 Hi and welcome to Physiology Open. In this video we will discuss what are the various leads of ECG, how and why ECG is recorded differently in different leads and why we need different leads. We have discussed fundamentals of ECG in another video, the link for which is provided in the description section. You can check it out also. Anyways in brief I will tell that you too. So electrocardiography is the procedure of recording the spread of electrical activity in heart. Electrocardiograph is the machine which is used to record this electrical activity and the record which is obtained is known as electrocardiogram. Don't confuse, the record obtained is not known as graph, it is known as electrocardiogram. The machine is known as electrocardiograph. Okay, since electrocardiography is the procedure of recording the spread of electrical activity in heart that means for knowing ECG, first we should know how the electrical activity spreads in heart. Initially the impulse travels from SA node to AV node, depolarizing atria in between via gap junctions and the average direction of depolarization of atria is like this. Direction is always taken from negative to positive. See the depolarized area has become negative outside. The one still at resting membrane potential is positive outside. So the direction will be like this. Then the septum gets depolarized from left to right for a brief duration and average direction of depolarization that time becomes like this. Then for ventricular depolarization again the average direction is like this. But the final part of the ventricle to be depolarized is posterior basal portion of left ventricle. So while the rest of the ventricle is depolarized that is outside negative and this posterior basal portion of left ventricle is positive outside the average direction becomes like this. These four arrows represent the average direction of spread of electrical activity in heart. This is known as vector cardiogram. Now to record this spread of electrical activity we place electrodes around the heart. The way this electrical activity is recorded depends on two things. One obviously the average direction of spread of electrical activity and second where we put the positive and negative electrodes outside on the body. We can also call it what is the direction of the lead. Again direction taken from negative to positive electrode. Whenever the direction of the lead is same as the direction of the spread of depolarization it is recorded as a positive wave. So suppose we have a lead like this from right to left approximately 60 degrees from horizontal how the spread of depolarization will be recorded in this lead? See initial atrial depolarization is along the direction of the lead. So it will be recorded as a positive wave. Then septal depolarization is opposite to the direction of lead. Then how will it be recorded? See you have to apply the knowledge of vectors here. I will once again direct you to the video in the description section which describes how it will be recorded. So here septal depolarization is almost opposite to the direction of the lead. So it will be recorded as a negative wave. Then ventricular depolarization is again becoming along the direction of the lead. So it is again a positive wave and since ventricular tissue is larger than the atrial tissue so larger amplitude positive wave is recorded. Now see last part of ventricular depolarization is almost 90 degrees to the direction of the lead. Again vector knowledge no voltage will be recorded for this arrow if it is exactly 90 degrees. In this case it is a little obtuse. So some voltage will be recorded but negative. So we just saw how atrial and ventricular depolarization will be recorded in lead 2 of ECG. Yes the direction of lead shown here is on lead 2 of ECG and see we have just obtained our P wave which is recorded due to atrial depolarization and QRS complex which is obtained due to ventricular depolarization. Now during ventricular repolarization also the vector direction is like this. So it is along the direction of the lead so it is also recorded as a positive wave in lead 2. Now let us see other ECG leads and how all these directions of depolarization and repolarization will be recorded in different leads. But why do we need so many ECG leads? There are in total 12 ECG leads. So why do we need all these leads? We can't get full information about the spread of depolarization from a single ECG lead. See heart is a 3D organ and one lead only tells about 2D spread of depolarization. See till now we were talking in 2D terms vector direction pointing to left, right, up to down. But what about anterior to posterior direction and vice versa? So to fully understand the spread of depolarization we place multiple leads directed in multiple directions. So say if this is the heart and I draw a circle around the heart like this and we consider it a vertical plane and another plane like this this is horizontal plane. We place leads such that they are directed in both the planes. These leads are limb leads and chest leads. Among limb leads we have bipolar limb leads and unipolar limb leads and chest leads are all unipolar chest leads. So first let us see what is the meaning of these terms unipolar and bipolar and what is the direction of these leads? A lead records potential difference between its two electrodes. Both unipolar and bipolar leads do that. In bipolar leads both the electrodes are at some potential and lead records the potential difference between them. In unipolar leads one of the electrode is at zero potential. So if the potential at one electrode say v1 is subtracted from the other electrode potential that is zero we will get that potential only that is v1. So unipolar lead is said to record the actual potential. So okay what are these unipolar and bipolar leads? So what we do is that when one electrode is put on right arm which is negative electrode and another electrode put on left arm which is positive electrode the direction of lead is like this. You know if the direction is taken from negative to positive? This is known as lead one of ECG. When negative electrode is on right arm and positive electrode on left leg direction of lead is like this 60 degrees to the horizontal and this is lead 2 of ECG with negative electrode on left arm and positive electrode on left leg. The direction of the lead is like this. This is lead 3 of ECG. If we draw the direction of all these leads from a common center we can draw it like this also. It is nothing we are only just shifting the vectors we are just drawing it from a common center. Now these leads lead 1, 2, 3 are bipolar limb leads. There are three unipolar limb leads also. These are AVR, AVL and AVF. For these leads the electrode at one arm is considered positive electrode while the electrode at other two places is passed through high resistance such that the potential becomes zero there and it is taken as negative electrode. So for AVR, R is for right arm. The electrode at right arm is a positive electrode and that at left arm and left leg is passed through high resistance and considered as negative electrode. So direction of the lead is like this. For AVL the electrode at left arm is positive and that at right arm and left leg are made zero and considered as negative. So direction of lead is like this. Similarly for AVF the electrode on left leg is positive and the other two electrodes are made zero and the direction of lead is like this. So we have discussed six limb leads of which three are unipolar and three are bipolar. See these limb leads look at the depolarization of the heart in vertical plane only that is how depolarization is moving from up to down and right to left. To study how the depolarization is moving in antropostere direction we need chest leads. So there are six chest leads. V1 to V6 all these chest leads are unipolar leads. These chest leads look at the depolarization in antroposterior plane that is this plane. So what are the direction of the leads? For chest leads the electrodes placed on the limb are passed to high resistance and made to zero and all combined they act as a negative electrode. The positive electrode is placed on the chest wall. For lead V1 electrode is placed in fourth intercoaster space just right to the sternum. For V2 in fourth intercoaster space just left to the sternum. So direction of these leads is like this. This is V1 and this is V2. Now V4 is placed in fifth intercoaster space in mid-clavicular line and V3 is placed between V2 and V4. So their directions are like this. V5 is placed in fifth intercoaster space in anterior axillary line. These are all on left side and V6 in left fifth intercoaster space in mid axillary line. So only V1 electrode is placed right to the sternum and rest all electrodes are placed on the left side. Okay now let's go back to basics again. We have seen how the ECG looks in lead 2 and we know that when depolarization is along the direction of the lead it is recorded as a positive wave. Now by looking at the direction of the different leads can you think in which of the lead ECG will be recorded like this? Look at the direction of the lead. So here ECG is recorded as complete opposite to that of lead 2. If you are thinking about AVR you are totally correct. So to understand just take the direction of AVR lead which is like this and for each direction of depolarization and repolarization see how it will appear in this lead. Yes it will be in opposite direction. Now let us see how it will appear in chest leads. We will take two chest leads V1 and V5. Look at V1 direction it's like this and look at V5 direction it's like this towards the left side anteriorly. Now this vector cardiogram we have to see how this potential spread is moving in antero posterior plane. So this is the vector cardiogram you already know of. The first one the atrial depolarization we have seen that it is down and left plus also it is moving little bit anteriorly. Then the septal depolarization is moving bit posteriorly. Then major part of ventricular depolarization again it is anteriorly and the last part of ventricular depolarization is again upward left hand bit posteriorly. So how this spread of depolarization will be recorded in leads V1 and V5. First let us see atrial depolarization. You just see how it is with respect to the direction of lead V1 it is around 90 degree but it's still acute. So a positive P wear will be recorded but very small since it is near to 90 degrees. In V5 it is along the direction of V5 so a positive P wear will be recorded but of larger magnitude than in V1. Then septal depolarization again almost 90 degree so maybe no recording. For V5 septal depolarization is in opposite direction to V5 so it will be recorded as a negative wave. Then ventricular depolarization again it is close to 90 degrees but it is still acute so positive wave will be recorded but very small amplitude. But in V5 it is along the direction of V5 pointing anteriorly and towards the left so a larger amplitude positive wave will be recorded. Then last part it's almost opposite to V1 so large amplitude negative wave will be recorded but in V5 it is almost 90 degree but obtuse. So a small negative wave will be recorded in V5. Similarly ventricular repolarization in V1 it will be recorded as small positive and in V5 little larger magnitude positive wave. So in this video we have seen what are the different ECG leads, why we need different ECG leads and how ECG will appear in different ECG leads. In another video we will see in a very simple manner what is the duration of different waves the intervals and segments of the and what is their clinical importance.