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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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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ECG Waves

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ECG Intervals

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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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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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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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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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Premature V- Contraction

An accidental pacemaker within the ventricle causes an extra beat (extrasystole)

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Fibrillation

Atrial Fibrillation

Atria twitch irregularly

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Fibrillation

Ventricular Fibrillation

Ventricles twitch irregularly

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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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The Dipole Model

• a1,a2: Lead vectors

• va1= M.a1=|M|cosθ: ECG signal on lead a1

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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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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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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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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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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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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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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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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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Instrumentation

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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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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Inference in ECG

Effect of voltage transient from other devices, such as cardiac defibrillation

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Inference in ECG

60 Hz power-line inference

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Inference in ECG

Electromyographic(EMG) inference

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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