Transcript Introduction to Biomedical Equipment Technology

Biomedical Instrumentation I
Chapter 8 in
Introduction to Biomedical Equipment Technology: Electrocardiography
By Joseph Carr and John Brown
Schematic
Representation of
Electro-Conduction
System
• SA Node
• AV Node
• Bundle of
His
• Bundle
Branches
• Purkinjie
Fibers
From Berne and Levy Physiology 3rd Edition Figure 23-25
Pathway of Electro-Conduction System
of the calf heart starting at AV Node
• AV Node
• Bundle of
His
• Bundle
Branches
• Purkinjie
Fibers
From Berne and Levy Physiology 3rd Edition Figure 23-28
Electrocardiograph (ECG)
•
Components:
– P wave = Atrial
Contraction
– QRS Complex =
Ventricular Systole
– T Wave =
Refractory Period
•
•
Carr and Brown Figure 8-1
Typical measurement
from right arm to left
arm
Also see 1 mV
Calibration pulse
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
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
Carr and Brown Figure 8-2
Types of Leads
Bipolar Limb Leads: are those designated by
Lead I, II, III which form Einthoven Triangle:
– Lead I = LA connection to noninverting amp. input
And RA connecting to inverting amp. Input
– Lead II = LL connection to amp. Noninverting input RA connect to
inverting input and LA shorted to RL
– Lead III = LL connected to noninverting input LA connected to
inverting input
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 non-inverting 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)
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
LL
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
Variations in Chest Leads C with RA and LA connected
C1
C2
C3
1st Degree block RA LA LL connected
PR wave is prolonged >0.2 sec have a prolongation of delay
between atrial and ventricle depolarization
Normal
2nd Degree Block
Intermittent failure of AV conduction, such that not every P wave
is followed by QRS complex
Normal
3rd Degree Block
Complete failure of conduction between atria nd ventricles.
Common cause is AMI (Acute Myocardial Infarction
Normal
R Bundle Branch Block
Widened QRS complex abnormalities in R S as well as T wave Q is not
as affected because the left bundle branch initiates depolarization
Normal
Other ECG Signals
• Interdigital ECG: Signal taken between 2
fingers usually for home monitoring
• Esophageal ECG: electrode placed in
esophagus close to heart typically used to
record atrial activity where P and R wave
are used to determine position
• Toilet Seat ECG: used to detect cardiac
arrhythmias that can occur during
defecation
Block Diagram of ECG
ECG Pre-Amplifier
• 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.
Use of Feedback to reduce Noise
Derivation: V 1  Vin  Vo
V 2  V 1G1  Vn
V 2  Vin  Vo G1  Vn
Vo  V 2G 2
Vn = Noise
Vin +
Σ
+
V1
B Vo
V1G1 +
G1
Σ
V2
G2
V2G2
Β
• Thus Vn is reduced by
Gain G1
• Note Book forgot V in
equation 5-35
Vo Vo  Vin  Vo G1  Vn G 2
Vo  G1G 2Vin   G1G 2 Vo   G 2Vn 
Vo  G1G 2 Vo  G1G 2Vin  G 2Vn
Vo 1  G1G 2    G1G 2Vin  G 2Vn
Vo 
G1G 2Vin  G 2Vn 
1  G1G 2  
Vo 
G1G 2Vin

G1  G 2Vn

1  G1G 2   G1  1  G1G 2  
Vo 
G1G 2
G1G 2
Vn
* Vin 
*
1  G1G 2  
1  G1G 2   G1
Vo 
Vn 
G1G 2

V

1  G1G 2    in G1
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
• 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 


1
 1  50



1

 1
  10




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
High Pass Active Filters
Attenuates High frequency where
cutoff frequency is 1/(2)
=1/ 2RiCi
Rf
Ci
Ri
-
A
Vinput
Vinput
Ri
0
Rf
Voutput
Ri 
1
j 2(3.14)(1x10 6 )(1x10 3 )
Voutput
Vinput
0  Vinput

1
Ri 
jCi
 Rf

Ri 
IRf
When frequencies (w) is large gain ~ -Rf/Ri
Gain (1MHz, Ci=1mF)
 Rf
Rf
Voutput
Ii
Vout

Vin
Rf
0  Vinput
Ii 
1
Ri 
jCi
I Rf  Ii
Voutput  0
+
Ci
IR f 
Voutput  0

 Rf
 Rf

Ri
1.59 x10 4
Ri 
j
1
jCi
When frequencies (w) is small gain is reduced
Gain (1Hz, Ci =1mF)
Vout

Vin
 Rf
Ri 
1
j 2(3.14)(1)(1x10 3 )

 Rf
 Rf

159 Ri  number
Ri 
j
Low Pass Active Filters = Integrator
Attenuates High frequency where cutoff
frequency is 1/2=1/2RfCf
Cf
Rf
Ri
-
A
Vinput
+
Voutput
Cf
Vinput
Ri
0
Rf
Ii
ICf
Voutput
IRf
Voutput  0
Icf 
1
jC
Voutput  0
IRf 
Rf
0  Vinput
Ii 
Ri
Icf  IRf  Ii
Voutput  0 Voutput  0 0  Vinput


1
Rf
Ri
j C
Vinput
 j C
 jCRf  1 
1 
  Voutput 
  
Voutput 

Rf 
Rf
Ri
 1


Voutput  Rf 

1



Vinput
Ri  jCRf  1 
When frequencies (w) are high gain is reduced
Gain (1M Hz, C=1mF)
When frequencies (w) are low gain ~ -Rf/Ri
Gain (0 Hz, C=1mF)
Voutput
Vinput
 Rf

Ri

  Rf
1

 
j

CRf

1

 Ri
 1   Rf


0

1

 Ri
Voutput
Vinput

 Rf
Ri

  Rf
1

 
j

CRf

1

 Ri


1
 Rf

 
 j1M *1m * Rf  1  Ri  number
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 Defibrallitor 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 12 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
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
Schematic of Multi-channel Physiological
Monitoring System
•
•
Instrumentation Inputs:
–
–
–
–
–
•
•
•
•
Figure 8-11
Up to 12 leads to ECG
Lead 13 is for RL driver (feedback to patient and then machine needed to reduce common
mode voltage
Blood pressure
Body Temperature
Blood gases
Buffers which are noninverting amplifiers to give high input impedance or large
resistor
Wilson Network: series of resistors
Digitization of Signal
Serial data output to display
Instrumentation Amplifier using
OPA621
Differential resistors are the same
thus this stage of the circuit has a
gain of 1
 2 Rf
G  1 
Rg


2 * 249
  1 
5
124

CMR OPA621
 Vout 


Adiff
magnificationofdifferentialsig nal  V 2  V 1 
CMRR 


Acomm
magnificationofcommonsignal
 Vout 


Vin


Vin = V1 = V2
Frequency has an effect on CMR!
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
Digitization of Signal
DC Offset severely affect the resolution of your signal and if DC offset is too high
You may not see your ECG Signal
More bits to A/D board (10, 12, 19, 22) the more resolution to your signal because you
Can represent you signal with better resolution
Homework
• Read Chapter 9
• Derive the gain equation for an instrumentation
amplifier.
• What resistor values could be used to produce a
gain of 10 for an instrumentation amplifier?
• Why do you use non-inverting Op Amps in the
first stage of an instrumentation amplifier?
• Prove that feedback used for the right leg driver
can decrease the overall noise in your circuit.
• Problem 1 Chapter 8
Schedule
• Home Ch8 due 4/4
• Exam 2 on 4/11
– Material on Exam 2 Chapters 7, 2, 8, 9, studio
exercises, labs, homework, class notes
• ECG design labs due 5/2
• ECG team presentations 5/5 (Dr. Alvarez presenting at
conference, Florence Chua and class will grade presentations)
• Final Exam 5/13 from 2:30 to 5:00 in Colton 327
(same room that we meet)
– Cumulative, anything discussed during the semester will
be on the final, more emphasis on the material not
covered on Exam 1 & 2
ECG Example