hi this is Dr Nick with the ECG Academy with the second chapter in section five talking about the limb leads um well here we are going back to the idea that um the heart in the chest is creating this um electrical signal that's traveling in a direction usually down into the left of the patient okay that's the idea of a vector um a vector is an electrical signal that has a direction and so uh how do we record these vectors and how do these ECG leads make sense well we talked about einthoven's triangle and einthoven decided that he was going to use uh the right arm the left arm and the left leg to create this electrocardiographic um view of of the heart remember that lead one is moving anything that uh moves from the right arm to the left arm uh will be um in lead one um will appear to be upright in the cardiogram lead two are um signals traveling from the right arm to the left leg because that's where the wires are connected and those are the electrical signals that lead to is picking up and then lead three would go from left arm to left leg those are the wires that are used to um derive lead three and any signals going in this direction will be upright in the ECG lead of course conversely anything that's going in the opposite direction will show up as a negative deflection on the EKG okay just if as long I just want to make that clear so that gives us an idea if we look at any one particular lead we can tell what direction the signal is going in okay well a triangle is a little um difficult to understand so because we have the augmented leads in there as well AVR AVL and avf so I like to um I like the idea of a circle um the circle representing um the uh frontal leads which are the leads that we see in the front of the patient when we're viewing the front of him okay and this this circle um is divided into 360° by convention we usually divide this into four quadrants um and we consider that this angle to be 0 de um and the degrees go upwards from uh here and negative degrees are this way this will make a lot more sense when we talk about axis um but let's review um what the ECG leads that we have in Ein over's triangle are we have um lead one which is going from right arm to left arm and that's going straight across so we can draw it in this um Circle uh looking like this so lead one is drawn straight across and is point pointing to 0° and uh will record any activity that's going uh in a horizontal Direction um now lead two how do we remember that lead two is going from right arm to left leg well I always remind people that in English at least we read from left to right and um and uh from top to bottom so when you're ready to get to draw the second ECG lead in you want to draw it from left to right and top to bottom but it has to divide this the circle into six parts because there's three leads and they'll all BCT the circle so where do you draw it in this uh in this quadrant here you draw it 60° above the horizontal so that divides it like 60° remember there's 60° in every corner of an equilateral triangle for you guys who actually pass geometry and so lead two is drawn just like this um which is 60 degrees up from the horizontal going from the right arm to the left leg but it's beginning to divide this um circle into six equal parts now lead three which is from left arm to left leg we would naturally draw just like this and label it lead three and and thus we've divided the circle into six equal parts and we've defined angles that correspond to these three basic ECG leads now what about the augmented limb leads well the augmented leads kind of split these 60° um angles in half U because AVL which is um sort of the left arm compared with the the sum of the right arm and left leg is kind of um pointing towards the the arm itself and so what you get is this kind of an angle with the head of the uh Vector heading this way so this is AVL and likewise AVR is drawn 30° above the horizontal in this direction pointing towards the right arm so this is AVR and then of course avf would have to go right down the middle so that's avf and um and so we've now split this circle into 12 equal parts each one corresponding with one of the six limb leads that we see on a standard ECG uh remember what these look like uh we have 1 2 3 AVR AVL and avf those are the limb leads on the first part of a 12 lead ECG all right let me get rid of that for a second and go back to this drawing okay now um the idea behind um the leads being important has to to do with the direction of the electrical signal so let's say that in this normal person you have a vector that's traveling down and to the left so now if you happen to be monitoring let's say lead two lead two which would be going from the right arm to the left leg the signal is is traveling along lead 2 in that direction and so when you look at the EKG and Lead 2 you're going to see a large positive deflection of both both the p-wave and the QRS complex well let's say now you go over to that monitor and you switch the selector to AVL now AVL is going to be in this direction now we've already decided that the vector is traveling in this direction so AVL is um is almost perpendicular to the direction that the vector is traveling in and so when you look at AVL on your monitor you may only see a little tiny deflection and it wouldn't exactly be a good lead to monitor because the QRS complexes are so small you may want the monitor may actually double count the p and the QRS or it might not pick up anything at all and you'll get heart rates of zero and that makes the the patient's family really upset when they look up at the at the Monitor and see heart rates of zero um because the monitor is not picking up the C s complex so switching to a lead that will pick up a larger signal makes a lot of sense in those cases now the proof of what I was saying is in this ECG because here you have an example of where the signal is very large in lead 2 but it's much much smaller in AVL and so it's a similar situation to what I just uh described with the axis that's going in the direction of lead to because of that it's going in the perpendicular direction to AVL and the signal that you record is much much smaller doesn't it make sense now okay so the basic concept of a lead is that you can in fact have a get a sense of what direction things are traveling by switching leads you can get a better view an electrical view of the heart because the signal is traveling along that lead rather than perpendicular to it and in the intermediate course we'll we'll start to talk about axis which is basically the exact angle that the QRS happens to be at where you actually have a normal range of between Min -30 and um plus uh 120 where that that uh vector or the axis of the electrical signal is normal and anything above or to the right of that is going to be abnormal and Define axis deviation and uh allow you to diagnose crazy things like left anterior hemiblock and uh other kinds of advanced uh electrocardiographic diagnosis so with this discussion of the limb leads we'll get ready for chapter 3 which is going to be a review of the precordial or V leads and the kind of things we look at when we examine those and then in the next section we'll start to analyze the QRS complex itself using real Trace and start to talk about real electrocardiographic diagnosis this is Dr Nick with the ECG Academy thanks for watching