Introduction to Arrhythmias by Christine LaGrasta.
Welcome you to today's module on arrhythmias.
My name is Christine LaGrasta.
I work at Children's Hospital of Boston as a pediatric nurse practitioner.
Introduction.
In order for the heart muscle to contract and beat, it needs an electrical stimulus.
The stimulus follows a pathway beginning in the right atrium and traveling down to the
ventricles.
This is known as "myocardial conduction."
Myocardial conduction causes the heart to contract.
When there is an arrhythmia or a change in the normal electrocardiogram, there is a disturbance
in this pathway and the heart does not contract or beat efficiently.
To understand cardiac arrhythmias, it is necessary to learn the normal pathway and conduction
of the impulse.
Cardiac Conduction System.
Electrical impulses originate in the sinoatrial node, or SA node, located at the junction
of the right atrium and superior vena cava.
Each electrical impulse generated from the SA node travels through the right and left
atria, causing the atria to contract.
The impulse then travels to the atrial ventricular node, or AV node, then to the bundle
of His, and finally through the right and left bundle branches of the ventricles, causing
the ventricles to contract.
Interpreting an ECG.
Cardiac conduction is represented by the electrocardiogram, or ECG.
For the three-lead setup, the right arm, or RA lead, is placed directly below the clavicle
in the other right shoulder.
The left arm, or LA lead, is placed directly below the clavicle near the left shoulder.
The left leg, LL lead, is placed on the left lower abdomen.
The ECG is recorded on ruled paper.
Time is measured on the horizontal axis, and voltage is measured on the vertical axis.
The duration of any wave is determined by measuring along the horizontal axis.
The smallest divisions are one millimeter long, and one millimeter high.
Each small square represents 0.04 seconds.
There are five small squares between the dark lines, and the amount of time represented
by the distance between the dark lines is 0.2 seconds.
The P wave represents atrial conduction and contraction.
It is best viewed in lead two, where it is small and upright.
The PR interval represents the time from atrial contraction and conduction to the onset of
ventricular conduction.
It is measured from the beginning of the P wave to the beginning of the QRS complex.
The normal PR interval is 0.12 to 0.20 seconds.
The QRS complex represents ventricular contraction.
The normal QRS complex is 0.08 seconds or less.
The QT interval represents the time from the beginning of the ventricular contraction to
the end of the ventricular contraction.
The ST segment represents the time between the end of the QRS complex and the T wave.
It is isoelectric, meaning it is a flat line.
The T wave represents the ventricles recharging for the next contraction.
When interpreting ECG rhythms, it is important to follow an organized method to look at the
rhythm.
The following steps are one way to interpret rhythm strips.
One, evaluate the P wave.
Are the P waves present?
This indicates atrial depolarization.
Are the P waves normal shape?
They should be upright and round.
Are all the P waves the same shape?
Do you see one P wave for each QRS complex?
Step two, evaluate the atrial rhythm.
Measure the interval between two consecutive P waves.
This is known as the P to P interval.
Is the rate regular or irregular?
Three, calculate the heart rate.
We will discuss two methods of calculating the heart rate.
The first method is as follows.
Count the number of R to R cycles in six large boxes on an ECG rhythm strip.
On this ECG rhythm strip, there is one R to R cycle within six large boxes.
Next, take the number of R to R cycles in the six large boxes and multiply this amount
by the number 50.
For this ECG strip, we multiply 50 by one R to R interval and calculate that the heartrate
is 50 beats per minute.
The second method for calculating the heart rate is as follows.
Count the number of large boxes between two R waves.
On this ECG rhythm strip, there are four large divisions between the two highlighted R waves.
Divide the number of large boxes between the two R waves into 300.
For this ECG rhythm strip, there are four large boxes.
We divide 300 by 4, and calculate the heartrate to be 75 beats per minute.
Four, calculate the duration of the PR interval.
Count the number of small boxes from the beginning of the P wave to the beginning of the QRS
complex, and multiply by 0.04 seconds.
Does the PR interval fall within normal limits?
Is the PR interval constant for each QRS cycle?
Step five, evaluate the ventricular rhythm.
Measure the interval between two consecutive R waves.
This is the R to R interval.
Are the R to R intervals consistently the same?
If the intervals are not the same, then the rhythm is considered irregular.
Step six, calculate the duration of the QRS complex.
Count the number of small boxes from the beginning of the Q wave to the end of the S wave, and
multiply by 0.04 seconds.
Are the complexes all the same size and shape?
Do any of the QRS complexes appear different from each other?
Is there one QRS complex for each P wave?
Step seven, evaluate the ST segment.
The normal ST segment is flat, or isoelectric.
They can be an elevation or depression of two millimeters, or two small boxes.
This can be considered normal in pediatrics.
Abnormal changes are seen in pericarditis, myocardial ischemia, or infarction.
Evaluating a Child with an Arrhythmia.
General principles of evaluating children with arrhythmias.
One, look at the history.
The history will help point you towards or away from a possible diagnosis of an arrhythmia.
Questions to ask as you're evaluating the patient.
Does the fast or slow heart rate start or stop suddenly?
Does it get worse with exercise?
Are there other symptoms?
Does the patient have difficulty breathing?
Is there pallor, cyanosis, dizziness, or poor feeding present?
Do these symptoms occur before, during, or after the fast or slow heart rate?
What is the child's activity level?
Can they keep up with their peers when playing or exercising?
Family history is also important.
Are there any arrhythmias, sudden cardiac death, or congenital heart defects in any
other family member?
After taking a complete history, the most important tool to evaluating children with
possible arrhythmias is to always examine the patient.
Trust your physical examination.
Repeat the examination after every treatment and when there is a change in vital signs.
Questions again to always ask yourself.
How does the patient look?
Is he awake and alert, or sleepy and not responding to stimulation?
Is he of normal color, or is he pale or mottled?
Is the patient breathing?
How fast or slow is he breathing?
And also look at how much effort does it take for the patient to breathe.
Does the patient look comfortable?
Is there a pulse present?
Is it strong or weak?
Is it regular?
Is it equal in all extremities?
Another question to ask is chest pain.
Does the child have chest pain?
And your answer will depend on the age of the patient.
So children older than five years old describe arrhythmias as their chest is beating fast.
They could actually say chest pain.
They could have dizziness or shortness of breath.
Children younger than five years old may not be able to describe how they're feeling at
all.
And infants can be more fussy and may not eat as much as they should be, or they may
not be gaining weight.
So there could be a lot of different subtle signs that could go along with arrhythmias
too.
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