so to really understand the fundamentals
of ecg you must
first understand the electrical activity
of the heart
right so let's talk few words
about the electrical activity in the
myocardial cells
here i draw three
ventricular myocardial cells
and we see what is the electrical
activity
normally in these cells
let's suppose this is first myocardial
cells
ventricular myocardial this is second
cell and this is the third cell
right and we see how the electrical
activity goes through that
right then we'll discuss how that
electrical activity can be eventually
translated into ecg pattern
fine let's suppose that right now
these myocardial cells are not
stimulated they are unstimulated
there is no action potential passing
through them
is that right so we say these myocardial
cells are resting
if the cell is electrically
resting right it must have resting
membrane potential
now what is the resting membrane
potential resting membrane potential
is the electrical voltage difference
between the
inside of the membrane and outside of
the membrane
normally resting cells of the
ventricular myocardium
are electrically negative
inside right for example
the resting membrane potential is
minus 90 millivolt
in a resting cell when i say this
myocardial cell has
minus 90 millivolt resting membrane
potential
i mean that in the resting condition
this myocardial cell
has its membrane internally
polarized negatively right
now what really happens that as soon as
the cell is stimulated
right now when there is appropriate
stimulation to a myocardial cell suppose
ventricular myocardial cell if there's
appropriate stimulation
what will happen that its resting
membrane potential
will reach to the threshold right as i
said
resting membrane potential is minus 90
millivolt is that right
and this is minus 90.
if we stimulate the cell then some
cations will come in
some positive charges will come in let's
suppose that we bring
some sodium or calcium
ions n if we bring some electro positive
ions in what will happen that this
electro positivity which is coming into
cell
after the stimulation this will
take the resting membrane potential
towards
less negative value because these
positive charges
will neutralize the electronegativity is
the right to some extent
so what really happens as soon as
positive charges are coming in
resting memory potential is becoming
less negative
for example it become minus 80 and then
it become minus
70 millivolt right now
as soon as resting unit potential from
minus 90 millivolt
right it moves towards the
threshold potential and let's suppose in
this cell
threshold potential is minus 70
right threshold potential is a potential
at which membrane of
this cell will suddenly open
voltage-gated
sodium channels these are voltage-gated
sodium channels
and right now these channels are off
this channel they're not working but as
soon as you stimulate the cell
and some cations are coming in resting
membrane potential
start moving towards threshold as soon
as it touches the threshold
voltage-gated sodium channels open up
as soon as voltage-gated sodium channels
open in the
cell membrane lot of sodium comes in
lot of sodium is coming in why because
normally sodium is more outside the cell
is the right and normally cell is
electrically negative inside so as soon
as the resting manual potential goes to
the threshold voltage
right then voltage sensitive sodium
channels open
lot of sodium comes in this sodium as it
is rapidly coming into cell
is progressively neutralizing the
electronegativity
and the threshold potential which was
minus 70
it will become minus 60 minus 50 minus
40 and eventually it will become zero
it means as more and more sodium is
coming in this
membrane potential is progressively
going towards positive value
initially it is losing the
electronegativity and eventually it may
become
positive for example it is minus 60
minus
50 40 30 20 minus 10
0 and even it may become plus 10
so what has happened that once the
voltage gated sodium channels
open a lot of sodium come in this
movement of sodium
inside right neutralizes the
resting membrane potential and take it
towards
electro positivity and we say
the membrane which was previously
negatively polarized
it has lost its negative polarity what
has happened on
appropriate stimulation a lot of cations
came in
and these incoming cations neutralize
the electronegativity of the membrane
so inside of the membrane right which
was initially
negatively polarized it has lost this
negative polarity
or we simply say membrane is depolarized
when membrane loses the its negative
polarity we say membrane is
yes please membrane is depolarized so
this
event is called depolarization current
once depolarizing current is produced
and we say cell is depolarized
what is the next electrical element next
event is the depolarization sensitive
sodium sorry depolarization sensitive
calcium channels and
potassium channel both start
opening right now
these red channels are calcium channels
and this green channels are
potassium channels right and you must be
knowing
that in a normal healthy cell
right it has its normal healthy cell is
very very
rich in potassium intracellular
potassium is normally more
as compared to the extracellular
potassium and
extracellular calcium is more normally
as compared to intracellular calcium now
what really happens
listen you stimulated the cell cell
intervent
lot of sodium and flux that led to
phenomenon of
depolarization then depolarization
sensitive calcium channel then
potassium channels opened when potassium
channels opened i told you inside of the
cell is very rich in potassium
so potassium will start leaking outward
what is happening potassium will start
moving outward there
if e flux of potassium as soon as the
flux of the potassium start it means
cells start losing
cations cells start losing cation it
come towards the electro
negativity but as soon as it is around
zero voltage
calcium channels also open so for some
time for a very short time
potassium keep on moving out and as
calcium channels are open
so calcium keep on moving and it means
for a brief duration
potassium as positive charges are lost
and calcium
as positive charges are gained to the
inside of the cell
so it means for a brief duration cations
are lost as potassium
as well as cations are gained as calcium
so loss and gain of calcium balances the
potential right what really happens
that as potassium is being lost
at the same time calcium is being
gained so there is no net change in
potential
but as time passes by calcium channels
close as time is passing by calcium
start
calcium channels start closing but as
time passes by potassium
channels become more and more active so
what is the result
that by this time no more calcium is
coming in but because potassium channels
have
become more active so potassium losses
become very very heavy
there's a heavy potassium efflux
and as more and more potassium is going
out it means more and more positive
charges are going
out it means cell will again start
developing electronegativity because
cell is losing the positive
potassium so when potassium keep on
moving out
what really happens that cells due to
loss of
positively charged potassium become
progressively electro
negative and potassium is
progressively being lost potassium is
being lost until
membrane potential again becomes
at which value
resting membrane potential value that is
minus
90 volt is that right so what happened
during this time
what happens really that electrically
membrane has come back to its negative
polarization
if membrane has come back to its
negative polarization it means membrane
is
repolarized due to potassium efflux
membrane has gone to the phenomenon of
repolarization
let's revise it again what really
happens
this was a resting myocardial cell
it was having resting membrane potential
of minus 90 millivolt
is that right when it was appropriately
stimulated
some cations trickled in entered in
these cations took the resting membrane
potential at threshold potential
at threshold potential voltage-gated
sodium channels open
lot of sodium comes in and rest and then
potential goes towards the
it loses all the electronegativity and
goes towards the positive side
this phenomenon is called membrane is
undergoing which phenomenon
depolarization membrane is depolarized
after depolarization
in ventricular and arterial cells
repolarization process start and
repolarization process
initially potassium goes out but very
soon calcium start coming in
for a brief time potassium is going out
but at the same time calcium is coming
in
so cations are lost as well as cations
are gained so there's no voltage
difference so we say
that electrical potential is showing
plateau phase
but eventually calcium channels shut
down and potassium channel channels
become progressively
more open so lot of potassium leaks out
and due to heavy losses of positive ions
right membrane become progressively
electronegative inside
and potassium keep on going out until
the membrane voltage goes back to its
resting
negatively polarized state so we say
that there is
membrane has gone under the phenomenon
of repolarization
so these two processes together they are
called depolarization
and repolarization is that right
so by stimulation we say this cell
number one
has undergone process of depolarization
and after the depolarization
it has undergone repolarization is that
right
once the cell cell went under
depolarization
once the cylinder it was undergoing
depolarization
due to sodium and flux right
the sodium which was coming to the cell
some of the sodium
through gap junctions these are very
special
connections between the adjacent
myocardial cells
adjacent myocardial cells neighboring
myocardial cells
have electrical windows
so what happens when one cell undergo
depolarization
the cations which have come into the
cell some of these cations
trickle to the next cell from here they
go to the
next cell when cations from the first
stimulated cell
trickle into next cell what really
happens then resting memory potential of
this
next cell will reach up to yes please
reach up to
threshold and then what will happen
voltage-gated sodium channels will open
into this
then same thing will happen for short
time
voltage gated calcium and potassium
channels will be open
simultaneously eventually calcium
channels stop
but potassium channel keep on working
and losses of potassium
eventually take the membrane potential
back to its
resting situation so it means
depolarization of one cell
led to the depolarization of
second cell and when second cell is
depolarized when second cell gate gains
lot of
cations it trickles the cations into
third cell
and then third cell undergoes the
process of depolarization and
repolarization it means that this
fluctuating potential of the membrane
right the membrane
fluctuates from restriction potential to
depolarization
and then it fluctuates back to
repolarization
this depolarization and repolarization
they are moving
in the cell membranes because
depolarization and reposition the moving
so this fluctuation total electrical
fluctuation of depolarization and
repolarization because it is mobile
it is called action potential
is that right let me write it more
clearly
that there is yes resting membrane
potential
and with proper stimulation
there is stimulation resting
potential will lead to the membrane too
yes please
d polar rise asian
then depolarization will stimulate
initially calcium and potassium channels
but eventually
potassium channels become very active
and lot of potassium goes out
and that leads to re polarization
and as depolarization and repolarization
the moving on the membrane
so both of them together
they are called action yes please
action potential
is that right so in next discussion when
i say
that cells are undergoing depolarization
it means
cell is gaining cations usually
sodium in case of atrial and ventricular
cells they undergo during depolarization
they gain
sodium cations right and if there is
special point
if there is depolarization of acid and
av node
remember depolarization of sn node and
av node is not sodium dependent that is
calcium dependent so it means when i say
that atrial tissue or ventricular tissue
is undergoing depolarization
it means those tissues are gaining
sodium and if i say
astronomer and av node are depolarizing
it means they are gaining calcium
but anyway depolarization is whenever
cell is appropriately stimulated
a lot of sodium or calcium come in and
depolarizes the cell
but whenever cell is depolarized it is
followed by which activity
repolarization and this depolarization
in one cell
stimulates the next cell through the gap
junctions the next next cell becomes
depolarized and second cell will
depolarize the third cell and third cell
will depolarize the
next cell so in this way in the heart
because every cell has
electrical windows or gap junctions with
the next cell
so electrical stimulation of one cell
can lead to the electrical stimulation
of the next cell
am i clear if this basic concept
is clear now we'll talk about that
little bit more in detail
right eventually what we have to
understand today we have to understand
that how electrical activity of the
heart
can eventually be presented as
ecg pattern is that right now