 Current of injury is an important topic but it's a bit difficult to understand. For understanding current of injury, you need to know some fundamentals about electrocardiography. Well, I will try to simplify this concept of current of injury for you in this video with very little fundamentals of electrocardiography. Okay, so let's begin. See at resting membrane potential that means when there is no impulse, the cardiac muscle cell has positivity outside. Just like any other cell at resting membrane potential, it is positive outside and negative inside. So now if I place two electrodes anywhere, say suppose one is negative and other is positive, so these are placed on the surface of the body and since the charges are same everywhere, what happens? These electrodes don't record any potential difference, so there is zero potential between these two sides. So let us take an example of lead 2 whose direction is like this, back end being negative, the arrow represents a positive end. So this lead 2 will record zero potential when the cells are at resting membrane potential. So we'll focus just on the ventricles here because we'll be talking about the current of injury with respect to ventricles. So anyways at resting membrane potential, there will be a straight line or zero potential recorded. Now what happens when the ventricles depolarize, these are the direction of depolarization of the ventricles. So the concept is basically when the direction of depolarization is in the same direction as that of the lead or at an acute angle to lead, we get a positive deflection like this our wave and when the direction is at an obtuse angle to lead or to opposite direction to lead, we get a negative deflection. Also you see that at resting membrane potential, since the charges are same everywhere which we saw in the beginning, zero potential is recorded because there is no potential difference between two sides. Also if the heart is completely depolarized, you see everywhere there will be electronegativity outside with electropositivity inside. So then also there is zero potential between two sides. So again there is no potential because the charges are same everywhere. Okay, so with this concept let's proceed to current of injury. So what happens in an injured myocardium? So here we are seeing an example where left ventricular base is injured. So the injured area actually becomes more permeable to ions. Hence it remains in a depolarized state. So even at rest what happens see there exists a potential difference between different sides in the ventricle and the direction of potential difference is something like this from negative to positive we have to see. Okay, before we proceed just one thing that we are using the term current of injury and the books also say current of injury but here we are repeatedly talking about the voltage difference. See it's almost the same thing. Why? See voltage difference we are seeing in the direction from negative to positive but current we say that current flows from positive to negative, isn't it? But if you see in books they draw the current of injury like this only why so? Because you see if there is negativity outside actually inside the cell is positivity, isn't it? In depolarized state the inside of the cell is positive and all these areas will be negative so the current inside actually is flowing from positive to negative only. So it's the same thing but for understanding I prefer the term voltage difference more because in ECG we are actually recording the voltage difference. So I hope that even if you use the term voltage difference or current you'll be able to get it that how we are determining the direction. Okay, so we were saying that the injured area here is in depolarized and there is a voltage difference like that. Now when we consider lead tool it will record some voltage difference so we'll get a straight line but how do we know that this straight line is actually some voltage or is it is at zero? We don't know because it is just a straight line. We don't have any reference to compare this a straight line. So for that we need some reference so we'll see how we'll get a reference. So now when impulse comes the depolarization starts in the same direction what we discussed earlier. So if we see the direction of depolarization is obtuse to the direction of the lead so we get a negative wave then again the direction becomes like this like normal so we get a positive wave and lead to then you see in the end the direction of depolarization is different from what we saw in the normal. In normal we saw direction was like this but here because the left ventricles are already injured that is already they are depolarized so the only area to be depolarized which remains in the end is the base of the right ventricle. So this is how the direction of depolarization becomes at the end. Now if you draw this direction of depolarization with respect to the direction of the lead it is still recorded as a negative wave just like as a normal ECG but we should be clear about the fundamentals okay. Anyways so let's proceed now what happens that when the ventricles become fully depolarized so now you see there is no potential difference existing between different sides of the ventricle. So this is the actual zero and now the lead will not record any potential difference so what happens we get a straight line here. You see that this straight line is not along the previous straight line so here there will be a P wave so this represents actually PR segment and this area represents ST segment. So ST segment here actually represents when there is no voltage difference between two sides or when there is no current flowing between two sides of the ventricles. So this is known as isoelectric segment okay. Again now when there is repolarization what happens it goes back to the original line because again there is a voltage difference existing so this is the T wave actually which will not be clearly seen since it is moving from somewhere here to the level of this PR segment. So actually this is the wave loss somewhere. So fundamentally ST segment actually represents when there is no potential difference between two sides of the ventricles and the point which marks the beginning of the ST segment is known as J point and this is the one which actually gives us the reference for comparing the original voltage. So what we do is that we draw a straight line from this PR segment to the TP segment. TP segment is the end of T wave to the beginning of the new P wave. So straight line joining PR segment with TP segment and another straight line crossing through the ST segment via J point. So you see this is the actual zero potential and the potential which was recorded in the beginning at rest it is positive compared to the zero that is why this is known as positive current of injury but we always have to see it with respect to the direction of the lead because there may be other lead. So suppose if we are considering AVR whose direction is like this the entire ECG will be inverted in AVR. By the way to understand that how ECG is recorded in different leads I have made a series of videos whose link I have given in the description section below you can check those out also. So coming back to this topic the direction of the current of injury we always have to see with respect to the direction of the lead. So in this case in lead 2 we are seeing a positive current of injury but you see that with only one lead we will not be able to tell that to which site of the myocardium is injured. Why so? See I told you that if the direction of voltage differences along the direction of the lead or if it is at acute angle of the lead we get a positive recording. So that means in this case the direction of voltage differences like this so we are getting a positive current but even if it was like this then also we'll get a positive current if it was like this then also we'll get a positive current. So we have to determine that out of these many what is the actual direction of the voltage difference for that we need minimum two leads. So let's add one more lead here in this case I have taken a lead 3 and we'll see that how this will be recorded in both the leads now. So let's see so again this is the injured area and we'll get some recording in both the leads we don't know whether it is positive it is negative it is zero we don't know anything yet then when the impulse comes the depolarization starts so in lead 2 because it is at an obtuse angle we get a negative reading but in lead 3 it is an acute angle to the direction of the lead. So actually we get a positive deflection so there is no Q wave in this case in lead 3 okay then it moves forward like this so it is at an acute angle to both lead 2 and lead 3 so we get a positive deflection in both the leads then finally because of the injury the direction of depolarization is towards the right side and to the back so we get a negative wave in lead 2 but actually some positive value in lead 3 because of it being at an acute angle now when the heart becomes fully depolarized what happens the voltage comes back here and it goes till here in lead 3 then there is a straight line okay then finally with repolarization it comes back like this so now again we draw lines passing through the two places in the PR segment and TP segment or other line passing through the J point ST segment so we do that to recordings of the both the leads and what we see that there is a positive current of injury because this is the actual zero this represents the actual zero so there is a positive current of injury in both the leads but the magnitude is different in the lead you see in lead 3 it is more logically also you it's correct isn't it if you remember we said that this is the voltage direction of the voltage difference which exists so you will see this direction is almost along the direction of lead 3 so obviously this has to be recorded more in lead 3 while in lead 2 it is at an acute angle so the magnitude is little lesser in lead 2 so that's the reason that current of injury is more in lead 3 so now we have to determine the actual direction of depolarization which exists at rest to do that we will plot this voltage difference at the beginning just like we plot for the cardiac axis so for that also I've made another video you can have a look at that also so let's see how to plot it see this is the direction of lead 2 this is the direction of lead 3 arbitrarily I have plotted it so this is larger magnitude so let's put a point here and this is smaller magnitude so let's put a point here then after this we have to draw perpendicular from this point on the lead so let's draw those perpendicular and then from the central point we have to draw an arrow like this so this is the voltage difference direction which exists at rest with negativity at the back and positivity in the front and you know that negativity means that that area is actually depolarized at rest so we have determined the site of injury with this because you can actually try to place this axis on a kind of 3d imagery of heart and then you will be able to guess what is the site of injury okay so here I have one exercise for you where I've shown lead lead 2 and the ECG which is recorded in that basically this v2 is a chest lead which is directed from back to front so it is an antroposterior dimension and we are getting some current of injury in this now say suppose there is another lead of lead 3 there also we get a similar kind of recording maybe like this now can you tell what is the direction of the current of injury so again we'll follow our same thing we will draw two straight lines like this across the recordings of both the leads since this is the real zero in both so what we are getting here it is a negative current of injury so negative current of injury means that the current is directed in the opposite direction or at an obtuse angle to these leads so it may be like this like this in short we see it's actually directed from front to the back side so yes it's a negative current of injury directed from front to the back that means front is negative back is positive and negative we know that it is a depot rise or we can say front is anterior we can say that it is actually some anterior wall infarction so till now we were talking about injury injury is nothing in this case basically it is infarction so by looking at the current of injury whether it is negative positive in various leads and then deconstructing that you can actually determine the site of infarction in the ventricle so that's all for the video on concept of current of injury if you like the video to press the like button share the video with others and don't forget to subscribe to the channel physiology open thank you