Transcript in cardiac cells.

Cardiac
Electrophysiology
Qiang XIA (夏强), PhD
Department of Physiology
School of Medicine
Tel: 88206417, 88208252
Email: [email protected]
• The major types of cardiac muscle:
– Atrial muscle
– Ventricular muscle
Contractile cells
（收缩细胞）
– Specialized excitatory and conductive
muscle
Autorhythmic cells
（自律细胞）
Conducting system of the heart
Cardiac muscle
Sequence of cardiac excitation
The sinoatrial node is
the heart’s pacemaker
because it initiates
each wave of excitation
with atrial contraction.
The Bundle of His and other parts
of the conducting system deliver
the excitation to the apex of the
heart so that ventricular contraction
occurs in an upward sweep.
General process of excitation and
contraction of cardiac muscle
• Initiation of action potentials in sinoatrial node
• Conduction of action potentials along
specialized conductive system
• Excitation-contraction coupling
• Muscle contraction
Transmembrane
potentials recorded
in different heart
regions
Transmembrane potential of ventricular
cells and its ionic mechanisms
Resting Potential: -90 mV
Action Potential
•
Phase 0: Depolarization
•
Phase 1: Early phase of rapid
repolarization
•
Phase 2: Plateau（平台期）
•
Phase 3: Late phase of rapid
repolarization
•
Phase 4: Resting phase
Ionic mechanisms
• Resting potential
– K+ equilibrium potential
– Na+-inward background current
– Electrogenic Na+-K+ pump
The action potential of a
myocardial pumping cell.
 Phase 0
 Threshold potential (-70mV)
 Opening of fast Na+ channel
 Regenerative cycle（再生性循环）
 Phase 1
 Transient outward current, Ito
K+ current

activated at –20 mV

opening for 5~10 ms
 Phase 2
Inward current
Outward current
(Ca2+ & Na+)
(K+ current)
Types of Ca2+ channels in cardiac cells:
(1) L-type (long-lasting) (Nowycky, 1985)
(2) T-type (transient) (Nowycky, 1985)
Ca2+ channels
L-type
T-type
Duration of current
long-lasting
transient
Activation kinetics
slower
faster
Inactivation kinetics
slower
faster
Threshold
high (-35mV)
Low (-60mV)
cAMP/cGMP-regulated
Yes
No
Phosphorylation-regulated
Yes
No
Openers
Bay-K-8644
-
Blockers
varapamil
Tetramethrin
nifedipine, diltiazem
Ni2+
Inactivation by [Ca2+]i
Yes
slight
Patch-clamp recording
run-down
relatively stable
Outward current (K+ current):
(1) inward rectifier K+ current (IK1)
(2) delayed rectifier K+ current (IK)
 Phase 3
Inactivation of Ca2+ channel
Outward K+ current dominates
IK: Progressively increased
IK1: Regenerative K+ Outward Current
 Phase 4
Na+-Ca2+ exchange
Sarcolemmal Ca2+ pump
SR Ca2+ pump
Na+-K+ pump
a, The key ion channels (and an
electrogenic transporter) in cardiac
cells. K+ channels (green) mediate K+
efflux from the cell; Na+ channels
(purple) and Ca2+ channels (yellow)
mediate Na+ and Ca2+ influx,
respectively. The Na+/Ca2+ exchanger
(red) is electrogenic, as it transports
three Na+ ions for each Ca2+ ion across
the surface membrane.
b, Ionic currents and genes
underlying the cardiac action
potential. Top, depolarizing currents as
functions of time, and their
corresponding genes; centre, a
ventricular action potential; bottom,
repolarizing currents and their
corresponding genes.
From the following article:
Cardiac channelopathies
Eduardo Marbán
Nature 415, 213-218(10 January 2002)
doi:10.1038/415213a
Transmembrane
potentials recorded
in different heart
regions
Transmembrane potential of autorhythmic
cells and its ionic mechanisms
Purkinje cells: Fast response autorhythmic cells
4
Contractile cells
Autorhythmic cells
Phase 4 stable potential
Phase 4 spontaneous depolarization
（4期自动去极化）
Resting potential
Maximal repolarization potential
（最大复极电位）
Ionic mechanism
• Phase 0~3：similar to ventricular cells
• Phase 4：
– (1) If – Funny current, Pacemaker current（起搏电流）
– (2) Ik Decay（钾电流衰减）
Characteristics of If channel
• Na+, K+
• Voltage- & time-dependent
Activation── Repolarized to -60mV
Full activation── Hyperpolarized to -100mV
Inactivation── Depolarized to -50mV
• Blocked by Cs, not by TTX
Sinoatrial cells
Sinoatrial cells: Slow response autorhythmic cells
• Maximal repolarization
potential -70mV
• Threshold potential -40mV
• Phase 0, 3, 4
0
4
3
Ionic mechanism

Phase 0: ICa (ICa,L)
0
4
3

Phase 3:

Inactivation of L-type
Ca2+ channel
0

Outward K+ current (Ik)
4
3
• Phase 4：
 Ik
decay
Inactivated when repolarized to -60mV
 ICa,T
Activated when depolarized to -50mV
 If
The action potential of an
autorhythmic cardiac cell.
Electrocardiogram (ECG)（心电图）
The electrocardiogram (ECG) measures changes in skin
electrical voltage/potential caused by electrical currents
generated by the heart
The relationship between
the electrocardiogram
(ECG), recorded as the
difference between currents
at the left and right wrists,
and
an action potential typical
of ventricular myocardial
cells.
Electrocardiogram (ECG)
The standard 12 lead ECG
Einthoven’s Triangle
I
Limb leads (Bipolar) (I, II, III)
Augmented limb leads
aVR
aVL
(Unipolar) (aVR, aVL, aVF)
Chest leads (Unipolar) (V1, V2,
V3, V4, V5, V6)
V1
V2 V3
V4 V5 V6
III
II
aVF
Willem Einthoven: Dutch physiologist.
He won a 1924 Nobel Prize for his
contributions to electrocardiography.
Placement of electrodes in electrocardiography
Normal ECG
0.04 sec
ECG interpretation
•Measurements
•Rhythm analysis
•Conduction analysis
•Waveform description
•Comparison with previous ECG (if any)
• P wave: the sequential
depolarization of the right and
left atria
• QRS complex: right and left
ventricular depolarization
• ST-T wave: ventricular
repolarization
• U wave: origin for this wave is
not clear - but probably
represents
"afterdepolarizations" in the
ventricles
• PR interval: time interval from
onset of atrial depolarization (P
wave) to onset of ventricular
depolarization (QRS complex)
• QT interval: duration of
ventricular depolarization and
repolarization
• ST segment: the time period
between the end of the QRS
complex and the beginning of the
T wave, during which each
myocyte is in the plateau phase
(phase 2) of the action potential
Normal
Partial block
Complete block
Physiological properties of cardiac cells
• Excitability
• Autorhythmicity
• Conductivity
• Contractility
Electrophysiological
properties
（电生理特性）
Mechanical property
（机械特性）

Excitability（兴奋性）
 Factors affecting excitability
– Resting potential
– Threshold potential
– Status of Na+ or Ca2+ channels
Hyperkalemia（高钾血症）
•
The QRS complexes may widen so that they
merge with the T waves, resulting in a “sine
wave” appearance. The ST segments
disappear when the serum potassium level
reaches 6 mEq/L and the T waves typically
become tall and peaked at this same range.
The P waves begin to flatten out and widen
when a patient‘s serum potassium level
reaches about 6.5 mEq/L; this effect tends to
disappear when levels reach 7-9 mEq/L. Sinus
arrest may occur when the serum potassium
level reaches about 7.5 mEq/L, and cardiac
standstill or ventricular fibrillation may occur
when serum levels reach 10 to 12 mEq/L.
 Periodic changes in excitability
Premature systole & compensatory pause
(extrasystole)
A 39-year-old lady presenting with frequent palpitations
lasting a few months
•A 39-year-old lady presents to you with frequent palpitations
lasting a few months, which are not associated with dizziness,
syncope or angina. She has enjoyed good health and is not on
any medication or herbal medicine. She is a non-smoker and has
no known diabetes, hypertension or hypercholesterolaemia. Her
menses is regular and physical examination is unremarkable other
than a few premature beats. This is her ECG.
Answers:
Ventricular premature beats are noted.
Premature ventricular beats unmask the P waves

Autorhythmicity（自律性）

Autorhythmicity
SA node
100 times/min
AV node
50 times/min
Bundle of His
40 times/min
Purkinje fibers
25 times/min
Normal
pacemaker（正常起搏点）
SA node
Latent
pacemaker （潜在起搏点）
(Ectopic pacemaker [异位起搏点] under pathophysiological conditions)

AV node

Bundle of His

Purkinje fibers
The mechanisms of SA node to control latent pacemakers
– Capture（夺获）
– Overdrive suppression（超速抑制）
Factors Affecting Autorhythmicity
 Maximal repolarization potential
 Threshold potential
 The rate of phase 4 spontaneous depolarization
Sinus Bradycardia（窦性心动过缓）

Conductivity（传导性）
Gap junction
Conducting velocity
SA node
Atria
A-V node
0.05 m/s
0.4 m/s
0.02~0.05 m/s
His bundle
Purkinje fiber
Ventricle
1.2~2.0 m/s
2.0~4.0 m/s
1.0 m/s
Atrioventricular delay（房室延搁）: Asynchronization of
atrial and ventricular depolarization to provide
adequate cardiac output
Factors Affecting Conductivity
 Structural factors
• Diameter of cardiac cells
• Gap junctions at Intercalated disk
 Physiological factors
• The velocity and amplitude of phase 0
depolarization
• Excitability of adjacent region
First Degree AV Block
Definition: 1AVB is a rhythm in which the electrical impulse which leaves the SA node
and travels through the atria, AV node, Bundle of His to purkininjie fibers is slowed
down and takes longer than normal to arrive at its destination. The normal PR interval
is 0.12- 0.20 seconds. A 1AVBT is greater than 0.20 seconds. The cause ranges from
coronary heart disease, inferior wall MI's, hyperkalemia, congenital abnormalities, and
medications such as quinidine, digitalis, beta blockers, and calcium channel blockers.
Second degree AV Block type 1 (Mobitz)
Definition: Second degree AV block is also known as Second Degree Type I, Mobitz I, or
Wenckelbach. This arrhythmia is characterized by a progressive delay of the conduction at
the AV node, until the conduction is completely blocked. This occurs because the impulse
arrives during the absolute refractory period, resulting in an absence of conduction, and no
QRS. The next P wave occurs and the cycle begins again. Possible causes are acute
inferior wall myocardial infraction, digitalis, beta blockers, calcium channel blockers,
rheumatic fever, myocarditis, or excessive vagal tone.
Second degree AV Block Type II
Mobitz II is characterized by 2-4 P waves before each QRS. The PR of the conducted P
wave will be constant for each QRS. It is usually associated with acute anterior or
anteroseptal myocardial infarction. Other causes are cardiomyopathy, rheumatic heart
disease, coronary artery disease, digitalis, beta blockers, and calcium channel blockers.
Mobitz II has the potential of progressing into a third degree heart block or ventricular
standstill.
Third Degree -- Complete Block
A third degree atrial ventricular block is also know as a complete heart block artrioventricular block of
3degree AV block. It is a problem with electrical conduction. All electrical conduction from the atria are
blocked at the AV junction, therefore, the atria and the ventricles beat independently from each other.
This arrhythmia is dangerous because it significantly decreases cardiac output, and could lead to
asystole. Possible causes: acute inferior and anterior myocardic infraction, coronary heart disease,
excessive vagal tone, myocarditis, endocarditis, age, edema from heart surgery, and meditation toxicity
from digitalis, beta blockers, calcium channel blockers.
------------------------++++++++++++++++++++++++++++
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Reentry （折返）Model
Reentry can take place within a small local region within the heart or it
can occur, for example, between the atria and ventricles (global reentry).
For reentry to occur, certain conditions must be met that are related to
the following:
1.the presence of a unidirectional block within a conducting pathway;
2.critical timing;
3.the length of the effective refractory period of the normal tissue.
The End.