BASIC ELECTROPHYSIOLOGY

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BASIC ELECTROPHYSIOLOGY

• Contributors
• Jacques M. DeBakker, MD, Experimental Cardiology,
  Academic Medical Center, Amsterdam
• William G. Stevenson, MD, Cardiovascular
  Division, Brigham and Womens Hospital, Boston
• Antonio Zaza, MD, Dept. of Biotechnology and
  Bioscience, University of Milano-Bicocca
• correspondence to antonio.zaza_at_unimib.it

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1. CARDIAC ACTION POTENTIALS AND MEMBRANE
CURRENTS
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The ventricular action potential and underlying
currents
A
B
4
The nodal action potential and underlying
currents
A
B
5
Refractoriness and post-repolarization
refractoriness
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2. PROPAGATION OF EXCITATION
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The propagation circuit source and load
SOURCE
LOAD
_
Rm
INa

RGJ
Cm
Cm
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Velocity and safety-factor of propagation
dependency on intercellular coupling resistance
and membrane excitability
B
A
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ELECTRICAL ANYSOTROPY
A
RT gt RL
B
Velocity ?T lt ?L
Safety factor SFT gt SFL
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3. FOCAL RHYTHMS
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AUTOMATICITY
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TRIGGERED ACTIVITY I EARLY AFTERDEPOLARIZATIONS
LOW HR LONG AP
HIGH HR SHORT AP
0
outward
0
inward
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TRIGGERED ACTIVITY II DELAYED AFTERDEPOLARIZATION
S
Cai (nM)
0
ms
0
Im (pA)
ms
0
ms
Vm (mV)
DAD
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4. REENTRY
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Requirements for induction of reentry
trigger
exctitable gap (non-refractory tissue)
unidirectional block (prevents wavefronts
collision)
excitable gap
trigger
unidirectional block
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Why premature excitation triggers reentry
electrical asymmetry and Vulnerable Window
TIME (ms)
ERP
RRP
TW
fully depolarized tissue
0
fully repolarized tissue
ERP effective refractory period RRP
relative refractory period
VOLTAGE (mV)
excitable
refractory
SW
SPACE (mm)
propagation
block
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Excitable gap and circuit wavelength

• To avoid extinction, the circuit head must not
  collide with the circuit tail, which is still
  refractory to excitation
• The gap of excitable tissue between the head and
  the tail is named excitable gap (EG). The EG can
  be either fully or partially excitable.
• An EG is formed when the time required to travel
  the circuit exceeds the refractory period of the
  tissue
• The minimum length of the pathway which can
  accomodate a circuit (with an infinitely small
  EG) is called wave length (WL)
• The WL depends on the the tissue properties
  conduction velocity (CV) and refractory period
  (RP)
• WL CV RP
• (mm mm/s s)
• Since CV may not be constant in various points of
  the the pathway, the EG may continuously change
  as excitation travels along the circuit.

obstacle size gt WL
obstacle size ltWL
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Circuit size

• The physical lenght of any reentrant circuit (CL)
  is given by
• CL WL EG
• (mm mm mm)
• If the pathway is determined by an anatomical
  obstacle whose size exceeds WL, a fully excitable
  gap may be present
• CL of functional circuits is almost entirely
  determined by WL
• For reentry to occur, CL must not exceed the size
  of the tissue available to support it. Anything
  decreasing the ratio between CL and chamber size
  facilitates reentry
• A decrease in conduction velocity and/or
  refractory period leads to a smaller WL (i.e.
  smaller CL), which facilitates the occurrence of
  functional reentry.

sustained reentry
wavefront extinction
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Anatomic vs. Functional circuits
Anatomic
Functional
partially excitable gap
fully excitable gap
circular
circular
fully excitable gap
partially excitable gap
anisotropic
figure of 8
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Unidirectional block role of asymmetrical
myocardial injury
A)
B)
wave front
Wave front
stimulus
stimulus
fully excitable cells
inexcitable gap
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Unidirectional block influence of tissue geometry
A)
B)
S1
S1
S2
regular rhythm
premature stimulation
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5. ENTRAINMENT
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Entrainment of reentry
REENTRY
ENTRAINED REENTRY
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Verifying entrainment progressive fusion
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Measurement of Post Pacing Intervals (PPI)
stim site
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Entrainment diagnostics 1 circuit location
A
PPI 520
400
400
stim site
S
S
S
B
stim site
PPI 550
550
550
S
S
S
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Entrainment diagnostics 2 outer loop vs.
isthmus stimulation site
isthmus stimulation concealed fusion
outer loop stimulation overt fusion
stim sites