Lecture 3 ECG Inst', Respiration

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???? Lecture 3ECG Inst., Respiration

• ???????? ???
• ??? ????,IEEE Fellow

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The story so far

• The hearts chambers contract to pump blood
  around the body.
• The electrical signals which correspond to these
  contractions can be observed electrically and
  differentiated from common-mode noise using
  differential amplification.
• These signals are useful for a variety of
  clinical investigations.

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The story so far

• The hearts chambers contract to pump blood
  around the body.
• The electrical signals which correspond to these
  contractions can be observed electrically and
  differentiated from common-mode noise using
  differential amplification.
• These signals are useful for a variety of
  clinical investigations.
• For all of these applications a basic requirement
  is to determine the heart rate.

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Heart Rate Meters

• Heart rate is usually given in beats per minute
  (BPM).
• The easiest way to obtain this is to count an
  identifying feature in the ECG which occurs once
  per heart beat.
• The most obvious such feature is the QRS complex
  which is a sharp spike.
• Both averaging and beat-to-beat devices need to
  perform this detection

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QRS detection

• There are 3 main problems in detecting the QRS
  complex in ECG traces
• Artefacts due to electrode motion

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QRS detection

• There are 3 main problems in detecting the QRS
  complex in ECG traces
• Artefacts due to electrode motion
• Baseline wander
• T-waves with high frequency content

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QRS detection

• The solution to these problems is to use a
  band-pass filter to remove
• Low frequency changes such as baseline wander
• High frequency changes such as movement artefact.
• Most of the frequencies in the QRS complex are
  around 20 Hz.
• A pass-band of 10 - 40 (or 50) Hz should
  therefore be appropriate

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QRS detection

• Having removed the non-QRS parts of the signal,
  the QRS itself can be detected with a threshold
  detector.
• This should trigger a pulse generator so that a
  short pulse of a defined duration is generated
  once (and only once) for each QRS complex.

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Averaging HR meter

• The "average power" of the pulse train from the
  circuit just considered will be indicative of the
  Heart Rate.
• This can be determined using a "leaky integrator"
  (which is really a low-pass filter in disguise.

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Averaging HR meter

• The "average power" of the pulse train from the
  circuit just considered will be indicative of the
  Heart Rate.
• This can be determined using a "leaky integrator"
  (which is really a low-pass filter in disguise.
• The time-constant of the R-C circuit should be
  several beats long to minimise output ripple.

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Beat-to-beat Heart Rate meter

• This is best achieved using a digital circuit
  which
• 1 Counts the time between QRS complexes
• 2 Inverts this in some way

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Beat-to-beat Heart Rate meter

• This is best achieved using a digital circuit
  which
• 1 Counts the time between QRS complexes
• 2 Inverts this in some way

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Respiration
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Respiration

• Being pedantic, for a second or two
• Respiration is a technical term used by
  biologists and clinicians. It is one of three
  processes.
• It is not, technically, the same as breathing

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Respiration

• Gas exchange
• The movement of oxygen into an organism and
  carbon dioxide out of an organism.
• Breathing
• The ventilation movements that are needed in some
  larger animals so that efficient gas exchange can
  take place.
• Respiration
• The chemical process of releasing energy from
  (complex) organic compounds.

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Breathing

• We are primarily interested in breathing or the
  ventilation movements that draw air in and out of
  the lungs.
• This involves airways, ribs, intercostal muscles,
  the diaphragm and the lungs.
• These are known collectively as the respiratory
  system.

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The Lungs

• The lungs consist of a series of pipes which
  repeatedly fork into smaller pipes.
• Eventually they terminate in approximately 300
  million little sacks (per lung) called alveoli.
• Blood from the heart travels very close to the
  air in these sacks allowing gas exchange to take
  place.
• A surface area of approximately 75m2 is thereby
  provided for the exchange of gas.

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The Alveoli

• Are small thin-skinned sacks.
• Surrounded by capillaries.
• The blood comes very close to the air,
  facilitating gas exchange.

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Measuring Respiration

• Two traditional methods
• Measure the airflow by impeding it in some way
• Measure the chest volume restrictively
• Neither of these are particularly popular with
  patients.

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An electrical model of the lungs

• Air is an insulator, but ...
• Blood and tissue is broadly conductive.
• And so we can consider the lungs to have
  capacitance (an insulator with conductors on with
  side ...).
• But they have resistance in parallel with this
  capacitance.

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An electrical model of the lungs

• Air is an insulator, but ...
• Blood and tissue is broadly conductive.
• And so we can consider the lungs to have
  capacitance (an insulator with conductors on
  either side ...).
• But they have resistance in parallel with this
  capacitance.
• Furthermore, we need to allow for the resistance
  of the rest of the upper body

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An electrical model of the lungs

• We can simplify this somewhat

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An electrical model of the lungs

• We can simplify this somewhat
• However, the lungs are not a fixed capacitance ...

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Equivalent circuit

• At a specific frequency, the reactance of a
  capacitor is given by
• We can therefore simplify the model even more ...

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Overall equivalent circuit

• Electrodes will be necessary to inject the
  current to measure this reactance.
• The overall equivalent circuit can therefore be
  put together.

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Overall equivalent circuit

• Electrodes will be necessary to inject the
  current to measure this reactance.
• The overall equivalent circuit can therefore be
  put together.
• However, it too can be simplified.

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