Transcript BME 311: BIOMEDICAL INSTRUMENTATION I Lecturer: Ali Işın

FACULTY OF ENGINEERING
DEPARTMENT OF BIOMEDICAL ENGINEERING
BME 311: BIOMEDICAL INSTRUMENTATION I
Lecturer: Ali Işın
Lecture Note 4: ECG Systems
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ECG Systems
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What is an ECG?
The electrocardiogram (ECG) is a representation
of the electrical events of the cardiac cycle.
Each event has a distinctive waveform, the study
of which can lead to greater insight into a
patient’s cardiac pathophysiology.
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The Normal Conduction System
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The Normal Conduction System
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ECG
ECG is a surface measurement of the
electrical potential generated by
electrical activity in cardiac tissue.
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ECG
The human heart can be considered as a
large muscle whose beating is simply
muscular
contraction.
Therefore
contractions of the heart cause a potential
to be developed. The measurement of the
potential produced by cardiac muscle is
called electrocardiology.
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What types of pathology can we identify and
study from ECGs?
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Arrhythmias
Myocardial ischemia and infarction
Pericarditis
Chamber hypertrophy
Electrolyte disturbances (i.e. hyperkalemia,
hypokalemia)
• Drug toxicity (i.e. digoxin and drugs which
prolong the QT interval)
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Waveforms and Intervals
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2. Ventricular
depolarization
3. Ventricular repolarization
1. Atrial
depolarization
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Different Segments of ECG
P wave: the sequential activation (depolarization) of the right and left atria
QRS complex: right and left ventricular depolarization (normally the ventricles
are activated simultaneously)
ST-T wave: ventricular repolarization
U wave: origin for this wave is not clear - but probably represents
"afterdepolarizations" in the ventricles
PR interval: time interval from onset of atrial depolarization (P wave) to onset
of ventricular depolarization (QRS complex)
QRS duration: duration of ventricular muscle depolarization
QT interval: duration of ventricular depolarization and repolarization
RR interval: duration of ventricular cardiac cycle (an indicator of ventricular
rate)
PP interval: duration of atrial cycle (an indicator or atrial rate
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ECG basics
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Amplitude:
Bandwidth:
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Largest measurement error sources:
– Motion artifacts
– 50/60 Hz powerline interference
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Typical applications:
– Diagnosis of ischemia
– Arrhythmia
– Conduction defects
1-5 mV
0.05-100 Hz
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ECG Leads
Leads are electrode setups which measure the
difference in electrical potential between either:
1. Two different points on the body (bipolar leads)
2. One point on the body and a virtual reference point
with zero electrical potential, located in the center of
the heart (unipolar leads)
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Cardiac Axis by Different Leads
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Einthoven Triangle:
Note potential difference
for each lead of triangle
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Each lead gives a slightly different
representation of electrical activity of heart
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ECG Leads
The standard ECG has 12 leads:
3 Standard Limb Leads
3 Augmented Limb Leads
6 Precordial Leads
The axis of a particular lead represents the viewpoint from
which it looks at the heart.
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Standard Limb Leads
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Precordial Leads
Adapted from: www.numed.co.uk/electrodepl.html
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12-Lead ECG measurement
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Most widely used ECG measurement setup in clinical environment
Signal is measured non-invasively with 9 electrodes + 1 reference electrode (right
leg)
Einthoven leads: I, II & III
Goldberger augmented leads: VR, VL & VF
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Precordial leads: V1-V6
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12 Lead ECG System
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12 Lead ECG System
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Block Diagram of a Basic ECG System
Lead Selector
Isolated Power
Source
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ECG Pre-Amplifier
ECG pre-amplifier is a bioelectric differential amplifier.
It includes;
• High impedance input of bioelectric amplifier
• Lead selector switch
• 1mV calibration pulse source
• Means of protecting amplifier from high voltage
discharge such as a defibrillator discharge used on a
patient
• Pre-Amplifier contains an instrumentation amplifier
as well as an isolation amplifier for patient safety
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Isolation Amplifier
• Needed for safety! Isolates the patient from high voltages
and currents to prevent electric shock by introducing a
specific barrier between passage of current from the power
line to the patient.
• Can be done by using light (photo emitter and photo
detector) or a transformer (set of inductors that are used in a
step up / step down configuration)
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Common Mode Rejection
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ECG Amplifier Circuit With Right Leg
Driver
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Common Mode Voltage (CMV)
• If 2 inputs are hooked together into a differential
amplifier driven by a common source with respect to
ground, then the common mode voltage should be
the same and the ideal output should be zero.
However practically you will see a voltage.
• CMV is composed of 2 parts:
– DC electrode offset potential
– 50Hz AC induced interference caused by magnetic and
electric fields from power lines and transformers
• Capacitively coupled into circuit
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Analysis to reduce noise in ECG
• Right leg driver circuit is used in a feedback
configuration to reduce 50 Hz noise and to drive the
noise on the patient to a lower level.
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“6” ways to reduce Noise in ECG
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Common Mode Rejection (differential Amplifier)
Right Leg Drive (feedback loop to decrease noise)
Shielding of wires
Isolation amplifier
Notch filter to reduce 50 Hz noise
Bandpass filter to reduce noise below and above ECG
bandwidth (0.05-100 Hz)
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High Pass Active Filters
Attenuates frequencies lower than the cutoff frequency.
cutoff frequency is 1/(2) =1/ 2RiCi
Rf
Ci
Ri
-
A
Vinput
Vinput
Ci
+
Ri
Ii
0
Voutput
Rf
Voutput
IRf
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Low Pass Active Filters = Integrator
Cf
Attenuates frequencies higher
than the cutoff frequency
Rf
Ri
-
A
Vinput
+
Voutput
Cf
Vinput
Ri
0
Ii
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Rf
ICf
Voutput
IRf
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Defibrillator Protection Circuit
• Defibrillator = a high voltage electrical heart
stimulator used to resuscitate heart attack victims
• When the physician uses defibrillator, the high
voltages and currents discharged onto patient can
cause damage to medical equipment(specificly preamplifier circuitry), BUT physician still needs to view
ECG of the patient while defibrillating.
• How do you protect your medical equipment from
excessive voltages and currents?
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Protection Devices in ECGs: Neon Glow Lamps
• Neon Glow Lamps are pair of electrodes mounted in a glass envelope
filled with low pressured neon gas or a mix of other inert gases.
• Normally impedance across the electrodes is very high (so lamps are not
conductive) but when the potential across the electrodes reaches to the
ionization point of the gas, impedance suddenly drops making the
lamps conductive (they light up) creating a short circuit to ground,
transferring excess energy safely to the ground without damaging the
amplifier.
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Protection Devices in ECGs: Zener Diodes
• Zener Diode: is a diode which allows current to flow in the forward
direction in the same manner as an ideal diode, but also permits it to
flow in the reverse direction when the voltage is above a certain
value known as the breakdown voltage.
• In this configuration when a larger potential than the breakdown
voltage is applied to the system (i.e. Defibrillator discharge) the
zener diode allow the current to flow in reverse direction and shunts
it to the ground thus preventing any damage to the amplifier.
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Protection Devices in ECGs: Current-Limiting Diodes
• Current Limiting Diodes are electronic devices that limit current to a
maximum specified value for the device.
• These diodes consists of a n-channel JFET-transistor
• They act as a resistor as long as the current level remains below the
limiting value. If current tends to rise above the limit (as in the case of a
Defibrillator discharge), it will be clamped and limited preventing any
excess current reaching the amplifier thus protecting it.
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Electro-Surgery Unit (ESU) Filtering
• During surgery simultaneous ECG recording is also
required.
• ESU can introduce high frequency signals with
frequencies ranging from 100KHz to 100 MHz and
with magnitudes up to few kVolts into the ECG signal.
This interferance can heavily distort the ECG signal.
• ESU introduces:
– DC offsets
– and obscures the ECG signal
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• Even though the ECG frequency range is between 0.05-100 Hz
and all the higher frequency signals are attenuated by using
low-pass filters at filtering stage (slide 32), ESU signal still
manages to disort ECG.
• Because;
 Internal junction points of the ECG amplifier can rectify high
frequency signals like ESU signal and the parasitic capacitance
between the leg connections of the amplifier further filters
this signal to create a dc ofset potential. (Whenever the ESU
device is triggered during the surgery ECG baseline oscilates
up and down.)
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 And the ESU signal can leak through amplifier and
low pass filter layers obscuring the ECG signal.
• ECG needs to be of diagnostic quality so the ESU
noise should be eliminated
• Common technique is to use a pi-type 3-layered
RC filter
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RC Filters
Vs
Frequency
FH
• Low Pass Filters will pass frequencies lower than cutoff frequency of FH
=1/2RC
Vs
FL
•
High Pass Filters will pass frequencies greater than cutoff frequency of FL
=1/2RC
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3-Layered RC ESU Filter
RC Filter
RA
Defibrillator
Protection
Circuit
ECG
Differential
Amplifier
LA
LC filter design can also be used
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Some Sample ECG Circuits
High-precision analog front end of a portable ECG application. 3 patient
electrodes (only 3 bipolar leads)
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• ECG system with 5-patient electrodes (7-leads)
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Standalone ECG with Compact Design
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