When you assess a patient's heart rhythm by looking at an EKG strip, you are primarily looking for the following:
1) Heart Rate
2) Rhythm
3) Axis
4) Hypertrophy
5) Infarction
1) Rate
There is a way you can quickly determine a patient's heart rate by looking at a short section of an EKG strip, it involves memorizing the following numbers:
300, 150, 100, 75, 60,50
(If the heart rate is lower, you will have to count it out by the other method I will show you shortly)
As I mentioned earlier, the horizontal plane on an EKG strip represents time elapsed. Vertical lines mark out time intervals: with bolder lines marking 0.2 second (or 1/300 minute) intervals, and thinner lines marking 0.04 second intervals.
Using this we can determine the rate by counting the number of bold lines in between an interval of QRS complexes. The procedure follow thus:
First:
Find an 'R' wave that falls on (or nearly on) a bold virtical line.
Second:
Use the numbers! Count down with the numbers 300-150-100-75-60-50 using the bold vertical lines.
Find the next bold vertical line to the left of the first and say (or think) '300' (have you reached the next 'R' Wave in the neighboring QRS complex? No? Keep going!)
Find the next bold vertical line to the left of the '300' line and say (or think) '150' (have you reached the next 'R' Wave? No? Keep going!)
Find the next bold vertical line to the left of the '150' line and say (or think) '100' (have you reached the next 'R' Wave? No? Keep going!)
Find the next bold vertical line to the left of the '100' line and say (or think) '75' (have you reached the next 'R' Wave? No? Keep going!)
Find the next bold vertical line to the left of the '75' and say (or think) '60' (have you reached the next 'R' Wave? No? Keep going!)
Find the next bold vertical line to the left of the '60' line and say (or think) '50' (have you reached the next 'R' Wave? No? Whell that's too bad, cuz you have bradycardia and determining an accurate heart rate won't work with this method :P)
For Bradycardia-
Count the number of PQRST cycles on a six second EKG strip and multiply by 10 (ugh, this one is sooo hard!)
There is also a mathmatical method in which you take into account the fact that the thin lines are one millimeters apart and represent 1/1500th of a second:
Heart Rate=1500/Milimeters in between similar waves
2) Rhythm
I've already talked about rhythms (organized by their pacer orgin, in my previous blog post titled, 'Normal and Abnormal Heart Rhythms'
It is very importiant to know what all of those squiggly lines mean, because you can determine where the heart is having problems by how some of the waves are shaped, or if some are missing, or if they look like they're not supposed to.
3) Axis
Remember when I told you I wrote an arrow on my chest to get an idea of how the electrical activity in my heart moved, and how it the way each of the different leads would 'see' the PQRST complex?
Well I didn't know it at the time, but that arrow was an axis (actually, the arrow itself is a vector...but it represents an axis: the direction in which the depolarization is moving within the myocardium...particularly the ventricles)
In a normal, healthy heart, this conduction moves from the AV Node, through the bundle of His and the left and right bundle branches (located in the ventricular septum) to the perjinke fibers, and endocardal tissues. Because the heart is angled towards the left, and because the left ventricle is larger and thicker-walled than the right, more depolarization energy is used there. This contributes to the overall 'downward and to the left' vector.
Additional factors, affect the direction of the axis: including tissue hypertrophy and infarction. If the tissues in the heart are hypertrophied (enlarged) a greater degree of depolarization energy (and thus, the ventricular axis) is directed in the direction of the hypertrophied tissue.
If there is infarction, there is no electrical/depolarization activity in the affected tissue. Therefore, the direction of the vector/axis is deviated.
We measure the axis in degrees.
Imagine a circle on the front of your chest:
90 degrees is the direction of your belly button.
-90 degrees is the direction of your neck.
0 degrees is to the left side of your heart
180 degrees is to the right side of your heart
(In a 'normal healthy' heart, the ventricular axis is located approximately at 40 degrees)
If your heart was displaced (instead of facing towards the left, it was facing towards the right) the Vector would be displaced in the same direction. (This might sound far-fetched, but this type of deviation isn't unheard of. Sometimes electrodes need to be reversed in order to pick up the correct lead).
Now things get a bit tricky...
Remember in that blog post I wrote about monitoring the electrical activity in the heart? The one where I started talking about the polarity of all of the electrode-sensor leads? This factors into determining the axis in a specific lead.
I'll use an example:
The left side limb leads where the Left Arm has a positive electrode sensor are Lead I and AVL. This means that they are 'looking' at the positive wave of depolarization from the left side. In these leads, the QRS complexes of a 'normal healthy' heart will be upright (because the wave of depolarization is moving towards them!)
Imagine for a moment that you are looking at a full EKG page of squiggly lines. You first examine lead one and you notice (gasp!) that the QRS complexes are inverted!
This is called Right Axis Deviation (R.A.D) It can occur if the left side of the heart has been severely damaged, or the right side of the heart is severly hypertrophied.
The same principle applies to all of the limb leads, you just have to remember where the positive and negative sensors are located and you can find out where and if the axis has deviated from the normal range. This helps to determine areas of the heart that might be damaged.
4) Hypertrophy
For the definition of hypertrophy, please see the entry in the 'Axis' subheading.
The chest leads are usefull in identifying areas of hypertrophy in the heart. Because the chest leads consist of positive electrode sensors only, they can detect positive impulses moving in the direction of the sensor. This is usefull for determining the location of hypertrophy because the greater degree of depolarization energy used in this type of tissue will produce waves that appear abnormal in the nearby lead.
For example, in lead V1 a 'P' wave will appear as two waves (one positive and one negative) when hypertrophy is present where the V1 is positioned. You can further differientiate between right and left atrial hypertrophy by comparing the two 'P' wavelengths. If the first is larger, the hypertrophy is primarily on the right side of the atrium, if the second is larger; the hypertrophy is primarily on the left atrium.
Hypertrophy in both the ventricals is seen as an abnormally elongated 'R' wave in V1 (particularly visible, because this view is on the right side of the heart and the waveform of the 'R' is oriented upwards in this lead.)
In order to determine which ventricle is affected with hypertrophy, look at the other chest leads:
If the large R wave becomes smaller in leads V2, V3, V4 (as the sensors move to the left side of the heart) then right ventricular hypertrophy is present.
If the waveform is of very high ampletude, with exagerrated 'S' wave in V1, changing to a large 'R' wave in V5 (of greater then 35mm deapth total of the two) and an inverted and asymmetrical 'T' wave in leads V5 and V6, then left ventricular hypertrophy is present.
5) Infarction
I briefly defined infarction in under the subheading 'axis', it refers to an area of tissue that is dying: either the oxygen supply has been cut off and the tissue is in destress (Ischemia), the tissue has sufferd damage due to loss of oxygen (Injury), or the tissue has died in the affected area (Necrosis).
It is most commonly the left ventricle that suffers from myocardial infarct, therefore in order to assess if infarct has taken place we will need to look at the leads on that side and note the altered electrical/depolarization energy flow in the QRS (ventrical) complexes.
A common finding in ischemia of the left ventricle is a symmetrically inverted T wave.
In myocardial injury, the ST segment will be elevated (unless the injury occured just below the endocardial lining, then the ST segment will be depressed).
In myocardial necrosis, the ST segment is elevated and the Q wave is prominent (at least 0.04 seconds in length, or one small 1mm square on the graph). If this type of Q wave is present in the lead views (except for the AVR lead, because that one apparently will LOOK like a Q wave when its actually an R wave...cuz it looks at the heart from the wrong angle...yeah, its confusing), note the leads on which the promenent Q wave is visible (except for the AVR cuz its probably not a Q wave at all)
More notes on necrosis infarction: Reguardless of where the infarct is located, there are electrochemical markers that will help identify the area. Necrotic myocardial tissue CANNOT conduct electricity, therefore the cannot conduct a depolarization impulse and a chest electrode will not be able to sense electrical activity when it is placed over he area of infarct...
It WILL, however, pick up on the electrical activity on the OTHER SIDE OF THE HEART!
For example, a elecrode sensor placed in front of necrotic tissue on the left side of the left ventricle will pick up the depolarization impulse on the RIGHT side of the ventricle (in the septum). And the impulse from this area of the heart will be moving from the endometrial lining (on the inside chambers of the heart where the perjinke fibers are) TO the epicardium on the outside of the heart. This means the depolarization wave (as seen from this lead) will be moving in the opposite direction compared to the the depolarization wave seen in a 'normal healthy' heart.
Prominent Q waves in leads V1-V4 indicate anterior infarction. (The Q wave is promenent because the chest leads are picking up the depolarization of the septum better then they normally would because of the necrotic "hole").
Promenent Q waves in leads I and AVL indicate lateral infarction.
Promenent Q waves in leads II, III or AVF indicate inferior infarction.
Posterior infarction is easy to miss (because we don't place chest leads on the back of the heart!) However, it is very severe so we need to look for them carefully. They will produce ST depression and large R wave in leads V1, V2. The large Q wave may be present in lead V6. If this type of infarction is suspected, it is importiant to do a mirror test, in which the EKG strip in leads V1 and V2 is inverted and read from the "blank" side of the strip (hold up to a light source to assist with visuallization.) Look for the "Q waves with ST elevation" in this mirror view. These will indicate posterior infarction (Which I think are really the R wave and the ST depression).
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