Lecture 2 ECG Instrumentation

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???? Lecture 2ECG Instrumentation

• ???????? ???
• ??? ????,IEEE Fellow

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The story so far

• The heart pumps blood around the body.
• It has four chambers which contact in a carefully
  controlled order (two pairs of contractions) to
  achieve this effect.

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The story so far

• The heart pumps blood around the body using a
  carefully controlled order of contractions of its
  four chambers.
• The contraction of a muscle cell is a result of a
  depolarisation and repolarisation cycle called an
  action potential during which the potential
  difference between the inside and the outside of
  the cell changes.

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The story so far

• The heart pumps blood around the body using a
  carefully controlled order of contractions of its
  four chambers.
• The contraction of a muscle cell is a result of a
  depolarisation and repolarisation cycle called an
  action potential during which the potential
  difference between the inside and the outside of
  the cell changes.
• Considering all of the cardiac cells together we
  can view the heart as an electrical generator
  which drives current into a passive resistive
  medium, the thorax.
• By taking voltage differences at different points
  on the thorax the electrical activity of the
  heart can be observed.

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The Electrocardiocgram (ECG)

• Here is an example of an ECG signal

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Recording the ECG

• To record the ECG we need a transducer capable of
  converting the ionic potentials generated within
  the body into electronic potentials
• Such a transducer is a pair of electrodes
• Polarisable (which behave as capacitors)
• Non-polarisable (behave as resistors)
• Common electrodes lie between these two extremes

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Recording the ECG

• To record the ECG we need a transducer capable of
  converting the ionic potentials generated within
  the body into electronic potentials
• Such a transducer is a pair of electrodes
• Polarisable (which behave as capacitors)
• Non-polarisable (behave as resistors)
• Common electrodes lie between these two extremes
• The electrode most commonly used for ECG signals,
  the silver-silver chloride electrode is closer to
  a non-polarisable electrode.

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Electrode placement

• There exists a convention prescribing electrode
  placement.
• "12 lead" ECG placement is a well developed tools
  for recording 12 different ECG traces from an
  individual
• For simple ECG recording, however, a three "lead
  combination is possible in which electrodes are
  placed on the right arm (RA), the left arm (LA)
  and the left leg (LL)

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Electrode placement

• In this context, "lead" means a pair of
  electrodes The three electrodes above result in
  three possible differences
• (Potential at LA) - (Potential at RA)
• (Potential at LL) - (Potential at RA)
• (Potential at LL) - (Potential at LA)

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Silver-silver chloride electrode

• Electrodes are usually metal disks and a salt of
  that metal.
• A paste is applied between the electrode and the
  skin.
• This results in a local solution of the metal in
  the paste at the electrode-skin interface.
• Ionic equilibrium takes place when the electrical
  field is balanced by the concentration gradient
  and a layer of Ag ions is adjacent to a layer of
  Cl- ions.
• This gives a potential drop E called the
  half-cell potential (normally 0.8 V for an
  Ag-AgCl electrode)

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Silver-silver chloride electrode

• This double layer of charges will also have a
  capacitative effect
• Since the Ag-AgCl electrode is primarily
  non-polarisable there is a large resistive
  effect.
• This gives a simple model for the electrode.

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Silver-silver chloride electrode

• This double layer of charges will also have a
  capacitative effect
• Since the Ag-AgCl electrode is primarily
  non-polarisable there is a large resistive
  effect.
• This gives a simple model for the electrode.
• However, the impedance is not infinite at d.c.
  and so a resistor must be added in parallel with
  the capacitor.

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Movement artefact

• Movement disturbs the physical positioning of the
  ions in the ionic equilibrium.
• The half-cell potential will therefore
  momentarily change.
• The potential difference between two electrodes
  will therefore vary.
• This variation, which is unrelated to the
  underlying signal we wish to observe, is known as
  movement artefact
• It can be a serious cause of interference in the
  measurement of ECG

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Overall Equivalent circuit

• Using the simple model we used earlier for the
  thorax we can build up an overall circuit for the
  heart, the body and the electrodes.
• The resistors and capacitors may not be exactly
  equal.
• E and Eshould be very similar.
• Hence V should represent the actual different of
  ionic potential between the two points on the
  body where the electrodes have been placed.

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ECG Amplifiers

• The peak output voltage V of our equivalent
  circuit is around 1 mV.
• Therefore, amplification is required if we are
  going to store or display this information in
  some way.
• There are three problems which must be overcome
  which we will now consider

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Electrical Field Interference(Problem 1)

• To put it in hand-waving terms the human body is
  a good aerial and so the electrical signal at the
  electrodes is not pure, unadulterated, ECG
• More specifically, capacitance between power
  lines and the system couples current into the
  patient, wires and machine.

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Electrical Field Interference

• This capacitance varies but it is of the order of
  50pF.
• This corresponds to 64mW at 50Hz.
• If the right leg is connected to the common
  ground of the amplifier with a contact impedance
  of 5k, the mains potential will appear as a 20mV
  noise input.

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The solution

• The key is to remember that the ECG is a
  differential signal.
• The 50Hz noise, however, is common to all the
  electrodes.
• It appears equally at the Right Arm and Left Arm
  terminals.
• Rejection therefore depends on the use of a
  differential amplifier in the input stage of the
  ECG machine.
• The amount of rejection depends on the ability of
  the amplifier to reject common-mode voltages.

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Differential Amplifiers

• There have been covered in the core course.
• Lets have a look at the standard circuit.

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Differential Amplifier
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Q Why dont we use this amplifier for ECG
instrumentation?
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Differential (Instrumentation) Amplifer
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Differential Amplifiers

• So, the standard circuit gives a gain of
• ie a differential gain of

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Differential Amplifiers

• So, the standard circuit gives a gain of
• ie a differential gain of
• and a common-mode gain of

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Differential Amplifiers

• The overall common-mode rejection ratio is given
  by

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Magnetic Induction(problem 2)

• Current in magnetic fields induces voltage in the
  loop formed by the patient leads
• Either
• Lower the magnetic field strength (rather hard)
• Minimise the coil area (eg twist the wires
  together)

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Source Impedance Unbalance(problem 3)

• If these impedances are not balanced (ie the
  same) then the common-mode voltage of the body
  will be higher at one input to the amplifier than
  the other.
• Hence, a fraction of the common-mode voltage will
  be seen as a differential signal.
• Therefore, make sure the electrodes are on
  correctly!

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The signal

• So, the signal at the input to the amplifier will
  have three components
• The desired differential ECG signal
• An unwanted common-mode signal
• Unwanted common-mode signal appearing as a
  differential input

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The signal

• The output of the amplifier will therefore
  consist of three components
• The desired output (ECG)
• Unwanted common-mode signal because the
  common-mode rejection is not infinite
• Unwanted common-mode signal due to source
  imbalance

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A Modification

• There is a better approach than setting the
  system ground to the common-mode voltage.
• The common-mode voltage can actually be
  controlled using a Driven right-leg circuit.
• A small current (lt1?A) is injected into the
  patient to equal the displacement currents
  flowing in the body.
• The body sums these currents and the common-mode
  voltage is driven to a low value.

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A Modification

• There is a better approach than setting the
  system ground to the common-mode voltage.
• The common-mode voltage can actually be
  controlled using a Driven right-leg circuit.
• A small current (lt1?A) is injected into the
  patient to equal the displacement currents
  flowing in the body.
• The body sums these currents and the common-mode
  voltage is driven to a low value.
• Also improves patient safety.

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Diagnostic use of the ECG

• As has been seen, the ECG can provide diagnostic
  information to clinicians.
• Ectopic beats originate somewhere other than the
  SA node and often have different shapes
  (morphologies).
• Abnormal heart rates (arrhythmias) can be treated.

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Diagnostic use of the ECG

• Post heart attack (Myocardial Infarct) the ECG is
  highly informative.
• Cardiac muscle damage (infarcts) generally
  correspond to loss of amplitude.
• Insufficient blood supply to cardiac cells
  (Ischemic heart condition) changes the S-T level.

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Acquiring ECG for Diagnosis

• Two methods are in common use
• Exercise Stress ECGs
• Ambulatory monitoring

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Acquiring ECG for Diagnosis

• Two methods are in common use
• Exercise Stress ECGs
• Ambulatory monitoring

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Acquiring ECG for Diagnosis

• Ambulatory monitoring
• ECG monitored for 24 hours.
• Each beat analysed and either kept for records or
  ignored (because too normal!)
• Results printed out
• 1 page summary of conditions detected.
• 24-hour summary detailing the heart rate and ST
  segment changes over the period of the test.
• Findings pages with detailed information for each
  finding or symptom event.

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Foetal Monitoring

• It can be helpful to clinicians and midwives to
  have information about both the maternal and
  foetal ECG during childbirth
• Although this may add stress, the behaviour of
  the foetal ECG (and heart rate) is known to give
  information about potential foetal distress