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ah today's lecture on EKGs will cover EKG leads this lecture will be relatively packed with important information the learning objectives will include first to understand the basic concept of an EKG lead next to know the 12 conventional EKG leads and the angles from which each lead views the heart's electrical activity to know the proper location of the ten conventional EKG electrodes and finally to be able to correlate specific groups of leads with anatomic regions of the heart let me start by explaining what an EKG lead is a lead is a view of the voltage between two points known as poles as a function of time the positive pole for each lead is marked by a single recording electrode on the body the negative pole for each lead is marked by either a single recording electrode or a virtual electrode known as a central terminal which averages input from multiple other electrodes although they are frequently used interchangeably a lead is not the same as an electrode an electrode is the physical conductive sticker attached to either the chest wall or a limb through which electrical potential is measured different leads that is different combinations of positive and negative poles allows the clinician to view the heart's electrical activity from different directions in the event that your personal background in circuits is a little shy of an electrical engineering student some of this may already be sounding difficult to grasp don't worry I'm going to slow down just a little bit and we will be revisiting this description throughout this lecture if you stick with it I guarantee it will make more sense by the end since this lecture is covering proper placement of electrodes I wanted to show what the setup looks like for recording an EKG for those of you who haven't actually seen it yet first we need a patient I hope this cartoon figure will suffice next we need an EKG machine the many different shapes and sizes of devices that record EKGs the ones used in most American hospitals are typically the shape and size of an unusually large laptop computer often attached to a small rolling cart as opposed to the coffee table pictured here now the patient needs some electrodes as mentioned earlier a conventional 12-lead EKG uses 10 electrodes these usually look like rectangular black stickers one on each limb and six across the anterior and left lateral chest wall each electrode is connected by a separate wire to a junction shown here which then connects to the machine the electrical recording then shows up on the screen and if satisfied with the quality of the data recorded the clinician can print a hard copy usually on red or pink grid paper here is what a 12-lead EKG looks like this is exactly what prints off the machine in the separate lecture on EKG waveforms I reviewed what electrophysiological event each of the individual little squiggles represents now let's talk about why the waveforms look different as one moves across the page that's because as one moves across the page the leads that are displayed changes typically every 2.5 seconds for example first are these three leads which are known as the standard limb leads or occasionally the bipolar limb leads they are designated by the Roman numerals for one two and three after 2.5 seconds of that recording next across are the augmented or unipolar limb leads AVR AVL and AVF I'll discuss what those letters stand for in a few moments then there are the six precordial leads labeled v1 through v6 finally on the bottom half of the EKG are the rhythm strips unlike the top half of the EKG in which each lead is recorded for two and a half seconds the bottom half shows ten continuous seconds of three specific leads this specifically helps with the identification of unusual rhythms particularly those that are irregular typically leads b12 and v5 are the three leads shown in the rhythm strips those are not random choices as v1 and lead two are the leads in which the HR activity is usually the most prominently seen I don't know this for a fact but I would guess that b5 is included as it comes close to having an orthogonal direction to the other two as we'll soon see I won't talk any more about the rhythm strips today but they will come up from time to time in future EKG lectures I'll now discuss the three major types of leads starting with the standard limb leads for reference here is a torso with a ribcage drawn in and I'll make it just slightly transparent to reveal the approximate location and orientation of the heart inside the standard limb leads date all the way back to 1903 when wilhelm eindhoven invented the first practical EKG he attached recording electrodes to the right arm left arm and the left leg in order to create the three standard limb leads first lead one has the right arm as the negative pole and left arm as the positive Pole next lead two has the right arm as the negative Pole and the left leg as the positive Pole finally lead three has the left arm as a negative pole and the left leg as the positive pole the electrode on the right leg isn't used as part of a lead but is rather used to ground the system if your knowledge of electric circuits is limited just remember that the right leg is not included in the leads but the EKG will not record unless the electrode is attached these three leads make a nice little equilateral triangle but I can imagine that some of you are scratching your heads because the left leg is obviously not located at the umbilicus is shown and therefore these leads as shown are a substantially inaccurate ideal realization of what the geometry actually is this is something that nermeen quite a bit when first learning EKGs seeing lots of diagrams very similar to this one all I can suggest is not to worry about it and just accept that this triangle is actually a much better description than it seems it should be in fact this triangle is so important that is given a specific name autobahns triangle and I know opens triangle demonstrates an important relationship called ein Tobin's law it is usually stated simply as lead one plus the three equals lead to what this means more specifically is that at any given time the voltage as measured by lead 1 plus the voltage as measured by lead 2 must equal the voltage as measured by lead 3 I'm going to set the standard limb leads aside for a moment and move on to the Augmented limb leads one of the early challenges I Tobin faced was that his three leads only provided a very limited view of the heart's electrical activity it could only observe that activity from three directions in order to increase the number of directions from which the heart can be viewed the EKG machine also uses a central terminal as mentioned before the central terminal is made by connecting the right arm left arm and left leg electrodes together this results in a virtual negative terminal that is composed of the average of the electrical potentials from the three limbs the creation of the central terminal allows the recording of three new limb leads one for each of the right arm left arm and the left leg with the F of a V F standing four foot by definition the leads ABR and AVL are 30 degrees away from the horizontal well see in a minute how that angle can be defined so precisely with the Augmented leads as you might guess the a in front stands for augmented this refers to the fact that the voltage recorded by these leads is too small to be compared alongside the other leads so they must be amplified this was originally done by manual rearrangement of resistors but is now done automatically with a microprocessor the V that sits in the middle of the three-letter acronym used for each augmented lead apparently stands for voltage and is present in any lead that uses the central terminal as the negative pole including the precordial leads as with I Tobin's triangle this collection of leads has an interesting and important relationship a VR plus a VL plus a VF equals 0 that is at any given point in time the voltages as measured by the three augmented limb leads must cancel each other out we can now combine the augmented limb leads and the standard limb leads into a unified reference system known as the hex axial system imagine a two-dimensional plane using polar coordinates such that zero degrees is defined as pointing to our right but the patient's left and positive 90 degrees is pointed straight down now superimpose the directions of those six limb leads this is the hex axial system let me give a simple demonstration as to how the hex axial system allows us to understand the patterns of electrical signals in the heart during ventricular depolarization the first major event is a left-to-right depolarization of the interventricular septum as mewed from the front of the patient the wave of depolarization from this event is angled down and to the right the next major event is the near simultaneous depolarization of the bulk of the right and left ventricles the next depolarization vector for this event will be the sum of the depolarization vectors for each ventricle as the left ventricle is normally much more massive the sum is weighted in a decidedly leftward direction what do these two electrical events look like as viewed from each of the six limb leads first in AVL the initial event of septal depolarization is in a direction nearly opposite that of the AVL lead therefore there will be an initial negative deflection on the EKG recording of that lead despite that the two directions are nearly antiparallel that is directly opposite the amplitude of the negative deflection is still only moderate because the amplitude of the depolarizing wavefront in the heart is relatively small the second depolarization event is of a sort of similar direction as AVL so therefore AVL will record a later positive deflection the amplitude of which is similarly influenced by both the angle and magnitude of the depolarization vector I'll now walk my way around the EKG leads because the first depolarization vector is not in as complete in the opposite direction from lead one as it was relative to AVL the initial negative deflection in lead one is not quite as deep as AVL however the second depolarization vector is in nearly the same direction as one so therefore the later positive deflection as recorded by this lead is taller than it was into a BL when it comes to lead to the lead is nearly perpendicular to the first depolarization vector therefore there is negligible initial deflection the second depolarization vector makes a similar angle to two as it did with AVL therefore the later positive deflection in lead 2 is similar in amplitude as that in AVL in AVF both the first and second depolarization vectors are in a sort of similar direction as the lead therefore both deflections are positive but are relatively low amplitude in lead 3 the first depolarization vector is in nearly the same direction as the lead so the initial deflection on EKG is positive and large relative to the initial deflection in other leads the negative depolarization vector is sort of angled away so later deflection on EKG is negative finally in the AVR the first depolarization is perpendicular to the direction of the lead so there is negligible initial deflection the second depolarization vector is in a nearly completely opposite direction so that second deflection on the EKG is negative and very prominent one way to check your understanding of how these leads look at the heart's electrical activity is to pause the video at this point and double check the mathematical relationships between the leads that is at any given point in time the amplitude of lead one plus that in lead three should equal that in lead two also at any time the amplitudes in AVR AVL and AVF should cancel one another interestingly modern EKG machines only directly measure leads one and two and the other four are calculated using these equations you should not bother committing these equations to memory I have only listed them here to help you understand how we can know the exact angles at which these leads intersect they aren't based on the individual anatomy of the patient as much as precisely defined mathematical relationships so far all of the leads I have been focusing on that is the six limb leads all lie in the same plane known as the frontal plane it is the imaginary plane that divides the body into front and back halves if these were the only leads available to us we would only have an understanding of the heart's electrical activity in two dimensions in order to observe and understand it in three dimensions another set of leads within a different plane are used these are the precordial leads there are six precordial leads which examine the heart's electrical activity in the transverse plane sometimes known as the axial plane the positive pole for each precordial lead is added location on the anterior or left lateral chest wall while the negative pole for each is the central terminal the leads are named v1 through v6 as moving right to left the specific location to place the electrodes for the precordial leads is very important electrode v1 should be placed in the fourth intercostal space just to the right of the sternum electrode v2 it should be placed in the fourth intercostal space just to the left of the sternum v4 goes in the fifth intercostal space in the midclavicular line b6 in the fifth intercostal space in the mid axillary line b3 goes halfway between v2 and v4 and finally v5 goes halfway between v4 and v6 I feel compelled to stress that the placement of these six electrodes by various healthcare professionals is usually incorrect the most common mistake is to place v1 and v2 in the second or third intercostal space instead of the fourth another frequent mistake is for v5 and v6 to not be lateral enough while these errors won't lead to erroneous conclusions about the rhythm and are unlikely to lead to erroneous conclusions about bundle branch blocks and ventricular hypertrophy small missed placements of precordial electrodes can dramatically change the appearance of t-waves the consequence of this particularly when comparing a correctly recorded EKG to an incorrectly recorded one on the same patient but at a different time will be in an erroneous conclusion about the presence or absence of myocardial ischemia this can lead to the inappropriate triage of someone with chest pain or possible angina equivalent so that is our set of 12 EKG leads 6 limb leads in the frontal plane and 6 precordial leads in the transverse plane how do these two sets of 6 relate to one another we'll need to understand that in order to understand the true representation of cardiac electrical conduction in 3-dimensional space here's the frontal plane with its leads and here's the transverse plane with its now here they are overlaid on one another I wish that was good enough with 3d modeling software to make this rotate around but hopefully this view provides you an idea of the spatial relationship between leads understanding the spatial relationship isn't just an academic exercise it also is necessary to appreciate the atomic correlation between groups of leads and specific parts of the heart the best EKG leads to probe a specific chamber or wall of the heart are usually those lying most directly over top of it because of this leads are often grouped anatomically for example leads to three and a VF are known as the inferior leads as they most directly evaluate the inferior wall of the heart v1 and v2 are known as the septal leads as they lie over that structure v1 and v2 also lie over the right ventricle and are the primary leads to examine when right ventricular pathology is suspected b3 and b4 are the anterior leads as they lie over the anterior wall of the left ventricle the tricky group are the lateral leads hopefully with a three-dimensional representation here you can appreciate that leads 1 AVL v5 and v6 are all in a similar direction and are the best leads to evaluate pathology of the lateral wall of the left ventricle and more generally the left ventricle as a whole finally as you can see AVR is sort of the odd one out with no other leads in a similar direction and thus it has no group assignment I'm going to end this lecture by returning to a full 12 lead EKG to review the anatomic correlations in this format once again leads to three and a VF are the inferior leads v1 and v2 are the septal leads v3 and v4 are the anterior leads 1 AVL v5 and v6 are the lateral leads and the finally AVR is ungrouped with some experience these anatomic groupings will become second nature that concludes this lecture on EKG leads please remember to like or share the video if you found it helpful or post a comment if you have any questions at this point I recommend continuing on to the lecture on rates and axis you you