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

[Music] [Music] hi everyone i'm dr meena assistant

professor department of physiology chettinad hospital and research institute

and 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 and we all know that electrocardiogram

it records the electrical activity of the mass of atria and the ventricular muscle cells

essentially the electrocardiogram it 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

and 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 to its resting

state and 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 repolarization always proceeds from the epicardium

towards the endocardium so this light 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 you can see here some of the way or 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 so we know that a lead it has a positive

pole and a negative if the mean depolarization wave is directed towards the 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 uptime would be a

negative complex or a downward deflection and 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 bay so what do you mean by standard

calibration is if you are going to apply a signal of 1 milli volt

it should produce a deflection of 10 millimeter this is a standard calibration and

modern ecg missions they are automatically calibrated and 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 the 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 2 times the normal so now coming to the waveforms of ecg so normally the waves of ecg they are

labeled as p wave qrs complex t wave and u wave so all these waves

they 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 so this is the conducting system of the

heart so the conduction pathways or the tract

which is followed by the impulses generated from the sa node till it reaches the ventricular

myocardium is depicted on the ecg

so your p wave the entire p wave it signifies the activity of your atria the cure is complex it signifies the

depolarization of ventricles and the t wave it 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 so now coming to the p wave so the reason for p wave is it is caused

due to atrial depolarization and 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 so the duration of p wave is normally

less than 0.12 seconds and the amplitude of p wave is about 0.1 to point 12 milli volt

so the corner of p wave it is smooth and this p wave remains negative in the lead abr

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 and the electrical activity that arises from your cyanoatrial node

it spreads towards the left and that is why you have a negative p

wave in the lead avr whereas in lead 2 your p wave remains always positive

so next is about your qrs complex so the qrs complex it represents your ventricular depolarization and this is

the difficult complex to interpret an acg 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 five millimeter in

limb leads so as we all know ventricular

depolarization is recorded as the qrs complex so this ventricular depolarization happens in two phases

so the first phase happens with the septal depolarization or

the interceptal 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 so to understand the morphology of qrs

complexes first you must understand the nomenclature of qrs complex or how the qrs complexes are named

so the first negative deflection of a qrs complex will be labeled as q wave and the first positive deflection will

be the r wave and the negative deflection that follows the r wave will be labeled as yes wave

so 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 and if the amplitude is

considered to be significantly more it will be denoted by a capital alphabet and you can see here

some alphabets they are mentioned as r prime and what do you mean by that is if you

have a second waveform or second positive deflection then you use the letter prime to differentiate between

the two waves so now coming to the q waves so q waves are produced by the activation of inter ventricular septum

which are produced by the septal branches of the left bundle branch and 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 b4 to b6 so the appearance of qrs complexes or

the morphology as i said it varies in all your lead systems so now we will see what is a morphology of qrs complex in

the chess leads so for this you must remember two points the first point is the two phases of ventricular

depolarization and second one is a nomenclature of curious complexes if you understand these two points

then you can interpret the morphology of cures complex in all the late systems so first we will see the morphology of

cures complex in the chest leads so you have the right sided chest leads which are v 1 and v 2 and the left other test

leads which are v4 to v6 so the first phase of ventricular depolarization happens with the

inter septum that is your septal depolarization and the direction of this wave is from

lift towards the right so what happens with v1 as the direction of depolarization is

towards the right you have a positive deflection and 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 so what happens in the lead b6 which is

oriented to the left side as a depolarization wave moves away from it you have a negative deflection

and according to the nomenclature the initial deflection of or the first negative deflection will be labeled as

q so this r wave in v 1 and q wave and v 6 they are called as septal r wave and

septal q waves because they are produced by the septal depolarization so the second phase of ventricular

depolarization is with the ventricular mass and 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 so now what happens in the right side of

just lead b1 as a second phase of depolarization wave moves away from it you have a negative

deflection which will be labeled as yes wave because it follows the r wave

and in the left-sided test lead v6 as you have the depolarization wave moving towards it you have a positive

deflection which is labeled as r wave so this is the morphology of qrs complex in the chess leads

so remember it is rs and v1 and in v6 it is qr so what you can interpret from this

normally here the r is denoted as small r because amplitude is less but in b6 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 so 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 b5

to v6 it is the vice versa the r waves represent your left ventricular activity and yes vapes represent your right

ventricular activity and 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 our wave progression

and at a certain point say for example either in lead v3 or b4 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 b4

and there are some pathological conditions where the transition may occur at a earlier lead or the later

leads so if it occurs before v3 that is with either v1 or v2 we call it as yearly transition zone

and if the transition zone happens with leads b5 or b6 we call it as delayed transition zone

so what is the importance of this rv progression so this it helps us to identify the

normal and abnormal ecg patterns so you will not have a normal rv progression with ventricular hypertrophy

it can be either a right ventricular hypertrophy or left ventricular hypertrophy as well as in cases of

myocardial ischemia where you have a loss of myocardium and here again you have a

loss of normal r wave progression so next is the appearance of qrs in limb leads

so you have limb leads which are labeled as lead 1 2 3 and augmented limb leads which are avr

avl and avf so remember leads 2 3 and avf they are oriented

downwards whereas the leads 1 and avl they are oriented horizontally and as i told you

lead avr it 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 so then what about the morphology of

cures complex in other limb leads so here the morphology it depends upon the

electrical position of the heart or the mean qrs axis so if the heart is going to be

electrically horizontal or if the mean qrs axis is directed horizontally then you have a predominantly positive

qrs complex in the leads 1 and avl because these two lead systems are oriented horizontally

and 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 to lead 3 and avf so next coming to the t wave

so t wave it represents your ventricular repolarization and the normal t wave it has an

asymmetrical shape and it peaks towards its end and the duration of t wave is about 0.27

second and the amplitude is about 0.3 milli volt and 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 main curs 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 formed

to be positive in lead b2 then it must be positive in the subsequent

leads from b3 to b6 and the third point is the amplitude of t wave should be at least

1 by 8 less than and not more than 2 3 of the r wave amplitude and the next wave which can be

recorded on ecg is the u wave and this wave it reflects the slow repolarization of papillary muscles and the duration of

you wave is normally about 0.08 second and the amplitude is about 0.2 millivolt and 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 so when you look at an ecg

you will not only have the waveforms but you also have a flat line which are called as the isoelectric line

so what do these flat lines or the isoelectric lines represent so 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 it includes your waveforms as well as the flat lines or the isoelectric lines whereas the

segments it includes 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 so the first interval will be your pr interval so this is measured from the beginning

of p wave to the beginning of qrs complex and this pr interval it represents the time taken by the impulse

to travel from the atria to the ventricles and it also includes the a b nodal delay which is normally about 0.01

second and the normal duration of pr interval is about 0.12 to point 20 seconds and if

it gets prolonged or if it is more than point 20 second it tells us that there is some conditional abnormality and 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 or in other words it indicates the total

systolic time of ventricles so the normal duration of qt interval is 0.4 seconds

and the important point about this interval is the qt interval it depends upon the

heart rate if heart rate increases your qt interval decreases and vice versa and the second

important point is the qt interval has to be measured in the lead which shows the longest interval

and because of these two reasons you have to have a formula which corrects the qt interval

and this we call it as corrected qt interval which can be calculated by two mathematical formulas

one is the basset formula or the square root method and the other one is hodz method

so the basset formula is tells us that the corrected qt interval can be calculated by dividing the

measured qt interval with the square root of rr interval and next one is with the help of hearts

method where you have to know the heart rate in beats per minute and you should also calculate the qt interval and

substitute in this formula so this qt interval if it is going to be prolonged so 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 so next is about the segment so as we have discussed segments of only the

isoelectric lines and it does not include any waveforms so 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 and

the important thing about st segment is a normal deviation of less than one millimeter is acceptable

if the deviation of st segment is more than one millimeter it is considered to be pathological so the coming to the

next important aspect which is the calculation of heart rate so heart rate calculation can be done by

using two methods one is the box counting method another one is qrs counting method

so when we say box counting method you can count either the number of large boxes or the number of small boxes

so for this you have to know that one large square which is equal to five millimeter

is equal to point two seconds and one small square which is of one millimeter

is equal to point not four seconds so 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 so 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 cures

complexes so you can look at this diagram you have two qrs complexes two consecutive qrs

complexes and you have you have to count the large squares so the number of large squares

will be four so here you are having one 2 3 4 4 large squares so if you are going to substitute in

this formula it will be 300 divided by 4 which is normally equal to 75 per minute that will be the heart rate

similarly you can count the number of small boxes between the two consecutive cures 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 cures 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 one large square is equal to 0.2 seconds so if you want to obtain a recording for

one second you have to count five 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 per minute so this is the qrs counting method so eb 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 three calipers one is the time caliper 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 cures complexes that is the peak of our babe

so adjust it accordingly and now we have to use the calibration setup to set the calibration

so the standard calibration will be 4 000 milliseconds now press ok

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 three

intervals and then adjust it for three intervals that is three 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 2 and v 1 and in v 1 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 millimeter next is pr interval which denotes the time taken by the impulse to travel to

the ventricles normally it is about 0.12 to point 20 seconds and q wave is caused by the septal depolarization and the

depth of q wave should be 25 percent of the amplitude of our wave and the width should not be more than

one millimeter 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 b6 whereas yes vape it

decreases in amplitude from v1 to b6 and qrs complex it should be at least 10 millimeter in chess leads and 5

millimeter 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 one millimeter is acceptable

and t wave it should be at least 10 percent of the r wave in the same lead and qt interval which is measured in the

precardial 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 point 42 seconds for male and less than point 44 seconds for

female so the taco 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 waveforms 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 waveforms which have a significant

amplitude and these are my references and i especially thank elsevier

publications for granting me permission to use the images of goldberg's clinical electro choreography and i also thank my

institute for giving me this opportunity thank you and i wish you all a happy learning