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)

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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
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Ventricular Hypertropy Enlarged Ventricle
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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