a
collection of specialized muscle fibers with the ability to produce rhythmic
electrical impulses. It is known as the pacemaker of the heart. SA node fire at
regular intervals of about 70 to 80 /min; however, the rate of firing can be
increased or decreased by nerves external to the heart that respond to the blood
demands of the body.
The electrical signal begin at the SA-node will initiates the
(Depolarization) of the nerves and muscle of both atria (left and right), then
the atria contract and pump blood to the ventricles after that the
(Repolarization) of the atria will occur.
After the impulse pass over atrioventricular node (AV-node) the electrical
impulse depolarizes the ventricles causing them to contract and pump blood
into pulmonary and general circulations after that the (Repolarization) of the
ventricles will occur.
2
Note the following:
1. The propagation of electrical activity (action potential) will be in the
wall of the heart.
2. The electrical phenomena of the heart not only spread from cell to cell,
but also are conduct through the entire body. Hence, they can be
detected by electrodes places on the skin.
3. The nerves and muscles of the heart are sources of electricity enclosed
in an electrical conductor.
The Heartbeat:
• Each heartbeat is called a cardiac cycle.
• When the heart beats, the two atria contract together, then the two
ventricles contract; then the whole heart relaxes.
• Systole is the contraction of heart chambers; diastole is their
relaxation.
• The heart sounds, lub-dup, are due to the closing of the
atrioventricular valves, followed by the closing of the semilunar
valves.
The electrodes:
1. The electrical potentials generated by the heart, which are recorded on
the skin, is called electrocardiogram (ECG).
2. The potentials measured on the surface of the body depend upon the
location of the electrodes.
3. The surface electrodes for obtaining ECG are located on left arm (LA),
right arm (RA), and left leg (LL). The right leg (RL) electrode
connected is used as earth.
4. These surface electrodes (LA, RA, and LL) form Einthovens triangle.
ECG leads:
They are 12- leads, which recorded routinely to get ECG signals, and
divided to three groups: 1. Three standard limb leads which measure the potential between:
a. Right arm (RA) and Left arm (LA) called lead I
b. Right arm (RA) and Left leg (LL) called lead II
c. Left arm (LA) and Left leg (LL) called lead III
3
The potential between any two gives the relative amplitude and direction of
the electrical dipole vector in the frontal plane.
A dipole is a potential force caused by a negative and positive electrical
charge in close proximately to one another.
2. Three augmented leads aVR, aVL and aVF in the frontal plane:
a. aVR lead is connected the recorder from one side to (RA) and
the other side to the center of two resistors connected to (LL)
and (LA) .
b. aVL lead is the potential difference between (LA) and the
center of two resistors connected (RA)and (LL).
c. aVF is the potential difference between (LL) and the center of
(RA) and (LA).
Note: We have six leads in frontal plane:
Lead I, Lead II, Lead III + aVR , aVL, aVF
3. Six transverse plane is connected to the center resistors of RA, LA, and LL
and the other electrode (positive) is moved across the chest wall to the six
different positions.
4
The major electrical events of the normal heart cycle are:
1234-
The atrial depolarization produce the p-wave.
The atrial repolarization (which is rarely seen and is unlabeled).
The ventricular depolarization produces QRS complex.
The ventricular repolarization produces T-wave.
In clinical examination:
Six-transverse plane ECGs + Six–frontal plane ECGs = 12-Lead ECG
(Standard leads).
The electrical signals from the brain,
The electroencephalogram (EEG)
EEG: It is the recording of the signal from the brain, which are primarily due to
the electrical activity of the neurons in cortex of the brain.
The electrode:
1. Electrodes for recording the signals are small discs of chloride silver (Agcl).
2. These electrodes attached to the head at locations that depend upon the part
of the brain to be studied.
3. The reference electrodes are attached to the ear (A 1 or A 2). In routine
exams (8 to 16) channels are recorded.
5
Normal EEG:
1-The Right side signals are often compared to the left side signals, because
asymmetrical activity is often an indication of brain disease.
2- The amplitude of EEG signals is very low (about 50μV) so that; interference
from external signals causes problems in EEG signals.
Example: eye movement can cause artifacts in record (i.e.; the patients keeps
his eye closed).
3- The various frequencies bands of the EEG signals are:
a- Delta wave (δ ) or slow
b- Theta wave (θ ) or intermediate
c- Alpha wave (α ) relax person
d- Beta wave (β ) or fast
0.5 to 3.5 Hz appear in deep sleep
4 to 7 Hz appear in light sleep
8 to 13 Hz relax and wake
> 13 Hz alert person
Application of EEG:
1- EEG is used in clinical diagnosis:
a- To determine the distribution of electrical activity over wide areas
of the cortex.
b- To observe activity arising simultaneously in different areas of the
brain.
c- It aids in confirming brain tumors. (The electrical activity is
reduced in the region of tumor).
d- It is used as a monitor in surgery when ECG cannot de used.
e- In surgery; for indicating the anaesthesia level of the patient.
2- It is used in sleeping research:
It is involves observing the EEG pattern for various stages of sleep, as follow:
a- As a person become drowsy (his eye closed), Then the frequency
(8 to 13) Hz (α -wave) dominated the EEG.
b- As the person moves from light sleep to deeper sleep, Then EEG
shows (The amplitude increases and frequency decreases (Deltawave).
c- During sleep the EEG-shows a high frequency pattern called
paradoxical sleep or rapid eye movement (REM) because the eyes
move during this period.
(REM will associate with dream).
d- EEG-can measure the evoked response:
Evoked responses:
The signals that result when the brain receives external stimuli
such as, flashing light or pulse of sound.