Comprehensive Guide to ECG Waveforms, Intervals, and Heart Rate Calculation

Hi everyone. I am Dr. Meena, Assistant  Professor Department of Physiology,   Chettinad Hospital and Research Institute.  I am here to discuss the waveforms,   intervals and segments of ECG. So the objectives of today's session is  

to understand the normal waveforms,  their duration and amplitude and ECG,   to understand the various intervals and segments  of ECG, how to calculate heart rate from ECG and   also to know the importance of the various  waveforms, intervals and segments.  

So before discussing the waveforms in detail, I  would like to tell some basic concepts related   to electrocardiogram. We all know that  electrocardiogram records the electrical   activity of the mass of atria and the ventricular  muscle cells. Essentially the electrocardiogram,  

records two basic events, which are termed  as depolarization and repolarization.   Depolarization refers to the spread of stimulus  through the heart muscle or in other words,   it is the electrical activation  or stimulation of the heart.  

When the stimulated heart muscle tries  to come back to its resting state,   we call it as repolarization. So these two terms  are derived from the word polarized state.   Normally, a resting cardiac cell is said to be  in a polarized state, which is meant that the  

exterior of the cell is positive and interior of  the cell is negative. This is what we call it as   a resting state or a polarized state.  When the heart muscle is stimulated,   what happens is, there is a shift in charge  across the exterior and interior of the cell.  

So whenever a heart is excited or activated,  outside of the cell becomes negative   and interior of the cell becomes positive,  which we call it as depolarization   and repolarization is the return of the stimulated  heart muscle towards resting state.  

You have a difference in the propagation of  depolarization and repolarization across the heart   cells. So the wave of depolarization it normally  proceeds from the endocardium to the epicardium   whereas the wave of repolarisation always proceeds  from the epicardium towards the endocardium.  

So this slide, it gives you the recording  of ECG from all the lead systems   and what you can appreciate from this ECG  is you will not have a uniform waveforms,   intervals or segments in all the lead  systems. So you can see here some of the wave  

is taller in one lead and smaller in the other  lead systems and some waves are positive in   some leads and negative in other leads. Why there occurs a variation in the morphology   of waveform in different lead systems? The answer  to this question is the morphology of the waveform  

that is whether it is positive or negative, it  depends upon the orientation of the lead.   So for this you have to understand three  basic principles of electrocardiogram.   We know that a lead has a positive  pole and a negative pole. If the mean  

depolarization wave is directed  towards a positive pole of any lead,   then the recording obtained in an ECG will be  a positive complex or an upward deflection.   On the other hand, if the wave of depolarization  spreads towards the negative pole of any lead,  

then the recording obtained would be a negative  complex or a downward deflection. The third   principle is that if the wave of depolarization  is directed at right angles to any lead,   the wave obtained will be biphasic in nature  that is, it has both the positive complex  

and a negative complex. So these are  the basic principles which you must keep   in mind while interpreting the ECG. So before discussing the waveforms in detail,   I would like you to know that the  first thing to look at an ECG is the  

calibration, whether the ECG is calibrated to  the standard way. So what I mean by standard   calibration is if you are going to apply a signal  of 1 mV, it should produce a deflection of 10 mm.   This is a standard calibration and modern ECG  machines, they are automatically calibrated.  

There are some situations where you can adjust  the calibration to either twice the normal or   half the normal. Say for example, in  individuals with ectopic pacemakers   or in patients with ventricular hypertrophy,  you may not have enough paper to record the  

ECG in all lead systems, because the morphology  of waves will be larger. So in such cases,   the standardization can be reduced to one half. Similarly, if you want to study the morphology of   Q waves in detail, then you can increase the  standardization to two times the normal.  

So now coming to the waveforms of ECG.  Normally the waves of ECG they are labeled as   P wave, QRS complex, T wave, and U wave.  All these waves reflect the activity   that is going in the atria and the ventricles.  So the P wave, it normally represents the  

depolarization of atrial myocardium. The QRS  complex it represents the depolarization of   ventricular myocardium and T wave it represents  the repolarization of ventricular myocardium.   So now one might get a question, what happens to  atrial repolarization? You do not have any waves  

to represent atrial repolarization. Why? This is  because you have atrial repolarization which is   happening in the right and left atria, but these  electrical activity are of low amplitude, which   cannot be picked up by the surface electrodes  and hence, you do not have any waveforms to  

represent the atrial repolarization. This is the conducting system of the heart.   The conduction pathways are the track,  which is followed by the impulses generated   from the SA node, till it reaches the ventricular  myocardium is depicted on the ECG. So your entire  

P wave signifies the activity of your atria. The QRS complex signifies the depolarization of   ventricles and the T wave represents  the ventricular repolarization.   So this is the lead to ECG where you have the  typical waveforms which can be appreciated.  

So now we will be discussing each waveform in  detail and the points which will be discussed here   are the contour of the wave, how it appears in  different lead systems, what is its amplitude,   duration and its significance. Now coming  to the P wave. the reason for P wave is  

caused due to atrial depolarization. Here the important thing which we must know is   the first half of the P wave is recorded  by the activity of your right atrium   and the second half of the P wave is recorded by  the activity of left atrium. So your right atrial  

and left atrial activity, summate to form the  P wave and the contour of P wave is smooth   and it can be either entirely positive or  entirely negative in all the lead systems.   The duration of P wave is normally less than  0.12 seconds and the amplitude of P wave is  

about 0.1 to 0.12 mV. The contour of P wave is smooth   and this P wave remains negative in the  lead aVR and it is biphasic in the lead V1.   This is the important point which we must  remember if you are looking at the morphology of P  

wave. So remember P wave will be negative in  the lead aVR and biphasic in the lead V1.   This is because the lead aVR is oriented  horizontally and it is also oriented to the   right of your shoulder. The electrical activity  that arises from your sinoatrial node spreads  

towards the left that is why you have a  negative P wave in the lead aVR, whereas in   lead II your P wave remains always positive. Next is about your QRS complex. The QRS complex   represents your ventricular depolarization and  this is the difficult complex to interpret an ECG,  

because you may not have all the  three waves in all the lead systems.   You may have one or two waves which  are absent in the QRS complex.   So the amplitude of this QRS complex should be  more than or equal to 5 mm in limb leads.  

As we all know, ventricular depolarization is  recorded as the QRS complex, this ventricular   depolarization happens in two phases. The first  phase happens with the septal depolarization   or the interseptal depolarization which  proceeds from left towards the right  

and the second phase is the depolarization  of the bulk of both the ventricles,   right as well as the left ventricle. But your  left ventricle is electrically predominant,   considered with the right ventricle. To understand the morphology of QRS complexes,  

first you must understand the nomenclature of  QRS complex or how the QRS complexes are named.   The first negative deflection of a  QRS complex will be labeled as q wave   and the first positive  deflection will be the R wave.  

The negative deflection that follows  the R wave will be labeled as S wave.   You can see here there are some letters which are  represented in capital and some letters which are   represented in small letters. This is because if  the amplitude of the recorded wave is small, then  

it will be denoted by a smaller alphabet, if the  amplitude is considered to be significantly more;   it will be denoted by a capital alphabet. You can see here some alphabets they are mentioned   as R prime. What you mean by that is, if you have  a second wave form or second positive deflection,  

then you use the letter prime to  differentiate between the two waves.   Now coming to the Q waves, they are produced  by the activation of interventricular septum,   which are produced by the septal branches of the  left bundle branch. The current to this septum  

flows from left to the right direction. Normally,  the Q waves are absent in many leads, and it is   present in the left sided leads V4 to V6. The appearance of QRS complexes or the morphology   as I said, it varies in all your lead systems.  Now we will see what is the morphology of QRS  

complex in the chest leads. So far, you must  remember two points. The first point is the two   phases of ventricular depolarization and second  one is the nomenclature of QRS complexes.   If you understand these two points, then you can  interpret the morphology of QRS complex in all the  

lead systems. First, we will see the morphology  of QRS complex in the chest leads. You have the   right sided chest leads, which are V1 and V2 and  the left sided chest leads which are V4 to V6.   The first phase of ventricular depolarization  happens with the interseptum that is, your septal  

depolarization and the direction of this wave are  from left towards the right. What happens with V1,   as the direction of depolarization is towards  the right, you have a positive deflection.   As per the nomenclature of QRS complex, the  first positive deflection will be labeled as r  

wave. As it is of lower amplitude, you are using  a smaller alphabet to denote the r wave.   What happens in the lead V6 which is oriented  to the left side? As the depolarization wave   moves away from it, you have a negative  deflection. According to the nomenclature,  

the initial deflection of or the first  negative deflection will be labeled as q.   So this r wave in V1 and q wave in V6, they  are called as septal r wave and septal q waves,   because they are produced by  the septal depolarization.  

The second phase of ventricular depolarization  is with the ventricular mass, as I told you,   both right and left ventricle they get stimulated  simultaneously. But your left ventricle is   electrically predominant over your right  ventricle and hence, the wave of depolarization  

will be directed towards the left. Now what  happens in the right sided chest lead V1?   As the second phase of depolarization wave moves  away from it, you have a negative deflection,   which will be labeled as S wave, because it  follows the r wave. In the left sided chest  

lead V6, you have the depolarization wave moving  towards it, you have a positive deflection, which   is labeled as R wave. This is the morphology of  QRS complex in the chest leads. So remember, it is   r S in V1 and in V6 it is q R. What you can interpret from this?  

Normally, here the r is denoted as  small r, because the amplitude is less.   But in V6 it is denoted by a capital R,  because the amplitude is more, which means that   as you move from V1 to V6 the amplitude of r wave  increases progressively, whereas the amplitude of  

S wave decreases progressively. This is  a point which you must keep in mind.   In leads V1 and V2 your R waves represent your  right ventricular activity and S waves represent   your left ventricular activity, whereas, in the  left sided chest leads V5 to V6 it is the vice  

versa. The R waves represent your left ventricular  activity and S waves represent your right   ventricular activity. As I told you, you have a  progressive increase in the amplitude of R wave   as you proceed from right towards the left. This is what we call it as R wave progression.  

At a certain point, say for  example, either in lead V3 or V4   there will be a point where the ratio  of R wave to S wave will be equal to 1   or in other words, the amplitude of R wave  will be equal to the amplitude of S wave.  

So this transition where R is equal to S  is called as a transition zone and this may   happen with either lead V3 or lead V4. There are some pathological conditions   where the transition may occur at an earlier  lead or the later leads. So if it occurs  

before V3 that is with either V1 or V2 we  call it as early transition zone and if the   transition zone happens with leads V5 or V6  we call it as delayed transition zone.   What is the importance of this R wave progression?  

It helps us to identify the  normal and abnormal ECG patterns.   You will not have a normal R wave progression with  ventricular hypertrophy. It can be either a right   ventricular hypertrophy or left ventricular  hypertrophy. In cases of myocardial ischemia,  

you have a loss of myocardium and here again you  have a loss of normal R wave progression.   Next is the appearance of QRS in limb leads. You  have limb leads which are labeled as lead 1, 2,   3 and augmented limb leads which are aVR,  aVL and aVF. Remember leads 2, 3 and aVF  

are oriented downwards, whereas the leads 1 and  aVL are oriented horizontally. I told you lead aVR   is also oriented horizontally and to the right. The important feature of aVR is   all the waveforms in aVR will be negative. So  you have a predominantly negative QRS complex in  

lead aVR. Then what about the morphology  of QRS complex in other limb leads? Here   the morphology depends upon the electrical  position of the heart or the mean QRS axis.   If the heart is going to be electrically  horizontal, or if the mean QRS axis is  

directed horizontally, then you have  the predominantly positive QRS complex   in the leads 1 and aVL because these two lead  systems are oriented horizontally. If the heart is   going to be electrically vertical, then you have a  predominantly positive QRS complex in the inferior  

leads, that is namely lead 2, lead 3 and aVF. Next coming to the T wave, it represents your   ventricular repolarization. Normal T wave has an  asymmetrical shape, and it peaks towards its end.   The duration of T wave is about 0.27  sec and the amplitude is about 0.3 mV.  

There are certain features which you must  remember while appreciating T wave in the ECG.   The first point is, the T wave always follows  the path of your mean QRS axis and if the T   wave is positive in any one lead it should remain  positive in the subsequent leads. Say for example,  

if the T wave is found to be positive  in lead V2 then it must be positive   in the subsequent leads from V3 to V6. The third point is the amplitude of T   wave should be at least less than 1/8 and not  more than 2/3 of the R wave amplitude.  

The next wave which can be recorded on ECG  is the U wave and this wave reflects the   slow repolarization of papillary muscles. The  duration of U wave is normally about 0.08 sec   and the amplitude is about 0.2  mV. This U wave is rarely seen in  

normal people and you can have a prominent U wave  in metabolic disturbances like hypokalemia.   Having discussed about the various waveforms, now  we are moving on to the intervals and segments.   When you look at an ECG, you will not only  have the waveforms, but you also have a flat  

line which is called as the isoelectric line. So  what do these flat lines or the isoelectric lines   represent? These lines represent that there  is no electrical activation of the heart.   If you are not recording any electrical  activation, you get a flat line.  

So the intervals include your waveforms as well  as the flat lines or the isoelectric lines,   whereas the segments include only the isoelectric  lines in the ECG tracing and it does not include   the waveform. So the important intervals  will be your PR interval, QT interval,  

and RR interval and the important segments  will be your ST segment and TP segment.   The first interval will be your PR interval. This  is measured from the beginning of P wave to the   beginning of QRS complex. This PR interval  represents the time taken by the impulse to  

travel from the atria to the ventricles and  it also includes the AV nodal delay, which is   normally about 0.01 second. The normal duration  of PR interval is about 0.12 to 0.20 seconds.   If it gets prolonged or if it is more  than 0.20 second, it tells us that  

there is some conditional abnormality. The next important interval is your   QT interval, which is measured from the start of  QRS complex to the end of T wave, which means that   it includes both ventricular depolarization and  ventricular repolarization. In other words, it  

indicates the total systolic time of ventricles.  The normal duration of QT interval is 0.4 seconds   and the important point about this interval  is, it depends upon the heart rate.   If heart rate increases, your QT interval  decreases and vice versa. The second important  

point is the QT interval has to be measured  in the lead which shows the longest interval   because of these two reasons, you have to  have a formula which corrects the QT interval   and we call it as corrected QT interval which can  be calculated by two mathematical formulas.  

One is the Bazett formula or the  square root method and the other one   is Hodges method. Bazett formula tells us that  the corrected QT interval which can be calculated   by dividing the measured QT interval with the  square root of RR interval. Next one is Hodges  

method, where you have to know the heart rate and  beats per minute and you should also calculate the   QT interval and substitute in this formula. So this QT interval and the normal corrected   QT interval is less than 0.43 seconds for  male and less than 0.44 seconds for female.  

If QT interval is prolonged it  signifies either ischemia, hypocalcemia   or ventricular conduction defects. Next is about the segment. We have discussed   segments are only the isoelectric lines and it  does not include any waveforms. This ST segment  

is usually measured from the end of QRS complex  to the beginning of T wave. So this ST segment   should be in level with the subsequent TP  segment. So TP segment is calculated from   the end of T wave to the beginning of P wave. The important thing about ST segment is a normal  

deviation of less than 1 mm is acceptable. If the  deviation of the ST segment is more than 1 mm,   it is considered to be pathological. Coming to the next important aspect which   is the calculation of heart rate.  Heart rate calculation can be done  

by using two methods. One is the box counting  method and other one is QRS counting method.   When we say box counting method, you can count  either the number of large boxes or the number   of small boxes. For this you have to know  that one large square which is equal to 5 mm  

is equal to 0.2 seconds and one small square  which is of 1 mm is equal to 0.04 seconds.   Normally you have to take the ECG recording, and  you have to take two consecutive QRS complexes   and count the number of either the  large boxes or the small boxes.  

You have two formulas. One is either you can  divide 300 by the number of large boxes between   two QRS complexes or if you are going to count the  number of small boxes, then the formula will be   1500 by number of small boxes between two QRS  complexes. You can look at this diagram. You have  

two QRS complexes, two consecutive QRS complexes  and you have to count the large squares.   The number of large squares will be 4. So  here you are having 1, 2, 3, 4 four large   squares. If you are going to substitute in  this formula, it will be 300 divided by 4,  

which is normally equal to 75/min. That will  be the heart rate. Similarly, you can count the   number of small boxes between the two consecutive  QRS complexes and substitute in the denominator   and then you can get the heart rate. So these are the two methods which are employed by  

counting the large boxes and small boxes. And you  also have a easy method to estimate the heart rate   for which you have to memorize these numbers  300, 150, 100, 75, 60 and 50 which means that   if you have one large square between two QRS  complex, the heart rate will be 300. If the number  

of large squares is 2 heart rate is 150. If the number of large squares is 3,   heart rate is 100. Number of large squares is  4, heart rate is 75. Number of large squares is   5, heart rate is 60 and vice versa. And the next method is the QRS counting method  

for which you have to obtain a ECG recording  for either 6 seconds or 10 seconds.   So remember 1 large square  is equal to 0.2 seconds.   So if you want to obtain a recording for 1  second you have to count 5 large squares.  

And if you want to have a recording for 6  seconds, then it must have 30 large squares.   And for 10 seconds it must have 50 large squares.  So if you are going to have a recording for 6   seconds, then you have to multiply it with 10.  On the other hand, if you obtain a recording for  

10 seconds, then you have to multiply it with  6 to get the heart rate. So in this picture,   this is a recording for 10 seconds. And now here  you can calculate the number of QRS complexes.   So here you have 3, 6, 9, 11. So you have  11 QRS complexes in a 10 second recording.  

So you have to multiply it with 6 to get  the heart rate which is equal to 66/min.   So this is the QRS counting method. So EP Caliper is an online software which   can be used to calculate the heart rate  and QT interval. So this is the EP Caliper  

app. So here you have the open option where you  can open the ECG images which are in the folders   and you have a caliper button where you have 3  calipers, one is the time caliper, an amplitude   caliper and the angle caliper. So if you want to calculate the intervals  

and heart rate you have to use the time  caliper. So I will demonstrate how to   calculate the heart rate. So I am using this  time caliper. And then you have a zoom button   with which you can magnify the image. So as we all  know, the heart rate has to be calculated between  

two QRS complexes. That is the peak of R wave. So adjust it accordingly and now we have   to use the calibration setup to set the  calibration. So the standard calibration   will be for 1000 milliseconds. Now press  okay and you have to calculate the rate.  

So here again you have to adjust  it. So between two QRS complexes   and you get the heart rate. Clear? So for measuring the QT interval,   you have to give inputs on how many intervals  we are going to measure for the QT interval.  

So I am going to measure for 3 intervals   and then adjust it for 3 intervals that  is 3 QRS complexes and then press measure.   So you have the calculated QTc  which is appearing on the screen.  

So the normal QT is about 0.2 seconds and the  corrected QT interval is about 0.17 seconds.   And you can also calculate the mean rate.   Again you have to feed the number of  intervals you are going to measure  

and you have the mean rate which is appearing  here which is about 80 beats per minute.   So nowadays you are having online application  with which you can find the heart rate,   you can also calculate the QTc etc. You have the  P wave, which is best seen in leads II and V1 and  

in V1 the morphology of P wave is biphasic.  And in lead aVR, all the waveforms will be   negative. So the P wave the height and width  of it should not be more than 2.5 mm.   Next is PR interval which denotes the time taken  by the impulse to travel to the ventricles.  

Normally it is about 0.12 to 0.20 seconds. And Q  wave is caused by the septal depolarization and   the depth of Q wave should be 25% of the amplitude  of R wave. And the width should not be more than   1 mm. And next is the R and S waves which are  produced by the ventricular depolarization.  

So the R waves, it progressively increases  in amplitude from V1 to V6. Whereas S wave,   it decreases in amplitude from V1 to V6.  And QRS complex, it should be at least 10 mm   in chest leads and 5 mm in the limb leads. And ST segment, it is an isoelectric line  

and it should be at the same level as the  subsequent TP segment or the PR segment and a   normal deviation of less than 1 mm is acceptable.  And T wave, it should be at least 10% of the R   wave in the same lead. And QT interval, which  is measured in the precordial leads and it also  

depends upon the heart rate and hence you have to  correct it by using the mathematical formulas.   So the corrected QT interval must  be less than 0.42 seconds for male   and less than 0.44 seconds for female. So the take home messages will be the ECG is  

a recording of only the electrical activity of  the heart and not the mechanical activity. That   is the pumping or the contractile activity of  the heart cannot be recorded with ECG. And the   second point is the ECG does not directly depict  the abnormalities in cardiac structure. So if a  

patient has mitral stenosis or ventricular septal  defect it cannot be made out with the ECG.   Instead it gives only the electrical wave forms  that are altered by this structural deformity.   And third one is ECG does not record all the  electrical activity of the heart. This is because  

your surface electrodes can pick up only the wave  forms which have a significant amplitude.   And these are my references. And I specially thank Elsevier   Publications for granting me  permission to use the images of  

Goldberger's Clinical Electrocardiography  and I also thank my institute for giving   me this opportunity. Thank you and  I wish you all a happy learning.