 Welcome to Physiology Open. In this video we will try to understand the concepts of ECG. First, let us see how the voltage will be recorded across a tissue if it is getting depolarized. So, here is a cell which right now is in resting state that is resting membrane potential with inside negative and outside positive. So, if we connect a voltmeter such that its one electrode is inside and one electrode outside, we will get the resting membrane potential say minus 70 millivolt. Since it is recording potential difference between its two electrodes but if we place both electrodes outside we will get zero volts as potential. See both its electrodes will have same potential and potential difference between the two electrodes will be zero. So, at this resting state the potential will be zero. Now, if the cell starts depolarizing potential difference will start developing between its electrodes and voltmeter will start recording some potential difference. As the potential spreads it will become more positive till half of the cell is depolarized. When the potential changes spreads further again algebraic sum will start decreasing and when it fully becomes depolarized again both the electrodes will be at same potential and so again it will become at zero potential. So, see how the spread of depolarization leads to rise in potential difference recorded and then fall coming back to zero. So, this is called a wave energy. Now we are here talking about a cell but in cardiac muscle the all the cells are connected together and there we will talk about in terms of entire atria and entire ventricle. So, just replace this word cell with the entire atria cell. So, here we are seeing the spread of depolarization in atria and a positive wave is being recorded. Now, what if this is a bigger tissue like the ventricles the magnitude of wave which will be recorded will be bigger. Now, see in this diagram we have placed the electrodes along the direction of spread of depolarization where the direction we say is from negative to positive. So, when both the direction of the lead and the direction of spread of depolarization are same we get a positive wave. Now, suppose I invert the electrodes that means where it was positive I make it negative and where it was negative I make it positive. We will record same magnitude of voltage but its direction will be opposite. Similarly, if we put the electrodes 90 degrees to the spread of depolarization what will be the voltage recorded will it be same as before? No, because we have to apply our knowledge of vectors here. Whenever electrodes are placed 90 degree to the direction of spread of depolarization the voltage will be recorded 0. So, you have to apply your knowledge of vector here so V cos 90 degrees. V is the voltage in the direction of depolarization cos 90 degrees so V into 0 is equal to 0. So, when the lead is 90 degree to the spread of depolarization it will record a 0. In summary we can say that the recorded voltage not only depends on the direction of the spread of depolarization the tissue marks but also where we put the electrodes. Now, let us bring on this concept to heart. So, first let us see the direction of depolarization in. The impulse starts from SA node, moves to AB node, depolarizing atria in between because the signals will spread via cap junctions in atria. But if we see the average direction of depolarization when atria is getting depolarized which will be like this. The direction of vector being taken from negative to positive. So, slowly whole atria gets depolarized. After a pause the impulse travels via bundle of risk and first there is a depolarization of ventricular septum from left to right. Then the impulse moves downwards to the apex in between there is depolarization of the ventricles again via cap junctions and the average direction of depolarization again is like this can taken from negative to positive. The last part of heart getting depolarized is posterobasal portion of left ventricle. Since it is positive rest of the area is negative finally the direction of depolarization will be like this. So, these four arrows show the direction of depolarization of heart in a single spread of electrical activity. This is known as a vector cardiogram and ECG is recording of this spread of electrical activity. Now suppose if we keep two electrodes such that the direction of lead is like this how will this spread of electrical activity of heart be recorded in this tree? Remember I will be initial recording see if the lead is in the direction of the depolarization it will be recorded as positive. If it is opposite it is recorded as negative. So, one by one we will see all these arrows of vector cardiogram how will they be recorded in this tree? See initial atrial depolarization is in the direction of the lead. So, it will be recorded as a positive way the peak represents half of atrial depolarization when it comes back to again baseline it is the atria is fully depolarized. Next septal depolarization you see that it is almost opposite to the direction of lead it will be recorded as a negative way. But very small because the potential difference developed is very small right now. Then again ventricular depolarization is in the direction of the lead. So, it will be recorded as a positive way but with larger magnitude because more ventricular depolarization has occurred and ventricular tissue is larger than atria tissue causing more potential difference. Then finally, posterior lateral part of ventricular depolarization is recorded as negative way because its direction with respect to lead is obtuse. Had it been 90 degree no voltage will be recorded. The small obtuse means small voltage will be recorded but on negative side. So, what are we just shown? We have actually drawn lead to ECG. The direction of lead shown here is lead to and we have just shown waves of ECG, P wave and QRS complex which represent atrial and ventricular depolarization respectively. Now how about repolarization? During atrial repolarization the part of the atria that gets depolarized first gets repolarized first. So, the direction of change in potential becomes opposite but this atrial repolarization is not recorded separately. It is merged with QRS complex because it is happening at the same time. What about ventricular repolarization? In ventricles the part of ventricle that gets depolarized first gets repolarized last. It is important to remember this because it determines the direction in which the voltage will be recorded. So, direction of vector remains this only like it was in ventricular depolarization. So, it is same as the direction of lead to. So, ventricular repolarization is recorded as a positive wave in lead to. So, in this video we saw the concepts of ECG and how ECG will appear in lead to. In another video we will see different ECG leads, why we need so many leads, how spread of depolarization is recorded in different leads and what are the different waves, intervals and their duration. If you like the video do not forget to subscribe to the channel with geology open.