by
for
Before explaining how the program
ECGSIM3 works I would like to try
to explain the reason for the ECG
and how it is taken. However I
will understand if you just bleep
over the first section and go
straight to the program info!
The heart is a hollow muscular
organ, situated retro sternally
( behind the breast bone ),
slightly to the left hand side
of the chest.
It's job is to pump blood around
the body distributing all the
ingredients neccessary for life.
Simplified, the heart can be seen
to operate in 2 stages:
Stage 1. The Atria (2 chambers at
the top), fill up with
blood received from the
rest of the body and
contract to squeeze this
blood into the bottom 2
chambers, the ventricles.
Stage 2. The ventricles contract
and squeeze the blood out
to the rest of the body.
The electrocardiograph or
E.C.G is a representation
of the electrical activity
of the heart, normally
associated with muscle
contraction. That is heart
beat.
The ECG is generally
observed by connecting
electrodes to the skin
of the subject, sticky
pads or metal plates
and rubber straps.
Electrical activity is picked
up by an electrode and
displayed on an oscilloscope,
LCD screen and or printer.
Movement towards the
electrode is displayed
as an upward movement
on the screen, and
movement away from the
electrode is displayed
as a downward movement
on the screen.
The activity happens in a
particular order through
the heart. It starts in
The atria, goes to the
juction of the atria and
ventricles, waits a while
and then goes down the
centre of the heart into
the ventricles, where it
then spreads up the
outside. The muscle then
repolarises ready for the
next depolarisaton. This
repolarisation is also
seen on the ECG.
Electrodes can be placed anywhere
on the body. Twelve leads placed
on the legs, arms and chest give
an extremely good picture of the
happenings in the heart. However,
only 1 lead is needed to observe
the rhythms of the heart. Lead 2
(II) is a commonly used lead. This
lead is placed in such a position
that it looks at the heart from
below, therefore as the heart
depolarises normally, across the
atria and down the centre (septum)
, movement on the ECG is upwards,
as the depolarisation spreads up
the outside of the ventricles,
the movement of the ECG is
downwards.
To interpret the ECG
it is best to split
it up into it's
component parts:
A small positive deflection, which is
called the P wave is the activity
associated with atrial contraction.
A short section of flat line which is
the delay allowing blood to flow from
atria to ventricles. This is known as
the PR interval.
A negative deflection as activity
passes through the septum, the Q wave.
A positive deflection as activity
passes down the septum, the R wave.
A negative deflection as activity
moves up the outside of the ventricles
, the S wave.
A flat line, the ST segment.
A positive deflection as the ventricles
repolarise, the T wave. Repolarisation
of the atria is hidden in the QRS
complex.
Very occasionally a further positive
deflection, the U wave.
Some of these waves may
be missing, there is not
always any great
significance to this.
The important points to note are:
1. Is the Rhythm regular or irregular?
2. What is the rate?
3. Are there P waves?
With every QRS complex?
Or QRS complex with every P wave?
4. What is the PR interval?
5. Are the QRS compexes wide?
6. Is the ST segment iso electric
(does the flat line go back to
the same level as the other flat
lines?).
7. Are the T waves of normal shape?
If you can answer all these questions
you can interpret an ECG and determine
if there is an arrhythmia, and if there
is, what it is caused by. There are
over 440 arrhythmias. This simulator
covers about 40 of those. Most are
variations on a theme.
The program adapts itself to run on
both the Master and model B. However
the model B version is slower than
the Master version. This means that
on the model B you cannot calculate
the rate by counting the number of
QRS complexes appearing in one minute.
However, the grid and hard copy options
are accurate on both versions.
This simulator emulates
lead II. There are two
sections to the program.
1. See all the arrhythmias.
2. Test your skills of
interpretation.
1. SEE ALL THE ARRHYTHMIAS.
Selected by pressing R from the
main menu. All the arrythmias
programmed are displayed in
turn. Press the spacebar to
display the next.
A description of the arrhythmia
appears in the top half of the
screen on two lines. The top
line is a full description.
When using the test skill
option, entering more than half
of this description will give
you one point. The description
below is an abbreviated one,
you must enter the whole of the
abbreviated description to
score.
There are a number of options
available which are summarised
on the screen. They are:
D - Dump
Dump the current graphics window
to the printer. The default
window is the strip accross the
center of the screen.
The instruction is in line 280
and is *GDUMP for Printmaster.
W - Define window.
This is the *WINDOW option of
Printmaster it allows you to
change the graphics window
whilst the program is running.
The command is in line 290.
If you do not have a dump
program, don't use these options!
P - Prompt
A list of all the arrhythmia
descriptions is shown in the
bottom half of the screen.
The information is shown in
pages, press shift to see
the next page. This option
is intended to give a prompt
when using the interpret
option.
G - Grid
Place a grid onto the screen.
Paper printed from an ECG
travels at 25mm per second.
The paper is marked in 1mm
squares and heavier lines at
5mm. Horizontal lines are
time, vertical lines are
electrical voltage. 1 small
square is .04 of a second.
A square made by the heavier
lines is 5mm and represents
.2 of a second. Two vertical
large squares represent 1mv.
This simulation ignores the
small squares and prints
just the 5mm squares.
Pressing G toggles whether a
grid is dumped to the printer
when option D is chosen.
The current state is
displayed after the word GRID
(after the option has been
used). The default is grid off.
You will notice that, on the
monitor, the boxes are not
square, however they are when
dumped.
When all the arrhythmias
have been displayed, you
are returned to the main
menu once more.
2.TEST YOUR SKILLS OF INTERPRETATION.
Selected by pressing I from the main
menu.
All the options described for option 1
are available for this option.
One further point is that you will
notice 6 small vertical lines
above and below the two horizontal
lines on the display. These small
lines are markings representing
25mm on the print out. Each mark
represents 1 second. It takes just
over 5 seconds for a wave to
appear on the right of the screen
and disappear off the left of the
screen on the master.
An arrhythmia is selected
at random and displayed.
When you have decided
what the arrhythmia is,
press the space bar.
Enter the description,
either the full shortened
description or at least
half of the long
description (any section,
but more than half the
number of letters).
An approximately correct answer
gains one point, an answer
matching the programs description
gains two points. Answering
incorrectly 4 times loses you
1 point, tells you the answer and
goes on to another arrhythmia.
Your progress is summarised in the top
half of the screen with the total
number of attempts at interpreting,
the number of those attempts that the
program saw as correct and the number
of points awarded so far.
If you use the dump option from this
mode, the description of the
arrhythmia is garbled to avoid you
accidentally seeing the answer.
Oh, yes! I think it only fair to
mention that two of the rhythms
selected at random do not stay
the same. One attempts to
simulate an ill person. The
other just changes at random.
Try to interpret the former
before your patient goes into
asystole! Or get the Lignocaine
out before you need the Atropine!
[----R R interval-----]
R R
P T P T
^
Q iso electric Q
S line. S
[---] []
P R interval S T segment