Chapter 8 in

1
Biomedical Instrumentation I

• Chapter 8 in
• Introduction to Biomedical Equipment Technology
  Electrocardiography
• By Joseph Carr and John Brown

2
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
3
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
4
Electrocardiograph (ECG)

• Components
• P wave Atrial Contraction
• QRS Complex Ventricular Systole
• T Wave Refractory Period
• Typical measurement from right arm to left arm
• Also see 1 mV Calibration pulse

Carr and Brown Figure 8-1
5
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

6
Typical LeadsRA right armLA Left armLL
left legRL right legC ChestDifferent
leads result in different waveform shapes and
amplitudes due to different view and are called
leads
7
Cardiac Axis by different Leads
Carr and Brown Figure 8-2
8
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
9
Einthoven TriangleNote potential difference for
each lead of triangle
Carr and Brown Figure 8-3
10
Each lead gives a slightly different
representation of electrical activity of heart

11
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
12
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
13
Wilsons 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

14
Electrocardiograph Traces from different leads
15
Normal ECG with RA, LA, LL connected
Artrial Tachycardia with RA, LA, LL connected
Ventricular Tachycardia with RA, LA, LL connected
16
Variations in Chest Leads C with RA and LA
connected
C1
C2
C3
17
1st Degree block RA LA LL connected
PR wave is prolonged gt0.2 sec have a prolongation
of delay between atrial and ventricle
depolarization
Normal
18
2nd Degree Block
Intermittent failure of AV conduction, such that
not every P wave is followed by QRS complex
Normal
19
3rd Degree Block
Complete failure of conduction between atria nd
ventricles. Common cause is AMI (Acute
Myocardial Infarction
Normal
20
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
21
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

22
Block Diagram of ECG
23
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

24
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)

25
Isolation of Signal of Patient from Power needed
for safety
26
Typical Representation of an Isolation Amplifier
27
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.

28
Simplistic Example of ECG Circuit
Would like to analyze what type of common mode
voltage (CMV) can be derived
29
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)

30
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

31
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.

32
Use of Feedback to reduce Noise
Derivation

• Thus Vn is reduced by Gain G1
• Note Book forgot V in equation 5-35

33
Analysis to reduce noise in ECG

• Isolation Amplifier also will attenuate noise
• Shielding of cables further reduce noise

34
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

35
How to overcome offset voltage
Instrumentation Amplifier Gain (A1,A2,A3)
Non-Inverting Amplifier A4
36

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.

37
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
38
Affect of High Pass Filter of A5

• Feedback through HPF has a time constant of RC
• 3 Modes
• Diagnostic Mode (most time) where
• RC 1x10-6F3.2x106? 3.2 sec
• Cutoff Freq 1/(2pRC) 0.05Hz
• Monitor Mode (medium time) where
• RC 1x10-6F318x103? 0.318 sec
• Cutoff Freq 1/(2pRC) 0.5Hz
• Quick Restore (least time) where
• RC 1x10-6F80x103? 0.08 sec
• Cutoff Freq 1/(2pRC) 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

Drawn Incorrectly R and C should be switched
39
High Pass Active Filters
Attenuates High frequency where cutoff frequency
is 1/(2??) 1/ 2?RiCi
Rf
Ri
Ci
-
A
Vinput

Voutput
Ri
Ci
Voutput
Rf
Vinput
0
Ii
IRf
When frequencies (w) is small gain is
reduced Gain (1Hz, Ci 1mF)
When frequencies (w) is large gain -Rf/Ri Gain
(1MHz, Ci1mF)
40
Low Pass Active Filters Integrator
Attenuates High frequency where cutoff frequency
is 1/2??1/2?RfCf
Cf
Rf
Ri
-
A
Vinput

Voutput
Cf
ICf
Ri
Rf
Voutput
0
Vinput
Ii
IRf
When frequencies (w) are high gain is
reduced Gain (1M Hz, C1mF)
When frequencies (w) are low gain -Rf/Ri Gain
(0 Hz, C1mF)
41
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?

42
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

43
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

44
Protection Devices in ECGs Current-Limiting
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

45
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 1-2
  years or sooner if ECG is in Emergency Room

46
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

47
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
48
Electro-Surgery Unit (ESU) Filtering

• While a surgeon is conducting surgery he/she will
  want to see their patients 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

49
Correct for high frequency noise using LPF so ECG
can function with ESU
50
RC Filters
Vs
FH
Frequency

• 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

51
Schematic of Multi-channel Physiological
Monitoring System

• Figure 8-11

• Instrumentation Inputs
• 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

52
Instrumentation Amplifier using OPA621
Differential resistors are the same thus this
stage of the circuit has a gain of 1
53
CMR OPA621

• Vin V1 V2

Frequency has an effect on CMR!
54
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

55
Block diagram of Entire ECG Circuit
56
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
57
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

58
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 230 to 500 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

59
ECG Example