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Electrocardiogram: The Basics

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Electrocardiogram: The Basics. BMEN 321. October 31, 2006. Background Reading. This lecture summarizes information in Bioelectromagnetism by Malmivuo and ... – PowerPoint PPT presentation

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Title: Electrocardiogram: The Basics


1
Electrocardiogram The Basics
  • BMEN 321
  • October 31, 2006

2
Background Reading
  • This lecture summarizes information in
    Bioelectromagnetism by Malmivuo and Plonsey and
    can be found on the web at butler.cc.tut.fi/malmi
    vuo/bem/bembook
  • We cover chapters 15 and 19

3
The Heart
  • The heart consists of four chambers
  • The right atrium and right ventricle responsible
    for delivery of deoxygenated blood to lungs
  • The left atrium and left ventricle responsible
    for delivery of oxygenated blood to the body

4
The Heart Phases
  • There are two phases of the cardiac cycle
  • Systole The ventricles are full of blood and
    begin to contract. The mitral and tricucuspid
    valves close (between atria and ventricles).
    Blood is ejected through the pulmonic and aortic
    valves.
  • Diastole Blood flows into the atria and through
    the open mitral and tricuspid valves into the
    ventricles.

5
ECG
  • The ECG records the electrical signal of the
    heart as the muscle cells depolarize (contract)
    and repolarize.
  • Normally, the SA Node generates the initial
    electrical impulse and begins the cascade of
    events that results in a heart beat.
  • Recall that cells resting have a negative charge
    with respect to exterior and depolarization
    consists of positive ions rushing into the cell

6
Cell Depolarization
  • Flow of sodium ions into cell during activation

Restoration of ionic balance
Depol
Repol.
7
ECG Leads
  • In 1908, Willem Einthoven developed a system
    capable of recording these small signals and
    recorded the first ECG.
  • The leads were based on the Einthoven triangle
    associated with the limb leads.
  • Leads put heart in the middle of a triangle

8
ECG Leads
  • The basic values
  • The lead values

Also note that by KVL VI VIII VII
9
ECG Electric Signal
  • Assumptions
  • Model cardiac source as a dipole producing an
    electric heart vector, p.
  • Model body as an infinite, homogeneous volume
    conductor
  • The leads will pick up the projection of the
    electric heart vector, p, along the lead

10
Propagating Activation Wavefront
  • When the cells are at rest, they have a negative
    transmembrane voltage surrounding media is
    positive
  • When the cells depolarize, they switch to a
    positive transmembrane voltage surrounding
    media becomes negative
  • This leads to a propagating electric vector
    (pointing from negative to positive)

11
Propagating Activation Wavefront
12
Propagating Activation Wavefront
Depol. toward positive electrode Positive Signal
Repol. toward positive electrode Negative Signal
Depol. away from positive electrode Negative
Signal
Repol. Away from positive electrode Positive
Signal
13
Propagating Activation Wavefront
  • When the activation does not align directly with
    the lead (or propagate directly toward and
    electrode), the signal is proportional to
    component of the activation direction along the
    lead direction.

14
ECG Signal
  • Heart behaves as a syncytium a propagating wave
    that once initiated continues to propagate
    uniformly into the region that is still at rest.
  • The depolarization wavefront defines a dividing
    line between activated and resting cells.
  • Elsewhere, the signal is zero
  • Will propagate along conduction paths sinus
    node AV node bundle branches Purkinjie
    fibers

15
ECG Signal
  • The excitation begins at the sinus (SA) node and
    spreads along the atrial walls
  • The resultant electric vector is shown in yellow
  • Cannot propagate across the boundary between
    atria and ventricle
  • The projections on Leads I, II and III are all
    positive

16
ECG Signal
  • Atrioventricular (AV) node located on
    atria/ventricle boundary and provides conducting
    path
  • Pathway provides a delay to allow ventricles to
    fill
  • Excitation begins with the septum

17
ECG Signal
  • Depolarization continues to propagate toward the
    apex of the heart as the signal moves down the
    bundle branches
  • Overall electric vector points toward apex as
    both left and right ventricles depolarize and
    begin to contract

18
ECG Signal
  • Depolarization of the right ventricle reaches the
    epicardial surface
  • Left ventricle wall is thicker and continues to
    depolarize
  • As there is no compensating electric forces on
    the right, the electric vector reaches maximum
    size and points left
  • Note the atria have repolarized, but signal is
    not seen

19
ECG Signal
  • Depolarization front continues to propagate to
    the back of the left ventricular wall
  • Electric vector decreases in size as there is
    less tissue depolarizing

20
ECG Signal
  • Depolarization of the ventricles is complete and
    the electric vector has returned to zero

21
ECG Signal
  • Ventricular repolarization begins from the outer
    side of the ventricles with the left being
    slightly dominant
  • Note that this produces an electric vector that
    is in the same direction as the depolarization
    traveling in the opposite direction
  • Repolarization is diffuse and generates a smaller
    and longer signal than depolarization

22
ECG Signal
  • Upon complete repolarization, the heart is ready
    to go again and we have recorded an ECG trace

23
Normal ECG Signal
  • P atrial depolarization
  • QRS complex ventricular depolarization
  • T ventricular repolarization

24
Cardiac Cycle
25
Augmented Leads
  • Three additional limb leads are also used aVR,
    aVL, and aVF
  • These are unipolar leads
  • Each lead uses the average of the average of the
    other two leads as reference
  • VR FR (FL FF)/2

26
Precordial Leads
  • Measure potentials close to the heart, V1- V6
  • Unipolar leads

27
ECG Information
  • The 12 leads allow tracing of electric vector in
    all three planes of interest
  • Not all the leads are independent, but are
    recorded for redundant information

28
ECG Diagnosis
  • The trajectory of the electric vector resulting
    from the propagating activation wavefront can be
    traced by the ECG and used to diagnose cardiac
    problems

29
Electric Axis of the Heart
  • This axis changes during cardiac cycle as shown
    earlier generally lies between 30º and -110º
    in the frontal plane and 30º and -30º in the
    transverse plane
  • Clinically, it is generally taken where the QRS
    complex has the largest positive deflection
  • Note Often use aVR
  • Deviation to R increased activity in R vent.
    obstruction in lung, pulmonary emboli, some heart
    disease
  • Deviation to L increased activity in L vent.
    hypertension, aortic stenosis, ischemic heart
    disease

30
Cardiac Rhythm Supraventricular

31
Cardiac Rhythm Supraventricular
32
Cardiac Rhythm Ventricular

33
Cardiac Rhythm Ventricular


34
Activation Sequence Disorders


35
Bundle-branch Block
36
Atrial Hypertropy Enlarged Atria
37
Ventricular Hypertropy Enlarged Ventricle
38
Myocardial Ischemia and Infarction
  • Oxygen depletion to heart can cause an oxygen
    debt in the muscle (ischemia)
  • If oxygen supply stops, the heart muscle dies
    (infarction)
  • The infarct area is electrically silent and
    represents an inward facing electric vectorcan
    locate with ECG
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