Introduction to ECGs, by Dr. Douglas Mah.
Hi.
My name's Doug Mah.
I'm a Heart Rhythm Doctor here at Boston Children's Hospital.
Today we will be reviewing ECG interpretation in the pediatric population.
Review of Cardiac Conduction.
The ECG provides a window to the electrical activity within the heart.
As a review, everything starts from the sinus node.
This releases electricity across the top part of the heart, and tells it to beat.
These are the atria.
Electricity is then funneled to the bottom part of the heart through the AV node.
Electricity then travels through the His Purkinje system into the right and left bundle.
And this is what gets the ventricles to beat.
ECG Leads and Placement.
To perform an ECG, leads are placed on the arms, legs, and across the chest.
The leads that are placed on the arms and legs create what is known as the frontal plane.
This includes Leads I, II, III, aVR, aVL, and aVF.
The frontal plane lets you understand electricity moving in the superior and inferior direction.
This creates the vector diagramming.
By understanding where electricity goes along these vectors, you can understand how the
heart depolarizes.
Electricity that moves towards a lead creates a positive deflection known as an R wave.
Electricity that moves away from a lead creates a negative deflection, or an S wave.
The leads that are placed across the chest are known as the precordial leads.
The precordial plane looks at electricity moving in a more anterior and posterior direction.
This includes Leads V1 through V6.
Not pictured on this diagram, but done frequently in the pediatric population, is V3R and V4R.
These are mirror images to Leads V3 and V4, but on the right side of the chest.
This is done frequently in pediatrics because patients with congenital heart disease are
more likely to have a right-sided heart disease.
This is a standard 12 lead ECG.
ECGs can be quite overwhelming because of the amount of information that they hold.
But if you understand where the leads are placed and the vector diagram we just reviewed,
you get a better understanding of which leads cover which parts of the heart.
Leads I, aVL, V5, V6, and V7 cover the left lateral part of the heart.
Leads aVR, V3R and V4R cover the right side of the heart.
Leads II, III, and aVF are classically known as the inferior leads.
The remainder of the leads are known as anterior leads since they sit on the surface of the
chest.
Leads V1 and V2, since they span across the sternum, also give you a better understanding
of how the septum depolarizes inside the heart, specifically the atrial septum and the ventricular
septum.
Basic Principles of ECGs.
When interpreting ECGs, you should always make sure that they are performed in a standard
fashion.
A lot of ECG reading is about pattern recognition.
So in order to make sure you recognize these patterns, you should always make sure the
ECG is done in a way you are familiar.
This means that ECG is normally performed at a paper speed of 25 millimeters per second.
This means that each small box on the EKG is 40 milliseconds in duration.
In terms of the amplitude, the ECG is performed at 10 millimeters per millivolt.
This refers to the size of the deflections on the ECG.
This is normally recorded on the bottom part of the page or is also seen on the scale on
the left-hand side.
You can see that this rectangle is 10 squares high, which corresponds to 10 millimeters
per millivolt.
You should always ensure that this is done for every EKG you look at, as modern machines
will try to change the amplitude of an ECG tracing in order to fit everything onto a
page.
This may ultimately lead you to miss a diagnosis that is important for the patient.
Interpreting ECGs.
Pediatric ECGs change as the patient ages.
In order to get a better understanding of this and make sure that you read ECGs accurately,
you need to understand what normal values are as patients grow older.
I keep this reference next to me when interpreting EKGs to make sure I'm not missing anything
when looking at tracings.
In school we are always taught a step-by-step approach to reading ECGs.
This includes rate, rhythm, access, et cetera.
You're more than welcome to memorize this if you wish, but the most important thing
is you develop a systematic way of reading ECGs that is comfortable for you.
When looking at the rates, there are two different ways of figuring out what a patient's heart
rate is on an ECG tracing.
The mathematical way is counting those number of small squares between each QRS complex.
Each small square is 40 milliseconds.
Counting the number of squares between each QRS complex and multiplying it by 40 milliseconds
gives you the cycle length of a patient's heart rate.
You then divide 60,000 by the cycle length, and this will give you the heart rate and
beats per minute.
Another way to do it is the estimation method.
If you presume a QRS complex starts on a thick line and then you count 300, 150, 100, 75,
et cetera, you can then see where the next QRS complex lands and get a rough estimation
of what the patient's heart rate is.
For example, what is this patient's heart rate?
If we do the mathematical method, we can count 21 small squares between each QRS complex.
21 multiplied by 40 milliseconds gives you 840 milliseconds.
If you divide 60,000 by 840, you get 71 beats per minute.
Now, if we do the estimation method, we then find the QRS complex closest to a thick line
on the EKG.
You then count 300, 150, 100, 75, and you can see the next QRS complex lands around
this area.
This gives an estimated heart rate of 75 beats per minute, which is pretty similar to the
71 beats per minute we calculated mathematically.
Another way to do it is divide 300 by the number of large squares between each QRS complex.
For this example, divide 300 by 4.
This gives an estimated heart rate of 75 beats per minute.
Moving on to heart rhythm, the most important rhythm we want to see in an ECG is sinus rhythm.
Again, as a review, everything is driven by the sinus node.
This tells the top part of the heart to beat, which gives you a P wave on the ECG.
Electricity is then funneled down to the ventricles, which gives you a QRS complex.
Thus, sinus rhythm has a P wave before every QRS complex and a QRS complex after every
P wave.
Looking at this ECG, we can see that this patient is in sinus rhythm.
If you scan your eye across the screen, you can see that there is a P wave before every
QRS complex and a QRS complex after every P wave.
Let's move on to the QRS axis.
In normal adults, electricity moves downwards and to the left towards the left ventricle.
As a result, electricity goes towards the southeast quadrant, as you can see here in
this diagram.
This makes lead I positive and thus will have an R wave in lead I and also makes AVF positive,
and thus that too will have an R wave.
In children, the right ventricle is more dominant.
It is the thicker ventricle in utero.
As a result, electricity tends to move downwards and towards the right side of the chest.
This gives a QRS axis of 120 degrees or in the southwest quadrant.
As patients age over the first six months of life, the QRS axis will then shift more
towards an adult norm, usually around 60 degrees but always within the 0 to positive 90 sector.
If a patient had a right axis deviation, electricity would be moving towards the southeast quadrant
and thus would be negative and thus lead I would have a prominent S wave.
This is an important ECG to remember for the pediatric population.
You can see in leads II, III and AVF that they all have prominent S waves and thus have
negative deflections.
This is what we call a superior axis.
A superior axis is defined as between minus 60 to minus 100 degrees.
The two most common diseases that cause this are an AV canal, which is commonly seen in
patients with Trisomy 21 or Down Syndrome, and thus you should look for this on their
ECGs when they're born, and tricuspid atresia is also very common to give a superior axis.
Moving on to the P wave, the P wave reflects atrial depolarization.
Electricity starts from the sinus node and spreads right to left across the atrium.
Thus the first half of the P wave reflects the right atrial contraction, and the last
half the P wave reflects the left atrial contraction.
You should always look at normal values when deciding whether or not a P wave is of an
abnormal size.
A rough number to keep in your head are three squares high or three squares wide or an amplitude
less than 0.3 millivolts or duration less than 120 milliseconds.
You should always look at the P wave in leads II and V1, as this gives the most prominent
deflections on an ECG.
Patients who have right atrial enlargement have large P waves in lead II, as you can
see here in this diagram.
Again the P wave is usually greater than 0.3 millivolts.
This is a patient who has left atrial enlargement.
Because the left atrium is the second half of the P wave, this gives a large notch P
wave in lead II.
This is usually greater than 120 milliseconds.
In lead V1, this gives a biphasic P wave.
Because V1 sits on the surface of the chest, when the right atrium depolarzies, electricity
moves towards V1.
However, as the large left atrium depolarizes, electricity moves away from V1 and thus gives
you a slurred terminal portion of the P waves.
Moving on to the PR interval, this is measured from the beginning of the P wave to the beginning
of the QRS complex.
This reflects intra-atrial conduction, meaning electricity that moves from the top part of
the atrium to the bottom part and then through the AV node.
Again, you should always look at normal values when interpreting ECGs, but a good number
to keep in your head is less than 200 milliseconds for a normal PR interval.
Examining the PR interval will help you determine whether a patient has some degree of heart
block.
There are three different types of heart block which reflect sluggish conduction through
the AV node.
First degree heart block refers to a PR interval greater than 200 milliseconds.
This means that electricity moves through the AV node, but in a more sluggish way, but
it does make it down to the ventricle and causes a QRS complex to occur.
This is a good example of first degree heart block.
As you can see here, the PR interval is greater than 200 milliseconds between the P wave and
the QRS complex.
But after every P wave, there continues to be a QRS complex.
In secondary heart block, there are two types.
There is Mobitz type 1, or Wenckebach, and Mobitz type 2.
There are subtle differences between the two, but what makes them both second degree heart
block is that electricity from time to time gets blocked in the AV node, and thus you
will have a P wave without a QRS complex.
This is AV Wenckebach-- PR intervals that get longer and longer, a dropped beat, and
then a shorter PR interval afterwards.
This can be normal in some patients.
In contrast, this is Mobitz type 2 heart block.
As you can see, there are P waves that do not have a QRS complex afterwards.
The difference between this and Mobitz type 1 is the PR interval.
Before a dropped beat, the PR interval stays the same throughout.
This is very different from Mobitz type 1, where the PR interval gets longer and longer
before there's a dropped beat.
Unlike Mobitz type 1, Mobitz type 2 is always a pathologic finding.
Third degree heart block means that there is no conduction between the top and bottom
part the heart, being the AV node has shut down completely.
This is what we term complete heart block.
As you can see, the atrium and ventricles are completely dissociated from each other,
and thus the atrium and ventricle are beating independent of each other.
Let's move on to the QRS complex.
Abnormal conduction through the ventricles can be seen by evaluating the QRS complex.
A wide QRS complex is known as something greater than 120 milliseconds.
An example of this is a right bundle branch block.
Electricity moves slower through the right for Purkinje fibers, and thus the left ventricle
activates first.
As a result, the initial portion of the QRS reflects the left ventricular depolarization.
V1 then has a small R wave.
V6 has a qR complex which is which reflects the left ventricle depolarizing.
The latter portion of the QRS reflects the right ventricular depolarization.
In V1 then, you see a tall slurred R prime.
This is what people term the bunny ear complex.
In V6, you see a slurred S wave, as electricity moves away from V6 as the right ventricle
contracts.
The opposite pattern is a left bundle branch block.
Electricity then moves right to left through the ventricles.
As a result, V1 has an rS pattern, whereas V6 now has the bunny ear pattern with a tall
notched R wave.
In terms of hypertrophy, the QRS complex can tell us about the mass of the ventricles based
on the size of the deflections on the ECG.
Right ventricular hypertrophy has more accepted criteria for its diagnosis.
This includes a large R wave in V1, a large S wave in V6, or abnormal T waves in the right
precordium-- we'll talk about that in a few minutes-- right axis deviation, qR pattern
in V1, or an RSR prime pattern in V1 can also be used to diagnose RVH.
When there is an RSR prime pattern, the R prime has to be quite large, and sometimes
the pattern goes out as far as V3 in the right precordium.
Left ventricular hypertrophy has less accepted criteria.
This includes a large R wave in V6, a large S wave in V1, or an RS ratio greater than
the 98th percentile.
T wave inversions in the left lateral precordium can also be looked for as can a left axis
deviation.
Moving on to the ST segment, this is measured from the end of the QRS complex to the beginning
of the T wave.
This is normally isoelectric in that it is flat running across the baseline.
Changes in the ST segment can reflect ischema or inflammation, the former of which is quite
unusual in the pediatric population.
Looking at the ST segments, you can see that all of them are elevated.
When you see diffuse ST elevations across all of your leads, this is a good example
of pericarditis.
The T wave reflects the heart repolarizing.
The direction of the T wave can change as the patient ages.
In general, in the frontal plane, the T wave follows the axis of the QRS.
They are usually within 60 degrees of each other.
For the precordial plane, the T wave changes along the right precordium.
From birth to seven days, there are positive T waves noted.
After one week to around adolescence, the T waves flip over and are negative during
this time.
After your teenage years, the T waves then flip over and then become positive in the
right precordium.
After one week, positive T waves in V1 reflect RVH.
Looking at the QT interval, this is measured from the beginning of the QRS complex to the
end of the T wave.
To measure it properly, you should not measure where the T wave tapers slowly down to the
baseline, instead you should draw a tangent from the steepest part of the T wave slope
and draw it down to the baseline.
The QT interval is then measured from the beginning of the QRS complex to where this
tangent intersects the baseline.
You should always try to use leads II or V5, as this provides the longest QT interval on
an ECG tracing.
The QT interval has to be corrected for the patient's heart rate.
We use Bazett's formula for this.
We divide the QT interval by the square root of the preceding RR complex, that is the number
of milliseconds between the QRS complex right before the QT interval of interest.
Again, you have to look at normal values to determine whether or not this calculated QT
interval is abnormal for your patient.
But in general, a number to keep in your head is less than 450 milliseconds.
It is important to measure the QT interval accurately, as patients with prolonged QT
intervals are at risk for a ventricular arrhythmias or sudden death.
This ECG represents a patient who has congenital long QT syndrome.
Their QT interval is 500 milliseconds.
Thank you for watching this presentation on the interpretation of pediatric ECGs.
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