 Hello everyone, welcome to Physiology Open. In this video we will see what are ECG waves, intervals and segments, how to remember duration of various intervals and segments and even ECG waves and how to read an ECG. ECG that is the electrocardiography is recording a spread of electrical impulse in heart and how this electrical impulse is recorded as various waves, intervals, segments in ECG. So, to understand these let's see a lead to ECG. So, here this picture is showing a lead to ECG. So, this is the thermal paper over which ECG is recorded. This portion is showing the calibration that is the voltage recorded as some distance on a paper. The standard calibration is that 1 millivolt is recorded as 1 centimeter on a thermal paper. This portion is the recording of the spread of the electrical impulse in the heart. So, here it is showing 2 electrical impulse recording. So, let's focus on recording of one electrical activity here. The first wave which is seen here is P wave. It occurs due to atrial depolarization. After atrial depolarization there is some time during which atrial depolarized and ventricular is still at resting membrane potential because there is a pause at AV node. So, no potential is recorded during this time. Then we get another wave or we can say a wave complex known as QRS complex. The QRS complex represents ventricular depolarization. Then there is some time in between when again no voltage is recorded that is the time in between the ventricular depolarization and ventricular repolarization. After that we get another wave that is the T wave which is occurring due to ventricular repolarization. Now, you may ask where is atrial repolarization recorded? Atrial repolarization occurs at the same time as that of ventricular depolarization so that the voltage change there is merged with QRS complex. Now, this is the convention which we follow while naming the waves that is starting with P wave till T wave especially you should focus on QRS complex. QRS complex represents ventricular depolarization right? Now, how is it named? The first negative wave that is recorded during ventricular depolarization is known as a Q wave. First positive wave is known as R wave and the negative wave which occurs after the first positive wave is S wave. But why we need to know all this? Because there may be some leads in which you don't see first negative Q wave directly there may be a positive wave. Then that wave we don't call as Q wave we call it as R wave because by convention first positive wave is known as R wave but whole complex we call as QRS complex only. Second as we'll see later in the video that we have something known as PR segments and PR intervals not PQ segment and PQ intervals but even though they are known as PR segment and PR interval we mark them till the beginning of QRS complex. Okay, so we have dealt with waves of ECG here now let us see various intervals and segments. First thing to remember is segments do not have any waves between them but intervals include waves. So what are these segments and intervals? We will talk about three segments and five intervals. The segments are PR segment, ST segment, TP segment. So PR segment remember segment do not include waves so PR segment is from the end of P wave that is this point to the beginning of QRS complex that is this point. So this much is PR segment. So see here even though we are saying PR segment we are marking till the beginning of QRS complex. So PR segment represents AV nodal pause. During this time atria is depolarized but ventricles are a trusting membrane potential. ST segment is from the end of QRS complex that is this point to the beginning of T wave so till here. So this is ST segment. See again that there is no wave in between. During ST segment ventricles are depolarized and no current is flowing through the heart. Now TP segment is from end of P wave of one record to P wave of record of another impulse. So this portion is TP segment. Now let us see the intervals. First is PR interval. PR interval is from beginning of P wave to beginning of QRS complex. As already told the intervals includes the waves. So PR interval is from beginning of P wave it includes the P wave to the beginning of the QRS complex. Next is ST interval. ST interval is from end of QRS complex to end of T wave. So it is including the T wave. This is ST interval. While QT interval is from beginning of QRS complex to end of T wave. So that means from here beginning of QRS complex to end of T wave. Now sometimes it becomes very confusing to remember these intervals from where they begin and where they end. So I remember it by one stupid mnemonic. I don't know it may help you also. As already told intervals include waves while segments do not include waves. So which waves do these intervals include? See PR interval is this side of ECG. So it includes this side of wave that is P wave. Now ST interval is this side of ECG. So include this side wave that is T wave. While QT interval includes both the waves. See it includes from the beginning of the QRS complex to end of T wave. That means it includes both QRS complex and T wave. Now there are two other intervals which are of significance for us. One is RR interval taken from the peak of one R wave to the peak of another R wave. So this is RR interval. While in certain abnormalities we also need PP interval. That is from the peak of P wave to peak of another P wave. So when we will talk about how to read ECG we will tell that where this PP interval is needed. Now let us see what are the normal durations of various wave segments and intervals. For that we will just see one concept and it will help you remember the various durations. See as signal starts from SA node it takes 0.03 seconds to reach AV node and further 0.06 seconds to depolarize the atria. So total time for atrial depolarization we are just adding this 0.06 and 0.03 it is 0.09 seconds. Since P wave is representing atrial depolarization so this P wave duration is 0.09 seconds. Now impulse stops at AV node for 0.1 second that is the AV nodal delay and then only the ventricular depolarization starts. So if we add this the traveling time 0.03 seconds and this 0.1 second it comes to 0.13 seconds. That means from the beginning of the atrial depolarization to the beginning of the ventricular depolarization it is 0.13 seconds. This is what? This is nothing but PR interval. Then ventricular depolarization it takes around the 0.03 seconds to occur from year to year and then further spread by Purkinje fibers also takes 0.03 seconds. So again it is 0.06 seconds. So this QRS complex see it represents the ventricular depolarization and we are seeing that ventricular depolarization is occurring in 0.06 seconds. Simple to remember right? You have to remember only this 0.03 everywhere and AV nodal delay has 0.1 seconds and you can easily tell how much is the duration of these waves and intervals. Now to calculate PR segment you just minus PR interval from P wave and you will get the duration of the PR segment that is 0.04 second. So basically you need to remember only two more values that is the ST segment and the duration of the T wave. So ST segment duration is 0.08 to 0.12 second and duration of the T wave is 0.12 to 0.2 seconds. So when you know the duration of the ST segment and T wave then you can easily calculate what will be the duration of the ST interval right? That is from the end of the QRS complex till the end of the T wave is your ST interval. Just add the two durations and you will get the duration of the ST interval. Similarly you can get also the duration of the QT interval. Now let's combine all this information and see how to read an ECG. Remember that when ECG is recorded on a thermal ECG paper at a standard speed of 25 millimetre per second then each small square of ECG horizontally represents time of 0.04 seconds. Similarly the calibration which we discussed in the beginning of the video about the voltage each small square vertically represents 0.1 millivolt. So this 10 squares represent 1 millivolt. So one small square will represent 0.1 millivolt. Why it is important to know this? Because once you know the calibration and a speed and how much each square represents horizontally and vertically to determine the duration of each segment interval wave you can simply count the number of squares. So suppose here P wave is having around one and a half squares so it will represent 0.06 seconds and for voltage you just count the number of squares vertically. So suppose here again it is around one and a half square so it will be 0.15 millivolt. So for reading ECG what we need to look at. So there are multiple things we need to look at. First thing is whether there is presence of sinus rhythm. That is whether signal generated at SNode is reaching ventricles or not. So that we see whether every P wave is followed by QRS complex or not. So for this you take any lead ECG and you just see whether P wave is followed by QRS complex. Secondly look at the waves. What is their shape, duration and voltages. Now voltages of the waves will vary in different leads. This we have discussed in some other video. Please check that out. The link for it is given in the description section. Why voltages of waves differ in different leads. But the duration will be same in all leads. So for checking the voltages you see how it is appearing in different leads. Now before going further we will just see how to calculate the voltage of QRS complex. See QRS complex is having a negative wave then positive then negative. So which voltage to take out of these. What we do is we take the voltage of the most positive wave. So from the baseline in this case the most positive wave is R wave. So we will take from baseline and count the number of squares. So how much it is say 5, 10, 11, 12. 12 squares. So it will be 1.2 millivolt and the most negative wave. So out of this S wave and Q wave actually Q wave is more negative. So we will take it as minus 0.1 millivolt and we take a algebraic sum of these. So the voltage of the QRS complex will be 1.1 millivolt. Okay then we have to look at the segments and intervals. What is their duration whether it is normal or not. Now again duration will be the same in all leads right. So just count the number of the squares. So suppose I have to calculate the duration of the PR segment. Then what I will do I will just see the number of the squares. So in this case number of a square is 1. So duration of the PR segment will be 0.04 seconds. Because one small square represents 0.04 seconds. Next we have to look at the SG segment and something known as the J point. J point is the point where QRS complex ends. In normal condition if we extend our TP segment. So here comes the significance of the TP segment. So if we extend our TP segment back it will coincide with the SG segment. Both will be in the same line. But sometimes this end of the QRS complex and this SG segment will be either above or below this line. This is known as SG segment elevation or depression. So that also we have to check whether there is any SG segment elevation or depression or not. Then next we have to look for the RR interval. Now this RR interval gives us information about the heart rate. And what we do is we count the number of the small squares between the peak of 2R waves. And to calculate heart rate we use one formula 1500 divided by the RR interval. Where RR interval is represented in the form of number of the small squares in between the 2 RR peaks. So this will give you the heart rate. Now this RR interval is used when the sinus rhythm that is a normal rhythm is present that is P wave is followed by the QRS complex. But if sinus rhythm is not present that means all P waves are not followed by QRS complex. So then in that case we use PP interval. So PP interval will give you the atrial rate. Sinus rhythm is not present and RR interval in that case will give you the ventricular rate. Now finally you have to look at the cardiac axis. Now cardiac axis represents the average direction of depolarization of the heart in one impulse. Now how much is normal cardiac axis and how to calculate it plus what is the shortcut of knowing the cardiac axis that we will reveal in another video. Well thank you for watching the video and do not forget to like the video and subscribe to the channel Physiology Open.