Cardiac Physiology (Slides 34-68)
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101 Terms
Two main types of AP are generated in…
The heart
Sinoatrial node and Atrioventricular node
Purkinje fibers and Myocytes
Action Potential in a Sino-Atrial Node
Phase 0
Depolarization
Na+ enters about 2 msec
Action Potential in a Sino-Atrial Node
Phase 3
Repolarization
K+ channels open
Action Potential in a Sino-Atrial Node
Phase 4
Ion distribution restored
Na+/K+ pump
Action potential in a Cardaic Muscle (Purkinje fibers and myocytes)
Phase 0
Depolarization
Na+ enters about 2 msec
Action potential in a Cardaic Muscle (Purkinje fibers and myocytes)
Phase 1
Early repolarization
Sodium channels start to inactivate
Action potential in a Cardaic Muscle (Purkinje fibers and myocytes)
Phase 2
Plateau
Ca 2+ enters about 200 msec
Action potential in a Cardaic Muscle (Purkinje fibers and myocytes)
Phase 3
Repolarization
K+ channels open
Action potential in a Cardaic Muscle (Purkinje fibers and myocytes)
Phase 4
Ion distribution restored
Na+/K+ pump
Sinus Node (S-A Node, Pacemaker)
… is a natural pacemaker of the heart
SA node
Sinus Node (S-A Node, Pacemaker)
Specialized tissue close to…
Superior vena cava
3mm x 15mm x 1mm
Sinus Node (S-A Node, Pacemaker)
Phylogenic remnants of…
Venous sinus of lower vertebrates
Sinus Node (S-A Node, Pacemaker)
Characterized by…
Specialized pacemkaer cells (P-cells)
Sinus Node (S-A Node, Pacemaker)
Fibers are…
3-5 micrometer in diamter.
Sinus Node (S-A Node, Pacemaker)
Pacemaker elsewhere is called…
An ectopic pacemaker
Sinus Node (S-A Node, Pacemaker)
A) Spontaneous depolarization
Phase 4 is characterized by…
Gradual diastolic depolarization (pacemaker potential).
Sinus Node (S-A Node, Pacemaker)
A) Spontaneous depolarization
Diastolic depolarization is caused by…
Three ionic currents
if
iCa
iK
Sinus Node (S-A Node, Pacemaker)
A) Spontaneous depolarization
…
INWARD “funny” current of Na+ (if)
Sinus Node (S-A Node, Pacemaker)
A) Spontaneous depolarization
SLOW INWARD Ca 2+ current (iCa)
Sinus Node (S-A Node, Pacemaker)
A) Spontaneous depolarization
…
OUTWARD K+ current (iK)
Sinus Node (S-A Node, Pacemaker)
A) Spontaneous depolarization
Potential is…
Slow to develop, and is slow to return.
Sinus Node (S-A Node, Pacemaker)
A) Spontaneous depolarization
At activation threshold at about…
40 mv
The nodal cells fire and action potential develops.
Sinus Node (S-A Node, Pacemaker)
C) Velocity/Rate
Conduction velocity in SA node
0.05 m/sec
Sinus Node (S-A Node, Pacemaker)
C) Velocity/Rate
Intrinsic rate
80/min
Sinus Node (S-A Node, Pacemaker)
B) Atria
Three bundles of atrial fibers conduct…
Impulses from SA node to A-V node
Sinus Node (S-A Node, Pacemaker)
B) Atria
Conduction velocity in atria
1m/sec
Sinus Node (S-A Node, Pacemaker)
B) Atria
Signal delay in atrias
0.03 sec
A-V Node
Critical element in conducting system
A-V Node
Performs function of…
A filter
A-V Node
Located in septum of…
Right atria close to tricuspid valve
A-V Node
A-V noded divided into three regions
Atrionodal (AN)
Nodal (N)
Nodal-His (NH)
A-V Node
Space just behind A-V node is supplied by…
Autonomic nerves
A-V Node
Electrophysiology closely resembles…
Sinus node
A-V Node
Capable of…
Self-excitation
A-V Node
Characterized by…
One-way conduction through the A-V bundle.
A-V Node
Delay of the impulse in A-V bundle
0.13 sec
A-V Node
Total delay of the impulse
0.16 sec
A-V Node
Cause of delay
Thin fibers and few gap junctions
A-V Node
Conduction velocity in A-V node
0.05 m/sec
A-V Node
Intrinsic rate
40-60/min
Purkinje Fibers
Resemble…
Embryonic muscle cells
Purkinje Fibers
…fibers
Large
Purkinje Fibers
… of gap junctions
High permeability
Purkinje Fibers
Delay of the impulse in Purkinje Fibers
About 0.03 sec
Purkinje Fibers
Conduction velocity of action potential
1.5-4.0 m/s
Purkinje Fibers
High velocity allows…
Almost immediate activation of ventricular system
Purkinje Fibers
Intrinsic rate
15-40/min
Electrical current flows from…
Depolarized (electronegative) area to the poalrized (electropositive) area of the heart.
Inside of ventricles is…
Electronegative
Outside of the ventricles is…
Electropositive
During depolarization current flows from…
Negative to positive (from the base toward the axis).
Electrical currents are generated by…
The cardiac muscle
Surface body electrocardiogram (ECG)… measured
Externally
ECG during…
Depolarization and repolarization
ECG is…
A complex recording
In ECG, comparisons in…
Voltage are detected (from 0.1mv to 4 mv).
ECG
Algebraic sum of all myocardial action potentials
In ECG, … used to detect waves
Different points of measurements (leads)
P-wave
Atrial depolarization (SA and atrias) before atrial contraction
QRS complex
Ventricle depolarization at the beginning of ventricular systole
T-wave
Ventricular repolarization, after T-wave-both ventricles are relaxed.
P-Q intervals
Between the beginning of atria contraction and ventricle contraction
0.16s
P-R inetrval
Time from the beginning of P wave to QRS complex
0.2 s
S-T segment
Time from S wave to T wave
0.1 s
Q-T interval
Time from QRS complex to end of T-contraction of ventricles
0.35s
Atrial contraction
From P wave to Q wave
Isometric Ventricular contraction
Starts with R wave
Ventricular ejection
From S to T wave
Isovolumic ventricular relaxation
Begins at the end of T wave
Passive ventricular filling
After T wave to P wave.
Electrical activity of the heart can be recorded by…
Placing electrodes on the surface of the body.
Lead
A combination of two wires/electrodes to form a complete circuit.
Bipolar
Two electrodes on a limb located on different sides of the heart.
Typically… to register ECG
3 leads
Lead I
(-) to right arm
(+) to left arm
Lead II
(-) to right arm
(+) to left leg
Lead III
(-) to left arm
(+) to left leg
Precordial leads
Chest lead
Precordial leads
… of the chest/over the heart
Anterior surface
Precordial leads
… on the chest
Six separate points
Augmented Unipolar Limb Leads
Two of the limbs through the electrical resistance are connected to…
The negative terminal of the electrocardiograph.
Cardiac Arterial Supply
… lie on the surface of the heart
Two coronary arteries
LCA and RCA
Cardiac Arterial Supply
… penetrate into cardiac muscle
Smaller arteries
Cardiac Arterial Supply
… can obtain substances from the blood in heart chambers.
Endocardial layer (0.1mm)
Cardiac Venous Return
Blood from left ventricle cells (75% of venous return)
Coronary sinus
Cardiac Venous Return
Blood from right ventricular cells (about 20%)
Anterior cardiac veins
Cardiac Venous Return
Very small amount of venous blood (about 3-5%)
Thebesian circulation
Heart wall and arterial vessels arrangement
Epicardial coronary arteries
Intramuscular ateries
Subendocardial arteries (flow primarily during diastole).
Blood supply of the heart wall
…
Arteries → Perpendicular small branches → Network of arteries and capilalries.
blood supply of the heart wall
…
Rich supply of capillaries
Blood supply of the heart wall
…
One capillary for one myocyte
During rest normal human coronary flow…
About 250 ml/min
4-5% of the total CO
The adult human heart has a mass…
About 250-300g
Thus, 1 ml/min/g w.w.
At normal rates, …
About 80% of coronary flow occurs during diastole
Heart is characterized by…
Intensive metabolism
Myocardial: Very high level of…
Oxygen consumption and oxygen extraction.
Arterial blood O2 content
21 mL O2/100mL blood
Mixed venous blood O2 content
About 15 mL/ 100 mL
Coronary venous O2 content at rest
5 mL/100mL
Coronary Blood Flow (Perfusion)
Myocardial Oxygen Consumption
