ECE 501 Introduction to BME
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Part IV Bioinstrumentation Electrocardiogram
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• Physiology •Medicine
•Instrumentation
Introduction
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Medicine
The Electrical Cycle
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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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Medicine
ECG Waves
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Medicine
ECG Intervals
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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
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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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Medicine
Heart Flutter
Paroxysmal Tachycardia
• An abnormal pacemaker in ventricle may discharge at a rapid rate
• Ventricles depolarize irregularly at a high rate
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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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Medicine
Premature V- Contraction
An accidental pacemaker within the ventricle causes an extra beat (extrasystole)
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Medicine
Fibrillation
Atrial Fibrillation
Atria twitch irregularly
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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
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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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Instrumentation
The Dipole Model
• a1,a2: Lead vectors
• va1= M.a1=|M|cosθ: ECG signal on lead a1
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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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Instrumentation
ECG leads –Limb leads
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Instrumentation
ECG leads – Augmented Limb leads
a: aVR b: aVL c: aVF d: Lead vectors
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Instrumentation
ECG leads – Chest leads
Wilson’s Central Terminal
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Instrumentation
ECG leads – Chest leads
Lead vector: Between the electrode and Wilson’s Central Terminal
Electrode positions Lead vectors
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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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Instrumentation
Block Diagram ofan Electrocardiograph
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Protection Circuit
Prevent high voltages from damage the electrocardiograph
Voltage-limiting devices Current-voltage characteristics of Voltage-limiting devices
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Instrumentation
Protection Circuit
• Parallel silicon-diode voltage limiting circuit
• Low voltage breakdown
• Vb: 600mV
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Protection Circuit
• Back-to-back silicon Zener-diode voltage limiting circuit
• Moderate voltage breakdown
• Vb: 2 to 20 V
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Protection Circuit
• Gas-discharge tube (neon light) voltage limiting circuit
• High voltage breakdown
• Vb: 50 to 90 V
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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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Calibration
A 1-mV calibration signal is momentarily introduced into the electrocardiograph for each channel that is recorded
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Preamplifier
• Differential amplifier with high common-mode-rejection ratio (CMRR)
• Analog filters
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Instrumentation
Preamplifier
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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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Isolation Circuit
Transformer isolation amplifier
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Isolation Circuit
Optical Isolator
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Isolation Circuit
Capacitively coupled isolation amplifier
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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
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Instrumentation
Driven right leg circuit
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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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Memory System
• Store digital electrocardiograms from different leads
• Store patient information
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Microcomputer
• Control the overall operation
• Perform a preliminary analysis to
• determine heart rate
• recognize some types of arrhythmia
• calculate intervals
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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
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Instrumentation
Inference in ECG
Effect of voltage transient from other devices, such as cardiac defibrillation
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Instrumentation
Inference in ECG
60 Hz power-line inference
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Inference in ECG
Electromyographic(EMG) inference
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Instrumentation
Inference in ECG
Electromagnetic inference
Twisting wires to minimize inference
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Instrumentation
Inference in ECG
A mechanism of electric-field pickup from the powerline
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Instrumentation
Inference in ECG
Another mechanism of electric-field pickup from the powerline