The Linear Progression Method

Learning to read EKG’s thoroughly and correctly may seem to be a daunting task at first. However, it is crucial for Emergency Medicine physicians to learn to read EKG’s accurately. To facilitate your task of learning to read EKG’s, we offer you a systematic approach on EKG interpretation. If you follow this algorithm, you should be able to detect most EKG abnormalities.

The Linear Progression method of EKG interpretation is quite simple and can be summed up in 5 simple words.

- Rate
- Rhythm
- Axis
- Widths
- Heights

If you can remember these 5 words, you can learn to read EKG’s thoroughly and correctly.

First, take the EKG and fold over the top part of the page where the computer has given you its “interpretation” of the EKG. It is often wrong and knowing what the computer thinks will never help you learn how to read EKG’s.

Next, look at the rhythm strip, which is usually the bottom-most line of a standard EKG. The rhythm strip is important because it is generated from one single lead (often lead II) and there are no “hash marks” found in the rhythm strip. Hash marks are the small vertical lines that separate different sections of the EKG where lead changes occur. This fact is important when you are trying to determine if a patient has pacemaker spikes on their EKG.

Look at the rhythm strip and determine the RATE of the EKG. To do this, find a complex whose apex falls on the line of a large box on the EKG paper. Then count the number of large boxes you cross until you reach the apex of the next complex. Take that number and call it N. Then, plug it into the formula:

RATE = 300 / N

If the complexes are regularly spaced across the EKG, this formula will give you the approximate RATE of the EKG. If the complexes are irregularly spaced (as in atrial fibrillation) then you will have to calculate your rate in a different manner. However, this simple method works for a large percentage of EKG’s.

If you use this formula enough times, eventually you will start to learn a pattern for the different rates as the complexes fall on 1 large box, 2 large boxes, 3 large boxes and so on. Three hundred divided by 1 is 300. Three hundred divided by 2 is 150. Three hundred divided by 3 is 100 down to 300 divided by 6 is 50. The set of numbers you will end up with is:

300, 150, 100, 75, 60, 50

If you mention these 6 numbers to any physician that reads EKG’s, they will recognize the pattern immediately.

If you want to get a more exact estimate of the rate, you can count the number of small boxes instead of large boxes. Call that “n” and plug it into the formula:

RATE = 1500 / n

This is intuitive since there are 5 small boxes for each large box on EKG paper and 5 times 300 is 1500.

The next thing to look at is the RHYTHM of the EKG. Most of the EKG’s that you will come across will be in sinus rhythm and will be driven by the sinus (SA) node. On an EKG, this will be evidenced by a P wave preceding every single QRS complex. Additionally, the P waves should all look the same since they are all being generated from the same point of origin, the SA node. They should also all have the same PR interval. Discussion of the other rhythms is more complicated and can be discussed later when looking at specific EKG’s that are not generated by the SA node.

The next piece of information to acquire is the AXIS of the QRS complex. Many books and physicians suggest different complex formulas and logical progressions to accomplish this task. However, many only answer the question “Is the axis ‘normal?’” According to the way that I was taught to read EKG’s, this was not adequate and a single number should be derived when asked to calculate the axis of the QRS complex.

To calculate the AXIS of the QRS complex simply look at leads I and aVF. Remember that lead I points horizontally to the right (on the paper, left on the body) and lead aVF points downward on the page. These are the only two leads that you will need to look at for this simple computation. Figure out the “net vector” of the QRS complex in lead I. To do this, add the number of positive boxes (above the horizontal axis) and subtract the number of negative boxes (below). Most of the time you should end up with a positive number. Plot your net number of boxes along an X axis. Keep in mind that a positive number falls to the RIGHT of the center and a negative number falls to the LEFT of the center. Then do the same thing and calculate the “net vector” for the QRS complex in lead aVF. Plot this on the Y axis with positive aVF pointing DOWN and negative aVF pointing UP. Now look at these two vectors. If they are approximately the same size, then the addition of these two vectors is around 45 degrees. If the horizontal vector is double the size of the vertical vector, then the axis is around 30 degrees. Remember that lead I is designated to be 0 degrees. Positive aVF is designated as 90 degrees and negative aVF (pointing up) is designated to be –90 degrees. Using this simple method, you can calculate the AXIS of any QRS complex. One fact to point out is that if the “net vector” turns out to be 0 (iso-electric) in either I or aVF, then you do not need to plot that on the axis. Your final QRS axis will be either 0 degrees, 180 degrees or +/- 90 degrees depending which lead had a net vector of zero.

Once you have those three pieces of information figured out, you can move on to read the rest of the EKG. Always begin from the left of the complexes and move to the right. Start with the WIDTHS of the complexes. If you move from left to right, the first width you come across is the P wave width. Look specifically in lead II. If the duration of the P wave is greater than 0.12 seconds (3 small boxes) left atrial enlargement (LAE) exists.

As you move across the complex from left to right, the next WIDTH you encounter is the PR segment. This segment encompasses the entire P wave from its beginning up until the first deflection of the QRS complex. It should be LESS THAN 0.2 SECONDS (1 large box) to be considered normal in adults. If the PR segment is greater than 0.2 seconds, first degree AV block exists.** **

The next WIDTH you encounter is the QRS complex width. It should be LESS THAN 0.12 SECONDS (3 small boxes). If it is greater than 3 boxes, either a bundle branch block (BBB) exists (partial or full) or the complex did not take the conduction pathway at all and had a ventricular origin as is seen in a premature ventricular complex or PVC.

The next WIDTH that we come across as we move from left to right is the QT segment. Prolongation or shortening of this segment is neither specific nor sensitive for any single diagnosis. However you should know that the corrected QT or the QTc should be less than 450 milliseconds. If prolongation occurs, it may be a tip-off that electrolyte abnormalities exist or some toxin is present. While this concept is more complex than depicted here, it is not so important and can be discussed later when looking at specific EKG’s.

Having looked at all the widths of the EKG, we can move on to the HEIGHTS. Again, as we move from left to right, the first HEIGHT we come across is the P wave height. Specifically look at the P waves in leads II and V_{1}. If the P in lead II is greater than 2.5 mV (small boxes) right atrial enlargement (RAE) probably exists. If the P wave in V_{1} is negative or biphasic, then LAE probably exists. This negative portion of the P wave in V_{1} should be more than 1 box wide and 1 box down to be considered significant.

The next HEIGHT we look at is the PR segment. The only classic significant PR segment abnormality that is encountered in the emergency department is PR depression. This is most often seen in the setting of pericarditis. Since there are multiple stages of pericarditis, these depressions are not always seen when this disease is present.

Next we look at the HEIGHT and shape of the QRS complexes. Here, we are looking for Q waves in any of the 12 leads and we are looking for BBB patterns. Go through all 12 leads systematically and look for Q waves. Remember that leads that point in the same general direction will classically have similar shapes. The groupings that you need to remember are:

II, III, aVF – Inferior leads

I, aVL – High lateral leads

V_{1-2} – Septal leads

V_{2-4} – Anterior leads

V_{5-6} – Low lateral leads

To recognize BBB’s, I almost always look at leads V_{1-3}. You should have already determined if the width of the QRS complex meets criteria for a BBB. Remember that the QRS complex needs to be greater than 3 small boxes (0.12 sec) for a BBB to exist. Now you need to determine which bundle is not conducting correctly. To diagnose a right BBB (RBBB) there should be an RSR’ in leads V_{1}, V_{2} or V_{3}. This is easy to remember if you imagine RSR’ look like “rabbit ears.” The “**R**” from **R**ight and **R**abbit are all similar and will help you remember this fact. To diagnose a left BBB (LBBB) you need a deep, wide Q/S wave in these anterior leads. This you will just have to memorize. There are other criteria to recognize to diagnose BBB’s but this should be sufficient to start you off.

After looking at the shape of the QRS complex, look at the magnitude of the HEIGHT of the QRS complex. Specifically, you are looking for signs of left ventricular hypertrophy (LVH). The two criteria I memorized are either a positive deflection in leads I or aVL greater than 11 mV, or a value greater than 35 mV when you add the absolute values of the more negative of V_{1} or V_{2} plus the more positive of V_{5} or V_{6}. The first one is easy to remember since 1 and L look like an 11 when they are side by side. For the second criteria, you will just have to look at the Q or S in leads V_{1} and V_{2} and see which is more negative. Take the absolute size of that complex and add it to the larger R of V_{5} or V_{6}. Remember, only one criterion is sufficient to diagnose LVH.

As you continue to move from left to right across your complexes, you need to determine the HEIGHT of the ST segment. Again, you need to check systematically through all 12 leads of the EKG looking for ST elevations or depressions. These findings are consistent with AMI or ischemia respectively.

The last HEIGHT to look at is the T wave height. Specifically, you are looking for flipped T waves that are pointing in the negative direction. This is also symbolic of coronary ischemia. Quickly glance at the shape of the T waves. If they are sharp and pointy instead of nicely rounded, hyperkalemia may exist.

Using this systematic method to approach EKG reading should enable you to become proficient at EKG interpretation. Practice by reading as many EKGs as you can get your hands on. We hope that this algorithm will soon become second nature to you which will allow you to take better care of your cardiac patients.