So to successfully manage a patient who is in cardiac arrest, the caregiver must carefully,
systematically and immediately identify the cardiac rhythm and choose the appropriate
and most effective treatment algorithm.
In this section we will be discussing the different cardiac dysrhythmias that lead to
cardiac arrest, their characteristics, and the appropriate therapies to treat and correct
the particular dysrhythmia if at all possible.
The patient’s past medical history and any or all events leading up to the medical emergency
will help the health care professional determine if there is any chance of reversing an underlying
cause for the cardiac arrest.
An example of this is assessing the patient with the 5 Hs and 5 Ts which are discussed
in detail in the ACLS secondary survey section of this program.
It’s important to remember that until the underlying cause has been identified and corrected,
pharmacological and electrical therapies may offer little to no help in the attempt to
resuscitate the cardiac arrest victim.
Now when assessing the electrical activity of a victim's heart, it will be vital to recognize
the underlying dysrhythmia and know how to treat it appropriately to restore a perfusing
cardiac rhythm.
A sinus rhythm is defined as any cardiac rhythm where depolarization of the cardiac muscle
begins at the sinus node.
It’s characterized by the presence of correctly oriented P-waves on the electrocardiogram.
An ECG waveform represents each electrical event in the cardiac conduction system during
a cardiac cycle.
This doesn’t mean necessarily that the muscle is reacting correctly or in correlation with
the electrical patterns, but simply shows that the electrical events that may stimulate
myocardial function are happening.
This will be discussed in more detail when we look at each rhythm individually.
Now for the following explanation, we’re going to be evaluating a normal waveform and
assume that normal mechanical function is occurring.
The P-wave, which is the first waveform in the complete complex is normally found upright
in most leads.
It represents the depolarization of both the right and left atria which occur at the same
time.
The segment between the P-wave and the R-wave represents the delay of the electrical circuit
in the AV node.
This is the time it takes from the end of the P-wave to the beginning of the ventricular
response represented by the QRS complex.
Now the Q-wave represents the first activity of the ventricular depolarization and is usually
the first negative deflection after the P-wave in the complete complex.
We’ll discuss the significance of Q-wave formation specifically as it relates to certain
dysrhythmias in each of the rhythm evaluations.
Now the R-wave is the first positive deflection after the P-wave and then is followed by the
S-wave which is the first negative deflection after the R-wave.
The timeframe between ventricular depolarization and repolarization is indicated via the ST
segment.
The ST segment is the baseline of the cardiac cycle and therefore is electrically neutral.
There should be no inflection or deflection as it’s isoelectric.
ST elevation or depression of more than 1mm can be clinically significant as it may indicate
underlying cardiac issues both acutely and chronically.
The T-wave, which represents repolarization of the ventricles should also be seen in the
same general direction as the QRS segment.
If there is an inversion of the T-wave, this too can be indicative of a cardiac problem.
I believe that it’s very helpful to healthcare professionals to have a repeatable and easy
way to interpret ECG rhythms, and therefore are going to follow a serial pattern for reading
and interpreting all ECG’s.
The pattern of interpretation most commonly used is to look at the following.
The rhythm: is the rhythm regular or irregular?
Next, the rate: what is the rate?
Is the rate normal, fast or slow?
Now remember, ECG paper grids are where time is measured along the horizontal axis.
And each small square is 1 mm in length and represents 0.04 seconds.
Each larger square is 5 mm in length and represents 0.2 seconds.
So a six second interval would be 30 large squares.
To determine heart rate, count the number of QRS complexes over a 6 second interval.
Multiply by 10.
This method works well for both regular and irregular rhythms.
Next we look at the P-wave: Are they present?
Do they occur regularly?
Is there one P-wave for each QRS complex?
Are the P-Waves smooth, rounded, and upright?
Do all P-Waves have similar shapes?
Next, the PR Interval: Does the PR-Interval fall within the norm of 0.12-0.20 seconds?
Is the PR-Interval constant?
Then we look at the QRS complex: Is the QRS interval less than 0.12 seconds?
Is the QRS wide or narrow?
Are the QRS complexes similar in appearance?
You see, in using a systematic approach to interpreting the ECG rhythms, we will help
ourselves and other ACLS team mates to efficiently, appropriately and effectively diagnose underlying
cardiac conditions.