Lecture 1 ECG

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???? Lecture 1ECG

• ???????? ???
• ??? ????,IEEE Fellow

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Course Overview

• The Electrocardiogram (ECG)
• Respiration measurement using Impedance
  Plethysmography
• Oxygen Saturation using Pulse Oximetry
• Non Invasive Blood Pressure

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The Electrocardiogram

• If you place two electrodes on someones upper
  body (or thorax) and record the electrical
  signals, something like this will be observed
• This is the electrocardiogram or ECG

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A deviation on terminology

• Every are of scientific endeavor develops its own
  terminology. The longer the field has been in
  existence the more arcane this terminology will
  seem to us.
• Medicine is no exception and has many terms which
  could be seen as unnecessarily complicating the
  field.
• However, many of the terms are used because they
  have precise meanings unlike the common English
  equivalent.
• As we go along, the medical terminology will be
  given and, where appropriate and useful, used.

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The Heart

• Pumps blood round the body
• 4 chambers (2 pairs of two chambers)
• The atria are holding cavities feeding into the
  ventricles
• The ventricles are the major pumps

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The Heart

• Two loops
• Pulmonary circulation Through the lungs (collect
  oxygen, lose Carbon Dioxide)
• System circulation And then round the body
  (deliver oxygen, remove Carbon Dioxide,
  distribute nutrients and other chemicals)

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Pulmonary circulation
Systemic circulation
Q Is it true that arteries carry only oxygenated
blood and that veins carry deoxygenated blood?
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Cells

• The body is made up of organs.
• Organs are made up of cells.
• Not surprisingly, there are many different types
  of cell.

• Cardiac Muscle from a rat
• (4) indicates the central nuclei of a cell
• Source Ratcliffe

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Cell Potentials

• Muscle cells and Nerve cells have excitable
  membranes.
• Energy is expended expelling Sodium ions and
  drawing in Potassium ions.
• Hence electric and concentration gradients across
  the cell membrane.

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Cell Stimulation

• When the cell is stimulated, the permeability of
  the cell membrane undergoes a cycle of changes.
• This is an all-or-nothing phenomenon
• Either the cycle triggers in full or not at all.
• This cycle of changes can also trigger other
  responses from the cell on top of the
  polarization cycle.

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Introduction of biomedical signal

• Electrochemical

• Electrical signal is transmitted by neurons,
  neurocyte

• The ions of cells exchanged (inside, outside)
  and generates action potential

Resting membrane potential(-70mv)
Na
Na
Cl-
K
K
Cell
Cl-
Stimulus
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Introduction of biomedical signal

• Electrochemical

• Electrical signal is transmitted by neurons,
  neurocyte

• The ions of cells exchanged (inside, outside)
  and generates action potential

Action potential
Na
Depolarizing
Na
Cl-
K
K
Cl-
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Introduction of biomedical signal

• Electrochemical

• Electrical signal is transmitted by neurons,
  neurocyte

• The ions of cells exchanged (inside, outside)
  and generates action potential

Repolarizing
Na
Na
Na
Na
Cl-
Cl-
K
K
K
K
Cell
Cl-
Cl-
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Introduction of biomedical signal

• Electrochemical

• Electrical signal is transmitted by neurons,
  neurocyte

• The ions of cells exchanged (inside, outside)
  and generates action potential

Resting membrane potential(-70mv)
Na
Na
Cl-
K
K
Cell
Cl-
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Cardiac Cells

• Generally, in a tissue, depolarization of one
  cell propagates to the adjacent cells until the
  entire tissue is depolarized.
• In muscle cells, depolarization also causes a
  mechanical response the cells contract (and
  hence the tissue contracts).
• The tissue becomes shorter in length after some
  delay following a depolarization.

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Introduction of biomedical signal

• Electrochemical

• Electrical signal is transmitted by neurons,
  neurocyte

• The ions of cells exchanged (inside, outside)
  and generates action potential

• Propagation of action potential

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Pacemaker Cells

• Put simply, there are a set of cells which tell
  the other cells when to trigger.
• Known as pacemaker cells, ionic leakage causes
  spontaneous excitation and depolarisation.
• These cells are said to be self-excitatory.

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Pacemaker Cells

• Put simply, there are a set of cells which tell
  the other cells when to trigger.
• Known as pacemaker cells, ionic leakage causes
  spontaneous excitation and depolarization.
• These cells are said to be self-excitatory.
• Actually, other cardiac cells are self-excitatory
  but at a lower frequency (ie they spontaneously
  trigger after longer time gaps) than the
  pacemaker cells.
• This provides a backup should the pacemaker cells
  fail or the signal not propagate.

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Propagation and Timing

• The pacemaker cells are to be found in the
  sinoatrial (SA) node in the heart.
• When they self-excite, a depolarisation wave
  spreads out from them.
• However, this wave does not immediately propagate
  beyond the atria there is a barrier of
  non-excitable cells.

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SA Node ? Atrial muscle cells ? AV nodes bundle
of His ? Bundle branches ? Purkinje fibers ?
Ventricles muscle cells
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Propagation and Timing

• So far
• The pacemaker cells in the SA node have
  depolarised.
• This depolarisation has spread to the atria which
  have contracted, pumping blood into the
  ventricles.
• Next
• The barrier of non-excitable cells is broken only
  by a bundle of specialised tissue called the
  Bundle of His.
• At the origin of this bundle is the
  atrio-ventricular (AV) node which propgates
  depolarisations slowly (10 of the speed of
  atrial cells).

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SA Node ? Atrial muscle cells ? AV nodes bundle
of His ? Bundle branches ? Purkinje fibers ?
Ventricles muscle cells
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Propagation and Timing

• So far
• The pacemaker cells in the SA node have
  depolarised.
• The atria have contracted.
• The depolarisation has been propogated through
  the AV node (slowly) and down the Bundle of His.
• Next
• The depolarisation reaches the ventricles and
  they contracts, pumping blood to the lungs (right
  ventricle) and body (left ventricle).

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SA Node ? Atrial muscle cells ? AV nodes bundle
of His ? Bundle branches ? Purkinje fibers ?
Ventricles muscle cells
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A brief summary so far

• Heart beats are in fact four contractions (two
  pairs of two contractions) which result in the
  blood being pumped around the lungs and the body.
• These contractions are the result of the
  carefully timed depolarizations of the cardiac
  muscle cells which form the four chambers.
• This timing is achieved using
• Pacemaker cells
• Propgation between muscle cells
• Non-propagating cells
• Specialised conducting cells

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Measuring the Action Potentials

• To a broad approximation, we can consider the
  heart to be an electrical generator.
• This current source drives current into the upper
  body (the thorax) which we can treat as a
  passive, resistive, medium.

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Measuring the Action Potentials

• Therefore, different potentials will be measured
  at different points on the surface of the body.
• A simple way to understand this is to consider
  the current flow lines and construct resistor
  chains.

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The Electrocardiocgram (ECG)(Also referred to as
the EKG)

• Here is the ECG trace we saw earlier
• It is a recording of potentials measured at the
  outer surface of the this resistive medium, the
  thorax
• And heres another one ...

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Introduction of biomedical signal

• Action potential of ECG

• Depolarizing(Na, Ca2 flows into cell),
  Repolazizing (K flows out of cell)

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Introduction of biomedical signal

• ECG wave and polarization

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Components of the ECG

• P-wave a small low-voltage deflection caused by
  the depolarisation of the atria prior to atrial
  contraction.
• QRS complex the largest-amplitude portion of the
  ECG, caused by currents generated when the
  ventricles depolarise prior to their contraction.
• T-wave ventricular repolaristaion.
• P-Q interval the time interval between the
  beginning of the P wave and the beginning of the
  QRS complex.

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Ventricles depolarization
Ventricles repolarization
Atrial depolarization
Q Why is atrial repolarization not seen?
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And the point is ...

• Really, clinicians are interested in how well the
  heart is performing a mechanical activity
  pumping blood.
• However, this is rather hard measure without
  sticking tubes in people.
• And the electrical activity gives clear
  indicators as to whether the mechanical action is
  being properly initiated and the rate at which
  the pumping is being initiated.

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