 Hi everyone, welcome to physiology open. Electrical axis of the heart or cardiac axis is average direction of depolarization of heart. We have seen in some other videos how direction of depolarization changes during the spread of the electrical impulse in heart. But when we take an overall mean or average of all these directions of depolarization, it is known as electrical axis of heart or cardiac axis. So, suppose the average direction of depolarization of heart is like this and these are the leads 1, 2, 3. The potential recorded in the different leads will be different depending on the vector projection of the direction of depolarization on these leads. So, it will be something like this in the lead 1 and 2. Okay, now let us take one more lead aviar. It will be something like this in lead aviar. See the amplitude is a smaller in lead 1 and aviar for little larger in lead 2, because the direction of depolarization is similar to the direction of the lead 2. Now, when we look at ECG, we do not know how current is flowing in heart or what is the average direction of depolarization. But what we do is that we use the voltages in these leads to calculate mean electrical axis in heart. So, we are kind of inverting the process. So, how do we do that? In this diagram, we have the ECGs of lead 1 and lead aviar and we have to find out the electrical axis of heart. So, for this, we will calculate voltage of QRS complex in these leads by subtracting the amplitude of R wave with most negative wave which can be either Q wave or S wave. Then we have to mark them on a paper along with the lead direction and determine the electrical axis. So, in lead 1, amplitude of QRS complexes, we have to count this R wave and then subtract the Q wave here. There is no negative wave. So, the amplitude of QRS complex will be 1.1 millivolt. Similarly, in lead aviar, it is 0.9 millivolt. Now, on a paper, draw the directions of the lead 1 and aviar and then mark with equal spaced marking numbers from 0.1, 0.2 till maybe 1.5, both in the directions of lead 1 and lead aviar. Now, mark the voltages of QRS complex which you have calculated on the respective lead direction. So, in lead 1, voltage is 1.1 millivolt. So, mark here and in lead aviar voltage is 0.9 millivolt. So, mark here. Now, drop a perpendicular from these points like this. These perpendiculars will meet at some point. Now, from the center, draw an arrow like this to meet the point of the intersection. This is the average direction of depolarization of the heart or electrical axis of the heart. You take the angle of this line from the horizontal, it will give you the electrical axis in degrees. Normal range of the cardiac axis is minus 30 degrees to plus 110 degrees. So, if we draw a circle and these are the directions of the lead 1 and aviar, from here to here minus 30 degrees to plus 110 degrees is normal axis. If the axis is more negative than minus 30 degrees, it is left axis deviation and if it is more positive than plus 110 degrees, it means right axis deviation. Okay, after all this, let us see a shortcut method of determining the cardiac axis so that you also just look at the leads and tell whether the axis is normal or not. For understanding, we will just draw a circle and divide into four parts like this. You know, this is the direction of lead 1 and this is the direction of lead avf. Let me just draw one table here. Okay, now see, if lead 1 and lead avf, both are positive. That means the prominent part is the positive r wave. That means it will lie somewhere in this quadrant. That is, it is normal axis. If lead 1 is negative and lead avf is positive, it will lie in this quadrant of the circle. So, it is right axis deviation. If lead 1 is negative and avf is also negative. So, it will come in this quadrant. It is known as extreme axis or not best axis. And finally, if lead 1 is positive and lead avf is negative, it will lie in this quadrant. Now you know, axis till minus 30 degrees is normal. So, this condition can be either when the axis is normal or there is left axis deviation. To confirm this, we just have to look at lead 2. If lead 2 is positive, then it is normal axis. But if lead 2 is negative, then it is left axis deviation. Just to summarize once more, if both lead 1 and avf are positive, it is normal axis. If lead 1 is negative, avf is positive, it is right axis deviation. Both negative, it is extreme axis. Lead 1 positive and avf negative. Then you have to look at lead 2. If lead 2 is also positive QRS complex, it is normal axis. If negative, it is left axis deviation.