Download cardiac AP and basis of ECG 2

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‫االسراء اية ‪58‬‬
By
Dr. Abdel Aziz M. Hussein
Assist Prof. Medical Physiology
Pulmonary circuits
Heart is the pump
Systemic circuits
• The heart is a hollow muscular organ
Site:
• Left side of the thoracic cavity partly behind the
sternum.
Size and weight:
• It is in the size of a man's fist
Atria
• It weighs about 320 gm in males.
• In female it weighs about 250gm.
Chambers:
Ventricles
• 4 chambers = 2 atria + 2 ventricles
• Its wall consists of 3 layers;
Epicardium
Endocardium
Myocardium
1. Nodal fibers
Atria
2. Conducting
fibers
Ventricle
3. Contractile
fibers
Myocardium is composed of 3 types of ms fibers;
a) Nodal ms fibers e.g. Sinoatrial (SA) node and
Atrioventricular node.
• SAN is the normal pacemaker of heart from which the
cardiac impulse initiated.
b) Conducting ms fibers include AV bundle, bundle
branches and Purkinje fibers.
• These fibers conduct the cardiac impulse from atria
to ventricles.
c) Contractile ms fibers
• They consist of atria and ventricles.
• The primary function of cardiac muscle cell is to
contract to pump blood into arteries
• Electrical changes within the cardiac muscle cell
initiate this contraction.
• There are 2 types of membrane potentials on the
cardiac muscle cell
1. One during rest called resting membrane potential
(RMP)
2. And the second during action called action potential
or cardiac impulses
• RMP is the potential difference across cell membrane
during rest i.e. inside the membrane is negative and
outside is positive
• Its value differs according to type of muscle fiber e.g.
in SAN it is -65 mv, and in ventricle ms is -90 mv
• It is measured by galvanometer with 2 microelectrodes
one is inserted inside the cell and the other is placed
outside the muscle cell
• Ionic basis of RMP
1. K+ diffusion to outside (main cause)
2. Na+ diffusion to inside (little effect)
3. Electrogenic Na+-K+ pump
• Action potentials occur when the membrane potential
suddenly depolarizes and then repolarizes back to its
resting state.
• So cardiac potential consists of 2 phases;
1. Depolarization phase (inside cell become +ve and
outside become –ve) mainly due to Na and Ca influx
(inflow inside cell)
2. Repolarization phase (inside cell become -ve and
outside become +ve) mainly due to K efflux (outflow
outside the cell)
RMP
Depolarization
Repolarization
Depolarization phase
Repolarization phase
• The excitation wave (depolarization followed by
repolarization) is initiated by the pace maker of the
heart (SAN).
• Then it spreads through the atrial fibers in a rapid rate
(1m/sec) to converge on the AVN.
• The conduction in the AVN is very slow (0.05 m/sec).
• The depolarization wave then travels rapidly down the
AV bundle, bundle branches (1m/sec) and the purkinje
fibers (4m/sec) to all parts of the ventricles.
• The body is a good conductor of electricity because
tissue fluids have a high concentration of ions that
move (creating a current) in response to potential
differences.
• The potential difference of about 120 mV between a
depolarized and a resting area of heart muscle
generates in the area of the heart an electric field that
projects and conducted to the body surface.
• These potential differences can be detected by
electrodes put on the skin surface and amplified and
recorded as a tracing on paper or on a monitor
Def.
•ECG is a record of the electrical activities of the cardiac
muscle
•Apparatus:
•The apparatus used for recording the electrical activities
of the heart is called electrocardiograph or ECG
•ECG apparatus consists of;
1. Recording galvanometer
2. Electrodes
Electrodes
Galvanometer
◊ Def.,
•It is the particular arrangement of 2 electrodes of the
ECG galvanometer.
◊ Types :
Bipolar Limb leads
Unipolar leads
They measure the potential difference between 2 limbs
at different levels of potential.
1. Lead I (RA (-) and LA (+))
2. Lead II (RA (-) and LF (+))
3. Lead III (LA (-) and LF (+))
Lead I
•Einthoven`s triangle is an equilateral triangle, the
sides of which represent the 3 bipolar leads & the
heart lies in its centre.
•Einthoven`s law the sum of voltage in lead I and
Lead III = voltage in lead II
Lead I
Lead II
Lead III
V leads
+
VR
+
VL
+
ECG
Central
t.
-
•They measure the potential at one point.
•The potential at one pole (-ve) of ECG is made zero by
connecting it to central terminal that receives electordes
from (RA, LA, and LF)
•Positive pole (exploring electrode may be placed at;
1. Limbs → unipolar limb (VR, VL and VF)
2. Chest → unipolar chest (V1- V6)
•They measure the potential at limbs (RA, LA and LF)
•The voltage of potentials recorded by these leads is
weak, so it must be augmented to form aVR, aVL, and
aVL
•This done by disconnection of electrode connected to
central terminal to limb to be augmented
aVR
VR
+
ECG
Central
term.
-
V1
V2
V3
V6
V4
V5
MAL
MCL
AAL
•6 unipolar chest leads from V1 to V6.
1. V1 (Rt 4th intercostal space)
2. V2 (Lt 4th intercostal space)
3. V3 (midway between V2 and V4)
4. V4 (Lt 5th intercostal space midclavicular line
5. V5 (Lt 5th intercostal space anterior axillary line )
6. V6 (Lt 5th intercostal space midaxillary line )
I
aVR
aVL
V1
V2
V3
V4
V5
III
II
aVF
V6
•ECG paper is composed of a number of 1 and 5 mm squares
•It calibrated so that,
•Each small square (horizontal)= 0.04 sec.
•Each small square (vertical )= 0.1 mv
• Consists of;
1. Waves (P,Q,R,S,T,U)
2. Segment (S-T segment)
3. Intervals (P-R interval, Q-T interval)
R
P
T
Q
S
U
1- P wave: represents atrial depolarization.
2- QRS complex: represents ventricular depolarization.
3- T wave: represents ventricular repolarization
4- U wave: represents repolarization of papillary muscles
•
•
•
•
B) S-T segment: normally is isoelectric
C) P-R interval:
It represents the time of conduction of impulse from
SAN to ventricles
Normally 0.12 to 0.2 sec
• It gives valuable information about;
1) The anatomical orientation of the heart
2) The relative size of the heart chambers.
3) A variety of disturbance of rhythm and conduction.
4) The extent, location and progress of ischemic
damage of the myocardium.
5) The effects of altered electrolyte concentrations
and drugs
•T wave is inverted
•Deep Q
•S-T segment is not isoelectric i.e. elevated or depressed
Remember
THANKS