Introduction to Clinical Electrocardiography

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Introduction to Clinical Electrocardiography

• Gari Clifford, PhD
• Andrew Reisner, MD
• Roger Mark, MD PhD

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Electrocardiography

• The heart is an electrical organ, and its
  activity can be measured non-invasively
• Wealth of information related to
• The electrical patterns proper
• The geometry of the heart tissue
• The metabolic state of the heart
• Standard tool used in a wide-range of medical
  evaluations

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A heart

• Blood circulates, passing near every cell in
  the body, driven by this pump
• actually, two pumps
• Atria turbochargers
• Myocardium muscle
• Mechanical systole
• Electrical systole

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To understand the ECG

• Electrophysiology of a single cell
• How a wave of electrical current propagates
  through myocardium
• Specific structures of the heart through which
  the electrical wave travels
• How that leads to a measurable signal on the
  surface of the body

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Part I A little electrophysiology
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Once upon a time, there was a cell
K
K
2 K
3 Na
ATPase
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a myocyte
time
Intracellular millivoltage
Resting comfortably
-90
8
time
Intracellular millivoltage
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Na channels open, briefly
time
Intracellular millivoltage
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Mystery current
time
Intracellular millivoltage
In Na
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Ca is in balance with K out
time
Intracellular millivoltage
In Na
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Excitation/Contraction Coupling Ca causes the
Troponin Complex (C, I T) to release
inhibition of Actin Myosin
time
Intracellular millivoltage
In Na
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Ca in K out
time
Intracellular millivoltage
In Na
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In Ca Out K
time
Intracellular millivoltage
In Na
Out K
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Higher resting potential Few sodium channels
reset Slower upstroke
time
Intracellular millivoltage
In Na
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a pacemaker cell
time
Intracellular millivoltage
-55
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a pacemaker cell
time
Intracellular millivoltage
-40
18
time
Intracellular millivoltage
19
time
Intracellular millivoltage
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time
Intracellular millivoltage
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How a wave of electrical current propagates
through myocardium

• Typically, an impulse originating anywhere in the
  myocardium will propagate throughout the heart
• Cells communicate electrically via gap
  junctions
• Behaves as a syncytium
• Think of the wave at a football game!

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The dipole field due to current flow in a
myocardial cell at the advancing front of
depolarization. Vm is the transmembrane
potential.
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Cardiac Electrical Activity
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Important specific structures

• Sino-atrial node pacemaker (usually)
• Atria
• After electrical excitation contraction
• Atrioventricular node (a tactical pause)
• Ventricular conducting fibers (freeways)
• Ventricular myocardium (surface roads)
• After electrical excitation contraction

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The Idealized Spherical Torso with the Centrally
Located Cardiac Source (Simple dipole model)
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Excitation of the Heart
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Excitation of the Heart
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Cardiac Electrical Activity
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Recording the surface ECG
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Clinical Lead Placement

• Einthoven Limb Leads

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Precordial leads
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12 Lead ECG
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(No Transcript)
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The temporal pattern of the heart vector combined
with the geometry of the standard frontal plane
limb leads.
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Normal features of the electrocardiogram.
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Normal sinus rhythm
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What has changed?
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Sinus bradycardia
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Neurohumeral factors
time
Intracellular millivoltage
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Neurohumeral factors
time
Intracellular millivoltage
. . . and the pacemaker current SLOWER. . .
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time
Intracellular millivoltage
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time
Intracellular millivoltage
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time
Intracellular millivoltage
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time
Intracellular millivoltage
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time
Intracellular millivoltage
Vagal Stimulation
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time
Intracellular millivoltage
Adrenergic Stim.
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Sinus arrhythmia
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Atrial premature contractions (see arrowheads)
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Arrhythmias
- Not firing when you should
- Firing when you shouldn't
- All of the above (Reentrance)
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Firing when you shouldn't

• Usually just a spark rarely sufficient for an
  explosion
• Leakiness leads to pacemaker-like current
• Early after-depolarization
• Late after-depolarization

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Whats going on here?
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Wave-front Trajectory in a Ventricular Premature
Contraction.
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Is this the same thing?
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Whats going on here?
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Whats going on here?
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Non-sustained ventricular tachycardia (3 episodes)
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Re-entry
Slow Refractory
Side A
Side B
KeyWords Heterogeneous, Circus,
Self-Perpetuating
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Re-entry
No Longer Refractory
Side A
Side B
KeyWords Heterogeneous, Circus,
Self-Perpetuating
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Re-entry
Side A
Side B
KeyWords Heterogeneous, Circus,
Self-Perpetuating
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Re-entry
Side A
Side B
KeyWords Heterogeneous, Circus,
Self-Perpetuating
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Re-entry
Side A
Side B
KeyWords Heterogeneous, Circus,
Self-Perpetuating
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Re-entry
Side A
Side B
KeyWords Heterogeneous, Circus,
Self-Perpetuating
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Timing is Everything
INCREASED Refractory
Side A
Side B
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Timing is Everything
INCREASED Refractory
Side A
Side B
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Timing is Everything
INCREASED Refractory
Side A
Side B
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Timing is Everything
INCREASED Refractory
Side A
Side B
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Timing is Everything
INCREASED Refractory
Side A
Side B
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Timing is Everything
INCREASED Refractory
Side A
Side B
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Ventricular Fibrillation
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Re-Entry-A-Go-Go
Phase I
V-Fib
Undulatory
( 1 - 2 sec)
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Re-Entry-A-Go-Go
Phase II
V-Fib
Convulsive
( 10 - 30 sec)
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Re-Entry-A-Go-Go
Phase III
V-Fib
Tremulous
( minutes )
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Re-Entry-A-Go-Go
Phase IV
V-Fib
Anoxic
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Heart attack
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Hyperkalemia
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Understanding the ECG A Cautionary Note

• Basic cell electrophysiology, wavefront
  propagation model, dipole model Powerful, but
  incomplete
• There will always be electrophysiologic phenomena
  which will not conform with these explanatory
  models
• Examples
• metabolic disturbances
• anti-arrhythmic medications
• need for 12-lead ECG to record a 3-D phenomenon

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Questions?