What is an ECG?
An ECG is a graphical representation of the electrical activity in the heart, or as I call it, squiggles on a piece of paper which makes me seem intelligent right? We use electrodes to connect the ECG monitor to the patient and these electrodes pick up the electrical activity and cause a positive, negative, or no deflection on the ECG paper. The part that is “baseline” is called the isoelectric line.
Intrinsic pathway
The heart is very unique in that it has automaticity which is the intrinsic ability to spontaneously depolarize itself. In a normal heart, there are a few pacemaker cells. The first one is the sinoatrial node or SA node. On its own, without any sympathetic or parasympathetic factors, it can cause beats from around 60-80 BPM. This node is at the top of the right atrium and is where the electrical activity originates. To depolarize the left atrium, there is this pathway that branches off of the SA node and heads past the septum to the left atrium. This is called the Bachmann’s bundle. To further depolarize the right atrium, the electrical impulses head down the internodal pathways to the second pacemaker node which is the atrioventricular node or AV node. This is the middle point between the atria and ventricles. This is an important structure because it has a slight delay to it which allows the atria to depolarize and to dump the blood into the ventricles which is important for our stroke. The AV node also acts as a gate keeper so it will only allow a certain amount of impulses down at a time which is important in patients in atrial fibrillation where there are 400-600 impulses a minute being produced in the atria. It helps cancel some of these out to prevent the heart from going into a deadly rhythm called ventricular fibrillation. This is important especially in patient’s with afib with WPW. Check out the post below to check up on that topic.
From the AV node, the impulse goes into the Bundle of His which is in the septum. The impulse then goes down the bundle branches. There are two, the left and the right bundle branch. The left bundle branch has two little fascicles that branch out. The left anterior fascicle and left posterior fascicle. From there the impulse heads down to the ventricles through the purkinje fibers and they cause the ventricles to depolarize.
Action Potential
Knowing the action potential helps us better understand why electrolyte disturbances can affect the 12 lead. The action potential starts off at roughly -90 mv and is separated into 4 phases. We start off with stage 4 because that is the end of the previous action potential. Phase 4 is noted at the bottom left and it shows the resting potential of the cell. When a myocyte gets an action potential from an adjacent cell, there will be a rush of positively charged ions into the cell. Sodium will rush in and the calcium will slowly come into the cell. These positive charges cause a massive change in the positive to negative charges in the cell and the cell becomes positive. This is shown as phase 0 which is the depolarization of the cell. At Phase 1, the sodium channels close which stops the positive ions from entering. Phase 2 is the plateau phase and is shown by the horizontal line on the graph. Potassium which is positively charged exists the cell and calcium enters at a rate that keeps the charge relatively constant. At phase 3, the calcium channels close and potassium dumps out of the cell and sodium slowly enters. This causes a shift to a more negative charge. Phase 3 shows repolarization of the cell. Phase 4 is in effect now and the cell is at the resting potential again. Now there is an interesting term we need to discuss here. From phase 0 to phase 3 the cell enters a state of absolute refractory period which means a second action potential cannot be initiated.
Electrodes
Now we will talk about our electrodes. When I went through school, vectors were not explained a lot to me and it honestly shortened my life expectancy significantly. But I will try to make it simple for you all. If an impulse heads towards a lead it will cause a positive deflection. If it heads away there will be a negative deflection. If the vector heads perpendicular to the electrodes or if there is extremely slow conduction (such as if the impulse is in the AV node), there will be no deflection from baseline. When the heart depolarizes, there will be a bunch of vectors but if you combine them all to produce the average direction, you will get something called the net vector.
12 lead vectors
We heard of a 12 lead right? Well what is it? It is a graph of 12 different views or leads of the heart. There are three types of leads. You have your bipolar which shows you I, II, III on the 12 lead. This is the coronal view and shows the view of the heart in more of a 2D way. The next set of leads are your augmented unipolar leads which also show you a coronal view of the heart and are shown in aVF, aVL, and aVR. The final set of leads are your 6 precordial leads or chest leads which show the heart with a transverse view. The leads associated with these are V1-V6.
Bipolar lead vectors
Here is your bipolar leads which means they have two charges to make a view. You can see the right arm has two negative charges, the left arm has a positive and negative, and the left leg has two positives. If you put your eye on the positive leads and look towards a negative lead it will show you the view from that direction. So if we put our eye on the left arm and look to the right arm, you get lead I which shows the electrical activity from the left side of the heart. What about leads II and III? Put your eye at the left leg and look towards the negative electrode on the left and right arm. You are looking at the heart from the bottom of the inferior part. And you see that leads II also lets you look at the electrical activity from the bottom of the heart.
Augmented vectors
Now we that we know about the bipolar vectors, lets look at the augmented vectors which is your aVR which means augmented vector right, aVL augmented vector left, and aVF which means augmented vector foot. So to find augmented vectors we need to see that they are located on the coroners of our triangle we made with our bipolar leads. aVR is in the right corner, aVL is in the left side of the triangle, and aVF is at the bottom or closest to the foot. If we wanted to find the vector for aVR, we need to find the two electrodes that put it in the middle which is the LA and LL. Each of these leads are found by having one positive reference point. Go right down the middle between them and put an arrow from the middle point on the line and go up to the upper right of the triangle. That is your vector for aVR. It views the heart from the upper right side of the heart.
We can do this with aVF and aVL as well.
Precordial leads
As you can see these leads show a transverse view of the heart. These leads are V1 through V6. Placement is very important for all leads but especially these ones.
V1- 4th intercostal right side of sternum
V2- 4th intercostal left side of sternum
We are going to skip V3 quickly. V4- midclavicular 5th intercostal space usually under the nipple
V3- between V2 and v3
V5- 5th intercostal space anterior axillary line
V6- 5th intercostal space midaxillary
Cardiac Axis
Now that we fully understand vectors. We need to determine the overall axis of depolarization which is called the cardiac axis. I won’t lie, this topic is a bit difficult but I have some tips for people who learn all kinds of ways. You must remember that if the impulse heads towards a lead, there will be a positive deflection and if it travels away it will cause a negative deflection. You also have to remember the normal depolarization through the heart. It starts at the SA node and heads at a downward diagonal path towards the ventricles. A normal axis is between 90 and -30 degrees. We want that impulse to head towards the AV node which is in that direction. We only care about two leads. Lead I and aVF. Using these two allows us to determine the axis more accurately. As you can see if lead I is positive, you will get the blue color and will tell you that the vector is from -90 to positive 90 degrees.
If avF is positive, the vector is heading down and the vector is between 180 and 0 degrees.
When we combine the two and see where the overlap is, it will be from 0 degrees to positive 90 degrees. You can obviously use the QRS deflection in lead II to see if the axis is within -30 to 90 degrees.
Thumb method
Alright guys this is my favorite way. It is super easy and it is called the thumb method. It is a down and dirty way of doing axis. And great for beginners. We are going to only care about leads I and aVF. And we will be broadly looking at Normal, LAD, RAD, and Extreme axis deviation. Your left thumb will be for lead I and your right thumb for aVF. Make sure you try it! If the QRS is mostly positive, you put that thumb up and if the QRS is negative, you put their thumb down. For normal axis, lead one is positive so put you left thumb up. Lead aVF is also positive so put your right thumb up. Two thumbs up! Your axis is good! For LAD, Lead I is positive so put your left thumb up and aVF is negative so put your right thumb down. Your thumbs have LEFT each other. So left axis deviation (LAD). For right axis deviation look at lead one which is negative so put your left thumb down and aVF is positive so put your right thumb up. The thumbs are heading RIGHT towards each other so right axis. For extreme lead one is down so put your left thumb down and aVF is also down so put your right thumb down. Your patient is going down down and is having a very bad day. See that’s a little easier and a down and dirty way to remembering axis.
And that is all to know for Axis and Vectors! Remember to do hood rat medicine with your hood rat friends.
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