The Heart
Location of Heart
흉골(胸骨)
• Surrounded by
pericardium심낭, 심막
• 1.5cm left from
center
• Size of a fist
• 250-300g
iPad mini: 308g
Anatomy of the Heart
• 4 Chambers
• 4 Valves
• Aorta
– Pulmonary artery
• Veins
– Superior vena cava
– Inferior vena cava
• Septum
– Thicker than RV walls
Cardiac Muscles
• Myocardium
– Striated like skeletal muscles
– Divided into 4 groups
• Spirally oriented
– 2 groups outside of RV & LV
– 3rd group around RV & LV
• Inside of 1st & 2nd group
– 4th group around LV only
– Lower resistance in muscle
fiber direction
Cardiac Muscle Cells
• Myocytes
• Same activation
like neurons
– AP about 100mV
– But, longer duration
 300ms
– Plateau phase
Contraction
• Propagation of AP
frog sartorius muscle cell
– From cell to cell
• In any direction
– Rather complex waveform
– Contraction within plateau
frog cardiac muscle cell
• Conduction barrier
– Btw atria & ventricles
– Conduction only through
conduction pathway
rat uterus wall smooth muscle cell
Generation of Pulses
• SinoAtrial node on RA
–
–
–
–
–
Special muscle cells
Shape of crescent
15mm long, 5mm wide
Self excitatory
Pacemaker cells
• 70 pulses/min
– Activation start to
propagate throughout the
atria rather slowly
Conduction to Ventricles
• AtrioVentricular node
– Only conducting path
– Btw atria and ventricles
– Intrinsic frequency
• 50 pulses/min
– Follow higher trigger
frequency
Propagation from AV Node
• Bundle of His
– Fast conduction
– Common bundle
– Separate into 2 bundle
branches
• Right bundle branch
• Left bundle branch
– Purkinje fibers
• Diverge to inner side of
ventricles
Propagating Ventricle Walls
• Formation of activation
wavefront
– Propagation through
ventricular mass
– Cell to cell activation
– From inner to outer side
• Shorter AP impulse in
epicardium(outside)
– Earlier repolarization than
endocardium(inside)
Electric Events in Heart
Location in
the heart
SA node
Atrium, Right
Left
AV node
Bundle of His
Bundle branches
Purkinje fibers
Endocardium
Septum
Left ventricle
Epicardium
Left ventricle
Right ventricle
Epicardium
Left ventricle
Right ventricle
Endocardium
Left ventricle
Event
Time [ms]
impulse generated
depolarization
depolarization
arrival of impulse
departure of impulse
activated
activated
activated
0
5
85
50
125
130
145
150
depolarization
depolarization
175
190
depolarization
depolarization
225
250
repolarization
repolarization
400
repolarization
600
ECGwave
P
P
P-Q
interval
QRS
T
Conduction
velocity [m/s]
0.05
0.8-1.0
0.8-1.0
0.02-0.05
Intrinsic
frequency
[1/min]
70-80
50
1.0-1.5
1.0-1.5
3.0-3.5
0.3 (axial)
0.8
(transverse)
0.5
20-40
Electric Potential Waveforms
• Intracellular
recording
– Microelectrodes
inside cardiac
cells
Isochronic Ventricle Activation
• By experiment with human heart
– Within 30 min post mortem
– 870 electrodes
• Initial phase
– Radial propagation
• From inside to outside
• In terminal phase
– Tangential propagation
Electrocardiogram: ECG
• Recording of electrical activity
– Generated by electric activity of the heart
– On the surface of thorax
– Extracellular electric behavior
• Properties in cardiac cells
– Propagation through low resistance gap junction
• Current freely into following cells
– Very restrictive extracellular space
• 25% of total volume, ro  ri  0
– Linear core conductor model
Linear Core Conductor Model
• Ii=- Io
𝜕𝑖
𝜕𝑜
–
= - Iiri= Iori,
= - Ioro
𝜕𝑥
𝜕𝑥
– 𝑖 =ri Iodx, 𝑜 =-ro Iodx
– Vm= 𝑖 - 𝑜 = (ri +ro) Iodx
Iodx= Vm /(ri +ro)
r
r
• 𝒊 = i Vm , 𝒐 =- o Vm
ri +ro
ri +ro
–
• “Voltage divider” condition
– Relationship of extracellular potential to membrane
potential
Depolarization
• Wavefront from right to left
• Negative 𝒐 during activation
ro
– 𝑜 =V =-(0.5)40mV=-20mV
ri +ro m
– Increased Vm during plateau
• Local im in depolarizing region
–
𝜕 2 Vm
im =
(ri+ro) 𝜕𝑥 2
1
• Positive VECG potential
– Equivalent with potential by dipole
pointing depolarization direction
Repolarization
• Different from depolarization
– Not propagating activity
– Repolarize after certain time after
• Approximate with propagating
wavefront
– On equal duration of action pulses
– Slow recovery: 100ms
• Wide recovery interval
• Negative VECG potential
– Equivalent with potential by dipole
pointing reverse direction
Dipole Orientation
• In the example:
– Opposite during depolarization and repolarization
– Different VECG polarity during depolarization and
repolarization
• In real situation
– AP pulse durations are not same
• Actually shorter AP in epicardium than endocardium
• Earlier repolarization in epicardium
• Same direction of dipole during depolarization and
repolarization
– Same VECG polarity during de/repolarization
Dipole Orientation
In the example:
depolarization
In real situation
repolarization