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so let's start with the most basic way
of interpreting the ECG the first thing
you do is you have to check the voltage
calibration what's this about this is
about the amplification of the image
that the ECG machine is going sometimes
with arrhythmias we want to enlarge the
complexes so we will double the the
amplification of the image but usually
for almost all routine ECGs the
calibration is so-called 10 millimeters
that's 10 little boxes and we'll show
you examples of that and it's always
good to check that to begin with because
if they had been using the Machine at a
higher amplification that complexes are
gonna look bizarre they're gonna be big
and so forth and you might make a
mistake in reading so first make sure
that that the calibration is correct
usually the technician takes care of
that that but it's there's a little box
that shows you the calibration is okay
and I'll point that out later the next
thing is to determine the rhythm and how
do you do that you're looking for P
waves followed by qrs's in other words
the normal progression remember starting
up with the sinus node atrial
depolarization P wave QRS ventricular
depolarization T wave resetting once you
know what the rhythm is in other words
is this a normal sinus rhythm or is it
not normal sinus rhythm but uh but in
arrhythmia and we're gonna have whole
lectures about the kinds of abnormal
electrical events that can occur when
it's not sinus rhythm you then calculate
the heart rate which is a there's a
normals there and I'm going to be going
over the normals as we go along you then
do the timing intervals
what's the PR interval from the
beginning of the P wave to the beginning
of the QRS what's the QRS duration
that's the period of ventricular
depolarization and then how long does it
take for the whole contraction and
resetting of the ventricle from the Q
wave to the end of the T wave we would
then determine the electrical axis in
other words what is the main vector
force of the electrical depolarization
wave what direction is it going in and
there's a certain normal area for that
and for example if you have
abnormalities of a ventricular mass you
can get abnormal of vectors abnormal
electrical vectors we want to look at
the p-wave morphology to see if it's
normal we look at its voltage and its
shape because certain abnormalities of
p-wave morphology can occur with certain
diseases we want to do the same for the
QRS morphology again certain diseases
and for example heart blocks things that
can lead to pacemakers will change the
morphology of the QRS will then look at
the ST segment and the T wave morphology
ischemia lack of blood flow in the heart
that can lead to angina or heart attacks
changes the ST segment and T wave
morphology and if we're lucky enough to
have an earlier baseline EKG we compare
it to see hey have there been changes is
something going on here that's something
acute so let's start here is a normal
ECG notice in the upper left corner
there is a little green box that's the
standard and that standard is if you
count the little tiny boxes ten little
boxes each one of the big boxes has five
smaller boxes so there'll be two of the
big boxes which constitute ten small
boxes each one of those boxes is one
millimeter and corresponds to 1
millivolt of electrical activity so
again this is the standard on all ECG
machines and if you had set the standard
to 1/2 then of course it would be each
one of those little boxes would be less
voltage if you doubled it they would be
more voltage but usually again almost
all EKGs use this green box of the 10
standard you'll notice how the leads are
placed here on the left-hand side
there's lead 1 2 & 3 remember zero
Degree + 60 + 120 the next 3 leads AV RL
+ F AV r + 210 AV el - 30 AV f + 90 and
then come the
cordial leads the ones that are sticking
through the heart like like needles in
the sagittal plane v 1 v 2 v 3 4 5 and 6
and this is a normal ECG notice there's
a p-wave in front of each QRS the QRS is
are nice and narrow there's a nice T
wave up right and and not way prolonged
after each QRS this is sinus rhythm
normal sinus rhythm that that's set off
by the sinus node passes normally
through the heart there's no evidence
here of ischemia or heart attack or or
hypertrophy of the heart muscle as I
said you'll notice that there's sinus
rhythm in the green box you'll see that
each QRS is preceded by a P wave the a
true the atrium depolarizes before the
ventricle all the P waves are followed
by a QRS there's no blockage of the beat
as it goes down through the heart
each QRS is preceded by a P and the peas
are all identical they're upright in
leads to and AVF and they're nice and
narrow they're not prolonged if any of
the comments just made the answer was no
then you're talking about an arrhythmia
and as I said we're going to have whole
lectures on the arrhythmias later so
right now we're just worrying about the
normal also you notice how nicely of the
QRS is progress in fact that whole strip
along the bottom even though they're
different leads it's continuous so
you're actually seeing one set of P
waves after another in order to obtain
the heart rate you count the number of
big boxes between two qrs's and you
divide by 300 so if there were two big
boxes between the two qrs's
that would be 2 into 300 or 150 if there
were three big boxes in between a QRS
that would be a rate of 103 into 300 if
there were four boxes in between the two
qrs's that would be 75 4 into 375 you
can also do it by counting the number of
qrs's in 3 seconds remember the ECG is
moving at a certain rate you can
calculate
a number of seconds and then multiplied
by 20 but usually what we do is we use
the rule of 300 mentioned before it's
important to note that the normal heart
beat is sixty to a hundred beats per
minute so now we have a heart rate by
the way the computer is almost always
right on the heart rate again let's talk
about the intervals so a normal PR
interval is 0.2 seconds to 0.20 seconds
that's three little boxes two five
little boxes
alright you remember there's five little
boxes within the big bigger box
so the normal PR interval from the
beginning of the P wave to the beginning
of the QRS is somewhere between three
and five little boxes 0.122 0.20 seconds
the normal QRS interval is less than 0.1
that's two and a half little boxes and
the normal QT which is corrected for
heart rate in a formula is somewhere
between point three and point four six
seconds remember the Qt from the
beginning of the Q wave to the end of
the T wave remember each small box is
0.04 seconds so a large box is five
times 0.04 seconds or 0.20 seconds and
there are five small boxes in each large
box as I've said before again what about
the axis well there's a general rule
that most med students use and that is
if the QRS is upright and leads one and
two it's a normal axis you can actually
calculate the axis because the axis is
perpendicular to any lead where the R
and the S or the upstroke and the down
stroke are equal in this example lead
three you see that pretty much the
amount above the line and below the line
is about the same so the axis is going
to be 90 degrees from lead three so 90
degrees plus 120 lead 3 is 120 remember
that's 210 and that would be in other
words the axis would be towards AVR or
minus 90 from 120 would be 30 that would
be somewhere needly near lead 2 which is
+60 well you can see how do we tell
where is the maximum our wave well the
maximum our wave is a round lead to it's
there's no up right our wave and AV are
so therefore the axis is actually
something like a plus 30 so again the
rule of thumb is you look to see where
the amount of voltage up and down is
equal it's 90 to the axis is 90 degrees
from that then you look for the lead
with the maximum R wave that's the
direction because you could go this way
on the 90 degrees or you could go that
way what tells you which way you go is
where's the maximum R wave in this one
it's lead to and again normal axis is
between minus 30 and plus 90 and if the
axis is not between plus minus 30 and
plus 90 then it's an axis deviation if
it goes more - that's so-called left
axis deviation if it goes more plus than
90 it's called right axis deviation and
we'll talk about how that's used in
reading various electrocardiographic
diagnosis so again just to reiterate the
axis for the mean frontal plane
electrical vector of the heart is near
the limb lead with the tallest R wave
and perpendicular to the lead where the
size of the upward deflection and the
downward deflection are equal remember
the upward deflection is called an R
wave the downward deflection is called
an S wave so let's take a look at the P
wave itself the normal P wave is going
to be three little boxes or less in
duration remember that the PR interval
that's the duration from the beginning
of the P wave to the beginning of the
QRS is going to be less than five little
boxes but the length of the P wave
itself should be only three little boxes
and it should be upright in lead one and
two and a negative deflection of less
than one box wide or one box deep in v1
if the P wave in lead v1 is more
negative than one box and wider than one
box that suggests that the atrium is the
left eight
trim is enlarged so-called left atrial
enlargement if the box is pointed and
higher than two millimeters then and
usually wider than two then that defines
right atrial enlargement now these
numbers are not anatomically perfect the
echo and the MRI and so forth would be
more perfect but they carry prognostic
information they're very important
because when they appear it really means
that there's quite significant either
left atrial dilatation or right atrial
dilatation let's look at the R wave now
the normal R wave should transit in the
precordial lead starting with lead v1
there should be a very small R wave and
then it gets a little bigger in v2 and
somewhere between v3 and v4 you have a
dominant R wave with not much s wave and
then it progresses out to v6 with
usually the maximum R wave somewhere in
v4 five and six
with the transition from more negative
to positive
somewhere around v3 or v for small Q's
that is initial downward deflections of
less than one little box can occur but
that they are never longer than one box
if they're wider than one box it
suggests that there's been damage to the
myocardium and the voltage should be
within a normal range also the ST
segment should be isoelectric that as it
should be flat you can have a little bit
of depression but if it's substantially
depressed more than a tiny amount it
suggests a number of things let's look
at what it suggests if there's a sort of
curved sagging of the ST segment as in
this example that means the patient is
usually taking digitalis did you tell us
has that affect if there's a ski MIA or
lack of blood flow you see a squared off
flattening of the ST segment we see that
with a positive exercise test and we see
that when patients come in and have a
so-called non-st elevation myocardial
infarct and we'll talk more about those
definitions later and then in hypo
Kaleigh Mia where the potassium is low
you may see a mildly down sloping ST
segment the T wave is often flattened
and as I mentioned before you may see a
little additional wave after the T wave
the you wave again here's the normal EKG
look at the ST segments here they're all
fine they're not depressed they're not
elevated they're in exactly the right
sequence
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