Heart revolution, cardiac action
potential, cardiac electrical
activity, ECG
Romana Šlamberová, MD PhD
Department of Normal, Pathological and
Clinical Physiology
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Slides from the lecture.
Respecting the copyrights it was not
possible to publish pictures showed at
the lecture at our website.
Heart
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The heart is a hollow, muscular organ responsible
for pumping blood through the blood vessels by
repeated, rhythmic contractions.
The heart is composed of cardiac muscle, an
involuntary muscle tissue which is found only within
this organ.
Metabolism of cardiac muscle mostly depends on
oxidative processes.
Source of energy are fatty acids, lactate, glucose
and sometimes aminoacids.
Cardiac muscle
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Myocardium = syncytium – muscle cells are
connected by plasmatic bridges.
Cell nucleus is located in centrum of the cell (as in
smooth muscles) and there is striated (as skeletal
muscle).
The thickness of heart cavities is different (the thickest
is in left ventricle).
Except muscle fibers that have as a main function
contraction, myocardium has also muscle tissue that is
specialized in production and spread of excitation =
conducting system of the heart.
Basic characteristic of
myocardium
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Chronotrophy = ability of myocardium to produce impulse
(excitation). Result = regular rhythmic muscle contractions
without external impulse.
Dromotrophy = impulse transfers to the entire muscular
unit (atriums and ventricles), providing synchronous
contraction of all muscle fibers.
Bathmotrophy = ability to induce muscle contraction that
is strong enough (supraliminal input). While input that is
lower than threshold does not induce contraction, supraliminals
input of different intensity induces the same response if it
comes in sensitive period.
Inotrophy = ability of muscle contraction and its
dependency on other factors, e.g. initial tension of muscle fiber.
Blood circulation
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Blood circulation are two separated circuits = Small
(pulmonary) circulation is powered by right heart ventricle
and large (systemic) circulation by left ventricle.
Volume of blood pumped in time unit to pulmonary and
systemic circulation is the same = Minute heart volume
Heart distribution is given by amount of systolic pulse volume
(volume of blood ejected by one heart contraction) and by
heart rate frequency.
Pulmonary and systemic circulation differs in their pressure
and resistence. Pressure in pulmonary circulation is about 4 –
5 times lower than in systemic one.
Heart works as pressure pump. It has two parts: static
component - breaks through the pressure gradient between
ventricle and artery, and kinetic component – induces
acceleration to ejected amount of blood.
Heart cycle
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Tension – isovolumic phase. At the beginning of systole the
intraventricular pressure increases and atrioventricular valves
close (systolic sound).
Ejection phase. Intraventricular pressure exceeds the
pressure in large arteries and the ejection of blood from
ventricles begins (systolic pressure in arteries). Ejection phase
ends when the intraventricular pressure decreases to a level a
little bit lover than the pressure in large arteries. Blood flow
turns around and semilunar valves close (diastolic sound).
Phase of isovolumic relaxation. Intraventricular pressure
decreases.
Filling phase. The pressure in ventricles decrease so that it is
lower than the pressure in atria and the atrioventricular valves
open. At the beginning – phase of fast filling, later phase of
slow filling. Systole of atriums = last phase of ventricle diastole.
Pressures and volumes
• Pulse (systolic) volume
(PV)= 70 ml
• Final diastolic volume
(FDV)= 120 ml
• Final systolic volume = 50
ml = functional reserve of heart
• Normal systole ejects around
60% of FDV = Ejection phase
= PV/FDV.
• Heart distribution = PV x
frequency
Rokyta: Fyziologie
Conducting system of heart (1)
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The entire heart muscle is able to produce impulses and to
induce contraction = Automacy. Myocardium of ventricles
and atria use the automacy function in pathological cases.
There is a system of heart tissue in the heart that differs from
myocardium of atria and ventricles. It specializes to
production and spread of impulses that induce contraction of
heart muscle.
Structure of cells of conducting system differs from cell of
myocardium in lower number of myofibres, high amount of
glycogen and especially electro-physiologist characteristic.
Nodal part of conducting system (sinoatrial node and
atrioventricular node), has low speed (0.02-0.1 m/s) of
impulse spread, but high ability of automacy.
Other parts of the conducting system have ability of high
speed of impulse spread – depolarisation wave (4 m/s).
Conducting system of heart (2)
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Sinoatrial node (Keith-Flack) = pacemaker – located in entrance
of right atrium (sinusoidal rhythm 60 – 80/min).
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If the SA node doesn't function, or the impulse generated in the SA node is
blocked before it travels down the electrical conduction system, a group of cells
further down the heart will become the heart's pacemaker.
Internodal tracks – connect SA node with AV node (The AV node
receives two inputs from the atria: posteriorly via the crista terminalis, and
anteriorly via the interatrial septum)
Atrioventricular node (Aschoff-Tawar) – located by base of
tricuspidal valve (nodal rhythm 30 – 40/min). Time needed for
impulse transfer through AV node is 130 ms.
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Unique for AV node is decremental conduction = property of the AV node
that prevents rapid conduction to the ventricle in cases of rapid atrial rhythms,
such as atrial fibrillation or atrial flutter.
The atrioventricular node delays impulses for 0.1 second before spreading to the
ventricle walls. Importance = to ensure that the atria are empty completely
before the ventricles contract.
Conducting system of heart (3)
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Bundle of His = a collection of heart muscle cells specialized
for electrical conduction that transmits the electrical impulses
from the AV node (located between the atria and the ventricles)
to the point of the apex of the fascicular branches. (the only
conducting connection between atriums and ventricles)
Right and left (anterior and posterior) bundle branches
(Tawar branches) – going to myocardium of ventricles.
Purkynje fibers – going to periphery and end in myocytes of
ventricles.
Together, the bundle branches and Purkinje network comprise the
ventricular conduction system. It takes about 0.03-0.04s for the
impulse to travel from the bundle of His to the ventricular
muscle.
Conducting system of heart (4)
Impulse spreads:
• from endocardium
to epicardium
• from apex to base
• Synchronous
activation of
myocardium – systole
of ventricles.
Conducting system of heart (5)
• Fibers of SA node are easy permeable for Na+,
that enter the cells and decrease their resting
membrane potential (only –55 to –65 mV).
• This process of decreased polarisation ends when it
reaches level of –40 mV – prepotential
(spontanous depolarisation).
• At this level the sodium-calcium channels opens
and action potential is produced.
• During action potential the K+ channels are closed
and again open during repolarisation.
Action potential
SA node
Myocardium
Myocardium – contraction
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Contraction – begins just after the start of
depolarization and lasts about 1.5 x longer than the
action potential
Absolute refractory period – cardiac muscle cannot
be excited again = therefore tetanus (as in the
skeletal muscle) cannot accur
Shortly after the end of repolarisation myocardium
response only to strong impulse and the power of
contraction is lower even during high intensity of
stimulation. This period is called relative refractory
period.
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Vulnerable period = period at the end of the action potential
during which the fibrilation of the heart may accur.
Myocardium – correlation length x
tension
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Frank-Starling’s law =
initial length of the fibers is
determined by the degree
of diastolic filling of the
heart, and the pressure
developed in the ventricle is
proportionate to the total
tension developed.
The developed tension
increases as the diastolic
volume increases until it
reaches a maximum, then
tends to decrease.
Ganong: Review of Medical Physiology
Electric expression of heart
action
Ganong: Physiology
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Record of summary electric activity of hear is called electrocardiogram
(ECG).
ECG curve is summary potential that is a result of all action potentials of
myofibers.
Beginnings of QRS complex and action potentials of ventricles are the same
and ending of ventricle action potentials is the same as the end of wave T.
History ECG
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Description of animal electricity by Luigie Galvani (1783) and
discovery of galvanometer by Hans Oerstad (1819) started the
period of experimental electrophysiology.
Electric changes dependent on heart action described as first
Italian physicist Carlo Matteuci (1842) by using frogs.
The term electrocardiogram used as first British physiologist
Augustus D. Waller, it came to our subconscious mind by
attempt of Dutch physiologist Willem Einthoven (1893).
Einthoven constructed fiber galvanometer (1901, Nobel price
1925) that showed the same curve as ECG. He described the
PQRST curve. At year 1906 he published atlas of normal and
abnormal ECGs, in which he described left and right
hypertrophy of ventricles and atriums, wave U (repolarisation
of Purkynje fibers), flutter of atriums etc.
ECG
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Limb leads:
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V1-V2 = right ventricle
V3-V6 = left ventricle
Eithoven’s leads I., II, III.
 V1 – 4th intercostal, right
parasternal
 V2 – 4th intercostal, left parasternal
 V3 – between V2 and V4
 V4 – 5th intercostal medioclavicular
 V5 – between V4 and V6
 V6 – 5th intercostal, anterior axial
Goldberg’s leads aVL, aVR,
aVF
 V8 – 5 intercostal, scapular
 V9 – 5th intercostal, paravertebral
Bipolar limb leads (measure
changes in potential between
two electrodes)
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Red – right arm
Yellow – left arm
Green – left leg
Black – right leg (grounding)
 Thoracic leads (Wilson’s)
Unipolar limb leads
 it is possible to use also
(measure changes in potential Nehb’s leads
between electrode and
 V7 – 5th intercosta, posterior axial
Wilson’s clamp)
th
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Leads and electrodes
Limb
Thoracic
Trojan: Fyziologie
Evaluation of the ECG
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Pulse: regular, irregular
Rhythm: sinusoidal or other (nodal = from AV
node)
Frequency: Normal 60-90 pulses/min
Heart electrical axis inclination: normal (the
same way), to left (outside), to right (inside) just
generally from limb leads I and III. Exactly by
using Einthoven’s triangle.
Description of waves, their duration and
intervals.
Frequency
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Heard frequency = 72 pulses/min, = pulse interval
0.83 s
During relaxation the frequency changes based of
the respiration (RESPIRATION ARYTMIA) =
inspiration - increased frequency, expiration –
decreased frequency.
Bradycardia = fysiological = deep long-term
inspiration, deep forward bend and knee band =
reflex changes of vagal tonus.
Tachycardia = fysiological = swallow (decrease
of vagal tonus), change of position from lying or
sitting to standing (ORTOSTATIC REACTION).
Description of curves
P = Impulse spread through atria
PQ = neutral (isoelectric) line after
depolarisation of all atria
myocardium
QRST = ventricle complex
Q = negative oscillation –
beginning of ventricle
depolarisation in septum
R = continue of depolarisation
wave through the ventricle
S = negative oscillation –
activation of last part of ventricle
myocardium in left ventricle base
ST = neutral (isoelectric) line after
ventricle depolarisation (plató
phase in action potential)
T = repolarisation from epicardium
to endocardium
Examples of pathological ECG
Sinusoidal rhythm
and fibrilation of atria & AV block
Fibrilation of ventricles
Heart attack
Electric expression of heart
action
Ganong: Physiology
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

Record of summary electric activity of hear is called electrocardiogram
(ECG).
ECG curve is summary potential that is a result of all action potentials of
myofibers.
Beginnings of QRS complex and action potentials of ventricles are the same
and ending of ventricle action potentials is the same as the end of wave T.
heart electrical axis
average (largest) electrical heart vector)
if positive in the lead
aVF (←) and
positive in the lead I
(→)
then must lie where
overlap
Heart electrical axis inclination
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Each myocyte produce dipole
during action potential - vector
with specific dimension and
direction.
Cell vector head from
depolarised to polarised part, i.e.
in direction of action potential
spread.
If the cell is completely
depolarised (plató phase) or
polarised (resting phase), vector
is neutral.
Electrical heart vector is a
summary of all cell’s vectors in
one time point
Normal value is -30° až +105°
Shift of the axis to right above 105 °
= hypertrophy of RV or long and
slim
Shift of the axis to left below -30°
= hypertrophy of LV or obese
people
Examination of the heart
Mitral valve (bicuspidal)
– 5th intercostal space in
mediclavicular line (apex)
Tricuspidal valve
– 5th intercostal space right
parasternal line
Semilunar valve of aorta
– 2nd to 3rd intercostal space
righ parasternal line
Semilunar valve of
pulmonary artery
– 2nd to 3rd intercostal space
left parasternal line