E C G Simulation E C G Simulation by C.J.Richardson. C.J.Richardson. 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. INSTRUCTIONS INSTRUCTIONS 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! The PQRST complex. The PQRST complex. [----R R interval-----] R R P T P T ^ Q iso electric Q S line. S [---] [] P R interval S T segment