Download ECE 501 Introduction to BME

ECE 501 Introduction to BME
ECE 501
Dr. Hang
Part IV Bioinstrumentation
Electrocardiogram
ECE 501
Dr. Hang
Introduction
• Physiology
•Medicine
•Instrumentation
ECE 501
Dr. Hang
Medicine
The Electrical Cycle
ECE 501
Dr. Hang
Medicine
The Electrocardiogram
• The electrocardiogram (ECG) is a standardized
way to measure and display the electrical
activity of the heart.
• Physicians can diagnose problems with the
heart by analyzing its ECG and comparing it to
the ECG of a healthy heart.
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Dr. Hang
Medicine
ECG Waves
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Dr. Hang
Medicine
ECG Intervals
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Dr. Hang
Medicine
A-V Blocks
Complete Hear Block
• The cells in A-V node are
dead and A-V node does not
conduct action potential at all
• Atria and ventricles beat
independently
• Ventricles are driven by
abnormal pacemakers
ECE 501
Dr. Hang
Medicine
A-V Blocks
First-degree Hear Block
• The A-V node is diseased
• The action potentials from
atria can still reach ventricles
• It is greatly delayed
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Dr. Hang
Medicine
Heart Flutter
Paroxysmal Tachycardia
• An abnormal pacemaker in
ventricle may discharge at a
rapid rate
• Ventricles depolarize
irregularly at a high rate
ECE 501
Dr. Hang
Medicine
Heart Flutter
Atrial Flutter
• An abnormal pacemaker in
atria may discharge at a rapid
rate
• Atria depolarize irregularly at
a high rate
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Dr. Hang
Medicine
Premature V- Contraction
An accidental pacemaker
within the ventricle causes an
extra beat (extrasystole)
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Dr. Hang
Medicine
Fibrillation
Atrial Fibrillation
Atria twitch irregularly
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Dr. Hang
Medicine
Fibrillation
Ventricular Fibrillation
Ventricles twitch irregularly
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Medicine
Ischemia
• Deficient in blood supply due
to coronary occlusion
• Early stage: ST elevation
• Late stage: ST elevation + TQ
depression
ECE 501
Dr. Hang
Instrumentation
The Dipole Model
• The electrical field generated by this dipole
represents the electric activity of the heart at a
specific instant
• Cardiac vector M: dipole moment, having a
magnitude of proportional to the amount of
charge multiplied by the separation of the two
charges
• The magnitude and direction of M vary over
time
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Dr. Hang
Instrumentation
The Dipole Model
• a1,a2: Lead vectors
• va1= M.a1=|M|cosθ: ECG signal on lead a1
ECE 501
Dr. Hang
Instrumentation
The ECG Model
• Electrodes are placed on points A and B
•RAB: Resistance between A and B
• RT1, RT2: Thoracic medium resistances
• ΦAB: ECG voltage signal on lead AB
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Dr. Hang
Instrumentation
ECG leads –Limb leads
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Dr. Hang
Instrumentation
ECG leads – Augmented
Limb leads
a: aVR
b: aVL
c: aVF
d: Lead vectors
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Dr. Hang
Instrumentation
ECG leads – Chest leads
Wilson’s Central Terminal
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Dr. Hang
Instrumentation
ECG leads – Chest leads
Lead vector: Between the
electrode and Wilson’s
Central Terminal
Electrode positions
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Lead vectors
Dr. Hang
Instrumentation
12 - lead ECG
• Simultaneous measurement from
 Three limb leads
 Three augmented limb leads
 Six chest leads
• Process all data from 12 leads to obtain ECG waveform
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Dr. Hang
Instrumentation
Block Diagram of
an Electrocardiograph
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Dr. Hang
Instrumentation
Protection Circuit
Prevent high voltages from damage the electrocardiograph
Voltage-limiting devices
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Current-voltage characteristics of
Voltage-limiting devices
Dr. Hang
Instrumentation
Protection Circuit
• Parallel silicon-diode voltage
limiting circuit
• Low voltage breakdown
• Vb: 600mV
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Dr. Hang
Instrumentation
Protection Circuit
• Back-to-back silicon Zener-diode
voltage limiting circuit
• Moderate voltage breakdown
• Vb: 2 to 20 V
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Dr. Hang
Instrumentation
Protection Circuit
• Gas-discharge tube (neon light)
voltage limiting circuit
• High voltage breakdown
• Vb: 50 to 90 V
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Dr. Hang
Instrumentation
Lead Selector
• Determine which electrodes are necessary for a particular
lead and to connect them to the remainder of the circuit
• Controlled by the operator or by the microcomputer of the
electrocardiograph
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Dr. Hang
Instrumentation
Calibration
A 1-mV calibration signal is momentarily introduced into the
electrocardiograph for each channel that is recorded
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Instrumentation
Preamplifier
• Differential amplifier with high common-mode-rejection ratio (CMRR)
• Analog filters
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Instrumentation
Preamplifier
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Instrumentation
Isolation Circuit
• Contains a barrier to the passage of current from the power line
• Transformer isolation amplifier
• Optical isolator
• Capacitively coupled isolation ampifier
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Dr. Hang
Instrumentation
Isolation Circuit
Transformer isolation amplifier
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Dr. Hang
Instrumentation
Isolation Circuit
Optical Isolator
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Dr. Hang
Instrumentation
Isolation Circuit
Capacitively coupled isolation amplifier
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Dr. Hang
Instrumentation
Driven right leg circuit
• Provides a reference point on the patient that normally is at ground
potential
• The connection is made to an electrode on the patient’s right leg
ECE 501
Dr. Hang
Instrumentation
Driven right leg circuit
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Dr. Hang
Instrumentation
Driver amplifier
• Amplify the ECG to a level at which it can appropriately
record the signal on the recorder
• Carry out the bandpass filtering
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Dr. Hang
Instrumentation
Memory System
• Store digital electrocardiograms from different leads
• Store patient information
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Instrumentation
Microcomputer
• Control the overall operation
• Perform a preliminary analysis to
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•
determine heart rate
•
recognize some types of arrhythmia
•
calculate intervals
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Instrumentation
Recorder-printer
• Provide a hard copy of the recorded ECG signal
• Print out patient information, results of the automatic analysis
• Analog oscillograph-type recorder, thermal recorder,
electrostatic recorder
ECE 501
Dr. Hang
Instrumentation
Inference in ECG
Effect of voltage transient from other devices, such as cardiac
defibrillation
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Dr. Hang
Instrumentation
Inference in ECG
60 Hz power-line inference
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Dr. Hang
Instrumentation
Inference in ECG
Electromyographic(EMG) inference
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Dr. Hang
Instrumentation
Inference in ECG
Electromagnetic
inference
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Twisting wires to
minimize inference
Dr. Hang
Instrumentation
Inference in ECG
A mechanism of electric-field pickup from the powerline
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Dr. Hang
Instrumentation
Inference in ECG
Another mechanism of electric-field pickup from the powerline
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Dr. Hang