Transcript ECG

E472
Hospital Instrumentation
Waleed Abdel Aziz Salem, PhD.
Electrical Department
Benha Faculty of Engineering, Benha University
Topic
• 1-Electrocardiography (ECG)
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2- Physiological pressure measurements
3- Defibrillator
4- Pacemakers
5- Intensive & Coronary Care Units
6- Electrosurgery Generators
7- Medical Ultrasound
8- The Human Respiratory Measurement
9- Computers in Biomedical Equipment
• 10- Experimental Work (Biomedical Measurement
System)
Introduction to Biomedical Equipment
Technology
By Joseph Carr and John Brown
Electrocardiography (ECG)
Schematic
Representation of
Electro-Conduction
System
• SA Node
• AV Node
• Bundle of
His
• Bundle
Branches
• Purkinjie
Fibers
• SA: serves as a pacemaker for the heart ,fires
electrical impulses but under control of the central
nervous system
• AV: Operate like a delay line to retard the
advance of action potential along the internal
electroconduction system toward the ventricles
• Purkinje Fibers: Excite the muscle cells of the
ventricles
• The contaraction of many muscles cells at one
time creates electrical signal that can detected by
electrodes
• Sequence: Depolarization occurs in the sinoatrial
(SA) node; current travels through internodal tracts
of the atria to the atrioventricular (AV) node; then
through Bundle of His, which divides into right and
left bundle branches; left bundle branch divides
into left anterior and posterior fascicles.
ECG Review
Electrocardiograph (ECG)
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Components:
– P wave = Atrial Contraction
– QRS Complex = Ventricular Systole
– T Wave = Refractory Period
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Typical measurement from right arm to left arm
Also see 1 mV Calibration pulse
Different Segments of ECG
Depolarization:Electrical
activation
of
the
myocardium.
Repolarization: Restoration of the electrical potential
of the myocardial cell.
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) 0.06 to 0.10 seconds
ST-T wave: ventricular repolarization
PR interval: time interval from onset of atrial
depolarization (P wave) to onset of ventricular
depolarization (QRS complex) P-R interval is 0.12 to
0.20 seconds
Different Segments of ECG (Cont.)
QRS duration: duration of ventricular muscle
depolarization
QT interval: duration of ventricular depolarization and
repolarization (0.34 and 0.42 seconds)
RR interval: duration of ventricular cardiac cycle (an
indicator of ventricular rate)
PP interval: duration of atrial cycle (an indicator or
atrial rate
Typical Leads
RA = right arm
LA = Left arm
LL = left leg
RL = right leg
C = Chest
Different leads
result in
different
waveform
shapes and
amplitudes due
to different view
and are called
leads
Cardiac Axis by different Leads
• ECG Electrodes: Two arrangements, bipolar and
unipolar leads.
• Bipolar Lead: One in which the electrical activity
at one electrode is compared with that of another.
• Unipolar Lead: One in which the electrical
potential at an exploring electrode is compared to
a reference point that averages electrical activity,
Cardiac Axis by different Leads
Standard Limb Leads: I, II, III; bipolar, form a set of axes 60°
apart
Lead I: Composed of negative electrode on the right arm and
positive electrode on the left arm.
Lead II: Composed of negative electrode on the right arm and
positive electrode on the left leg.
Lead III: Composed of negative electrode on the left arm and
positive electrode on the left leg.
Augmented Voltage Leads: aVR, aVL aVF; unipolar ; form a
set of axes 60° apart but are rotated 30° from the axes of the
standard limb leads.
aVR: Exploring electrode located at the right shoulder.
aVL: Exploring electrode located at the left shoulder.
aVF: Exploring electrode located at the left foot.
Types of Leads
Bipolar Limb Leads: are those designated by
Lead I, II, III which form Einthoven Triangle:
– Lead I = LA connected to noninverting
input and RA connected to
inverting Input
– Lead II = LL connected to noninverting input and RA connected to
inverting input and LA shorted to RL
– Lead III = LL connected to noninverting input and LA connected to
inverting input and RA shorted to RL
LL
LL
LL
Einthoven
Triangle:
Note potential
difference for each
lead of triangle
Carr and Brown Figure 8-3
Each lead gives a slightly different representation of
electrical activity of heart
Types of Leads
Unipolar Limb Leads:
augmented limb leads: leads that look at composite potential from
3 limbs simultaneously where signal from 2 limbs are summed
in a resistor network and then applied to an inverting amplifier
input and the remaining limb electrode is applied to the noninverting input
Lead aVR = RA connected to non-inverting input while LA and LL are summed at
inverting input
augmented (amplified) Voltage for Right arm (aVR)
Lead aVL = LA connected to non-inverting input while RA and LL are summed at
inverting input
augmented (amplified) Voltage for Left arm (aVL)
Lead aVF = LL connected to non-inverting input while RA and LA are summed at
inverting input
augmented (amplified) Voltage for Foot (aVF)
Types of Leads
Unipolar Limb Leads:
LL
LL
LL
Types of Leads
Unipolar Chest Leads: measured with signals from
certain specified locations on the chest applied to
amplifiers non-inverting input while RA LA, and LL
are summed in a resistor Wilson network at amplifier
inverting inputs
Types of Leads
Unipolar Chest Leads
Wilson’s Central Terminal
• Configuration
used with
Unipolar Chest
Leads where
RA LA and LL
are summed in
resistor network
and this is sent
to the inverting
input of an
amplifier
Electrocardiograph Traces from
different leads
Normal ECG with RA, LA, LL connected
Artrial Tachycardia with RA, LA, LL connected
Ventricular Tachycardia with RA, LA, LL connected
Block Diagram of ECG
ECG Pre-Amplifier
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High Impedance input of bioelectric amplifier
Lead selector switch
1mV calibration source
Means of protecting amplifier from high voltage
discharge such as a defibrillator used on a patient
• Amplifier will have instrumentation amplifier as
well as isolation amplifier
Isolation Amplifier
• Needed for safety! Want to isolate patient from high
voltages and currents to prevent electric shock where
there is specifically a barrier between passage of
current from the power line to the patient.
• Can be done using light (photo emitter and photo
detector) or a transformer (set of inductors that are
used in a step up / step down configuration)
Isolation of Signal of Patient
from Power needed for safety
Typical Representation of an
Isolation Amplifier
Common Mode Rejection
• Until now we assumed Amplifiers were ideal such
that the signal into each terminal would completely
cancel lead to complete common mode rejection
• However with practical Op Amp there is not
perfect cancellation thus you are interested in
what common mode rejection is.
Simplistic Example of ECG
Circuit
Would like to analyze what type of common mode voltage (CMV) can be derived
Common Mode Voltage (CMV)
• If 2 inputs are hooked together into a differential
amplifier driven by a common source with
respect to ground 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
– 60Hz AC induced interference caused by magnetic
and electric fields from power lines and transformers
• This noise is a current from in signal, common and ground
wires
• Capacitively coupled into circuit
• (Other markets that work at 220-240 V will experience 50Hz
noise)
Analysis to reduce noise in
ECG
• Common Mode Rejection:
– Instrumentation amplifier
(EX. INA128) using a
differential amplifier which
will cancel much of the 60
Hz and common DC offset
currents to each input
– If each signal is carrying
similar noise then the some
of the noise will subtract out
with a differential amplifier
Analysis to reduce noise in
ECG
• Right leg driver circuit is used in a feedback
configuration to reduce 60 Hz noise and drive
noise on patient to a lower level.
Analysis to reduce noise in
ECG
• Isolation Amplifier also will attenuate noise
• Shielding of cables further reduce noise
Review of Five 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 60 Hz noise
How to overcome offset voltage
Instrumentation Amplifier Gain (A1,A2,A3) =
Non-Inverting Amplifier A4
Vout ( A4)  Rf
  25K 
Vout ( A3)  2 Rf noninverting  Rf diff   2(25K)  25K 
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1
 1  50
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

1

 1
  10
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Vin
Vin
 Rin   510 
 Rin noninverting  Rin diff   5.5K  25K 
Problems of offset voltage
and how to correct
• If you had 300 mV of DC offset sent through two gains of
10 and then 50 you would have an offset of
(300mV)(10)(50) = 150V thus you would saturate your
amplifiers and not see any of your signal
• 3V offset after first set of noninverting amplifiers goes
through differential amplifier A3 which reduces the offset
voltage.
Other Corrections for Offset
• Feedback circuit where output of A4 goes
through HPF of A5 so only responses larger
than cutoff frequency pass through thus the DC
offset is attenuated
R and C
should be
switched
because this is
really a LPF
Affect of High Pass Filter of A5
• Feedback through HPF has a
time constant of RC
• 3 Modes:
– Diagnostic Mode (most time) where
RC = 1x10-6F*3.2x106Ώ = 3.2 sec
Cutoff Freq = 1/(2πRC) = 0.05Hz
– Monitor Mode (medium time) where
RC = 1x10-6F*318x103Ώ = 0.318 sec
Cutoff Freq = 1/(2πRC) = 0.5Hz
Drawn Incorrectly – Quick Restore (least time) where
R and C should be RC = 1x10-6F*80x103Ώ = 0.08 sec
switched
Cutoff Freq = 1/(2πRC) = 2Hz
With Feedback the DC offset is eliminated
and thus can have a gain of 50 on the
2nd Non-inverting Amplifier Stage
without Saturating the Circuit
Defribillator
• A Defribillator = a high voltage electrical heart
stimulator used to resuscitate heart attack victims
• When a physician applies this high voltage the high
voltages and currents can cause damage to medical
equipment BUT physician still needs to view ECG of
the patient
• How do you protect your medical equipment from
excessively voltages and currents?
Protection Devices in ECGs: Glow
Lamps
• Glow Lamps are pair of electrodes mounted in a glass
envelope in a atmosphere of lower pressure neon gain
or a mix of inert gases
• Typically impedance across electrodes is high but if
voltage across electrodes exceeds ionization potential
of gas then impedance drops so you create a short to
ground so vast majority of current goes safely to
ground and avoids your amplifiers
Protection Devices in ECGs: Zener
Diodes
• Diode: device that conducts electricity in one direction
only
• Zener Diode: “Turns-On” when a minimum voltage is
reached so in this configuration if a large voltage is
applied (ie defibrillator) the zener diode will allow
current to flow and shunts it to grounds thus current
goes to ground and not to the amplifiers
Protection Devices in ECGs: CurrentLimiting Diodes
• Diode: device that conducts electricity in one direction
only
• Diode acts as a resistor as long as current level
remains below limiting point. It current rises above the
limit, the resistance will change and the current will
become clamped
• Can also use a varistor (variable resistor) which
functions like a surge protector that clips spikes in
voltages
Types of Defibrillator Damage
• Defibrillator is 6X greater than normal working
voltage so damage will eventually occur
• Two forms of Damage:
– Both Amplifier inputs are blown thus readout is a flat line
– One amplifier input is blown so the ECG appears distorted
• Cause is from zener diodes becoming open or from
glow lamps becoming defective from an air leak, or
recombination or absorption of gases
• Recommended that lamps are changed every 1-2
years or sooner if ECG is in Emergency Room
Effect of Voltage Transient
on ECG
• Sometime a high voltage transient is applied to the patient
(defibrillator) which cause magnitudes much greater than
biopotential signal (ECG) which saturates the amplifier
• Once the voltage transient signal is removed the ECG
signal takes time to recover
Example of bandwidth and
magnitude of various biopotentials
ECG is approximately 1 mV and spans from DC to 500 Hz
Book assumes Diagnostic mode is 0.05 Hz to 100 Hz
Electromyography (EMG)
Electroencephalography (EEG)
Electrooculography (EOG)
Electro-Surgery Unit (ESU)
Filtering
Electro-Surgery Unit (ESU)
Filtering
• While a surgeon is conducting surgery he/she will
want to see their patient’s ECG
• ESU can introduce frequencies into the ECG of
100KHz to 100 MHz and with magnitudes up to
kVolts which can distort the ECG
• ESU introduces:
– DC offsets
– Obscures the signal
• ESU needs to be of diagnostic quality thus you must
eliminate higher frequencies which are noise
Correct for high frequency noise
using LPF so ECG can function with
ESU
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
Circuit Schematic of an example of ECG
•Lead I (LA – RA) means LA is going to the noninverting input and RA is going to
inverting input
•Precordial are the chest leads
Block diagram of Entire ECG Circuit
QUESTIONS?
THANK YOU
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