The cardiac axis made ridiculously easy

with the complicated geometry of the ventricles you can imagine that at each point in time there are vectors of different amplitudes showing in two different directions in the heart from all these momentary vectors an average vector can be constructed for each point in time the sequence of these average vectors over the time of ventricular depolarization enables us to construct the vector loop which you can see here the vector loop is the line that connects the tip of the arrows here the strongest or longest of these average vectors shown in red here is called the main vector and where this vector is showing determines the electrical axis of the heart and the frontal plane the most exact way would be to exactly determine and calculate the direction of the main vector in the frontal plane for example we could calculate if the main cardiac vector is showing into the direction of minus 60 degrees minus 90 degrees or plus 120 degrees on the Cabrera circle however this is way too time-consuming and really not worth the effort because there are only a few situations where knowledge of the X's really makes a difference and we'll get to those a little later but in almost all countries it is the rule to include information about the electrical axis in a complete ECG report so we should be able to find the most important abnormalities of the electrical axes and here is a simple trick how you can do it we should remember that if the cardiac axis is showing into the direction of a certain lead as lead 1 here in this example then the deflection in that lead will be mainly positive when the vector points away from that lead we will get a negative deflection in that lead of course the same also works fully to so if the main vector points into the direction of lead 2 then the 2 will be mainly positive taking these two informations together we can see what happens when lead 1 and need to have predominantly positive deflections let's check it out so we said lead 1 is mainly positive the Cardiac axis is showing somewhere in the direction of the yellow area now we take lead to and if Li 2 is also mainly positive we know that the Cardiac axis has to show somewhere at the intersection of the yellow and blue areas and the mixture of yellow and blue is green as we know so the Cardiac axis has to show somewhere in the direction of the green area which is minus 30 degrees to plus 90 degrees and since most vectors in humans show into that direction this is called a normal axis the terminology in nomenclature however varies in different medical schools and countries but we will use the terms mostly used in English and American textbooks so let's move on when we have lead 1 positive meaning that the Cardiac axis shows into the direction of lead 1 and lead to mainly negative meaning that the main cardiac axis shows away from lead - we know that the cardiac axis actually has to show at the intersection of these two areas at the dark yellow area here from minus 30 to minus 90 degrees and this area is called left axis deviation when lead 1 is negative we know that the main vector will be found in the gray area to be more precise we have to look at a VF now so when lead 1 is negative look at a BF and when lead 1 is positive we look at lead - as we've just seen so when lead 1 is negative we look at ADF and if ADF is mainly positive we know that the main cardiac vector has to show somewhere into the direction of this dark blue or dark gray area down here and this is called right axis deviation that's when the vector points into the direction of plus 90 to 180 degrees and lastly if lead 1 is negative and lead AVF is also mainly negative we know that the main cardiac axis has to show into this area up here and this area is actually called a Northwest axis it's actually quite rare to find an axis that points into the direction of the Northwest axis and you shouldn't bother about it too much for now so let's recap if the main cardiac vector points into the direction of the green area from minus 32 plus 90 degrees that's called a normal axis if the cardiac vector points into the direction of the yellow area from minus 30 to minus 90 degrees that's called left axis deviation and if the cardiac vector points into the direction of the blue area from plus 90 to 180 degrees that's called a right axis deviation and lastly if the main cardiac axis points into the direction of plus 180 or plus or minus 180 to minus 90 then that's called a Northwest axis you should only care about right axis deviation and left axis deviation for now why well actually if you have a normal axis then that doesn't help you that much in your diagnosis a Northwest axis is very rare but if you have a left axis deviation or right axis deviation that can really give you important clues as to the underlying diagnosis now let me show you a simple trick to determine the cardiac axis really really fast in most patients here's how it goes the first thing to do is to hold the ECG printout as shown here make sure that lead 1 is close to your left thump if lead 1 is positive then look at lead 2 which should be next to your right thumb if both leads are mainly positive as in this example and as we've already learned then the patient has a normal axis if lead one the lead to your left is mainly positive and lead to the lead to your right is mainly negative then you're dealing with left axis deviation if fleet one is mainly negative you should look at lead AVF instead of lead to make sure that the left thumb is next to Ligue 1 and that your right thumb is next to lead a VF in this case then we'll carry on just as before if the lead next to your left thumb that is lead one is mainly negative and delete next to your right thumb that is lead a VF is mainly positive then you're dealing with right axis deviation and if both leads one and AVF are negative then you're dealing with one of the rare cases of northwest axis some people also call this an extreme axis so here are the different instances again on the top half lead one is mainly positive so we have to look at lead to in addition to Ligue 1 if both leads one and two are mainly positive then the patient has normal axes if the left lead that is lead one is mainly positive and the right lead that sleeve 2 is mainly negative the patient has left axis deviation in the examples on the bottom lead one is mainly negative so we have to look at lead AVF in addition to lead one if the lead to your right that's lead a VF is mainly positive and delete to your left that's lead one is mainly negative the patient has right axis deviation and if both leads are mainly negative the patient has a Northwest axis one more thing sometimes you don't have access to lead a VF in which case you can use lead 2 instead of lead a VF so instead of switching to a VF when lead 1 is negative you stick with Li - that's not as ideal but you'll still get fairly accurate results as I've said there not a lot of clinical situations where knowledge of the cardiac axis really helps you come up with a better diagnosis but now let's turn to the clinical situations where knowledge of the cardiac axes really makes a difference situation number one this is the cardiac conduction system that's the AV node that's the bundle of his that's the left bundle branch which is subdivided into the left anterior fascicle left anterior fascicle and the left posterior fascicle left posterior classical and then there's the right bundle branch right bundle branch electrical depolarization that comes from the atria has to travel through the AV node and the bundle of his to the bundle branches to the Purkinje fibers now in any of these structures there can be so-called blocks that is when the depolarization cannot travel through the structure you can have an AV nodal block you can have a block in the bundle branches you can have a block in the left interior fascicle left posterior faster and also in the right bundle branch of course and then you can also have a combination of all those blocks and when you have a block of the right bundle branch and an additional block in one of the fastest we have what is called a by fascicular block and whenever that happens you get a deviation of the cardiac axes so let's assume you have a right bundle branch block you see this M form over the right ventricle in b1 and b2 and as we've learned that's a sign of right bundle branch block if in addition to that you also have a left axis deviation then that means that you not only have a block of the right bundle branch but also of the left anterior fascicle and this entity is easy to remember because there's a mnemonic that's called left left meaning when you have a block of the left interior fascicle you have a deviation of the cardiac axis to the left so left axis deviation is equal to left anterior Hemi block now let's turn to another scenario let's say you have right bundle branch block plus a right axis deviation what does that mean that means that you have a block in the right bundle branch plus a blow in the left posterior fascicle so the take-home message here is if you have right bundle branch block always look for axis deviation if you have left axis deviation that means that the left anterior fascicle is also blocked if you have right axis deviation that means that the left posterior vesicle is also blocked so that's the first situation in which knowledge of the cardiac axis helps you come up with more refined and better diagnosis now let's turn to the second situation whenever you suspect right ventricular strain or right ventricular hypertrophy it helps to look for the presence of right axis deviation so for example let's assume you have a patient who comes into the emergency room with symptoms that are compatible with a right heart problem with dyspnea and subtle signs of cyanosis you write an ECG and see signs of right ventricular hypertrophy with the tall R wave in v1 and a deep s wave in b5 and the R to s ratio is above 1 now this constellation of signs already suggests a high likelihood that something's wrong remember rrss criteria but when there is also right axis deviation you almost have certainty that something's wrong here and that the right heart has a problem so whenever you have a suspicion of a right heart problem of right ventricular hypertrophy for example with a V 1 and V 5 that look like this plus a right axis deviation then you have almost certainty that right ventricular hypertrophy is present in other words signs of right ventricular hypertrophy + right axis deviation increases the likelihood of right ventricle hypertrophy let's turn to situation number 3 let's assume you have a patient who has this ECG B 1 B 2 B 5 B 6 what's the problem here right this ECG suggests left ventricular hypertrophy the Sokolov index is positive the r MV v is 2.2 millivolt the s in v2 is 3.1 bit millivolt 3.1 plus two point two equals five point three and five point three is way over our threshold of 3 point 5 millivolts so we have a strong suspicion of left ventricular hypertrophy now let's assume that this patient also has right axis deviation what does that mean well it means or suggests that we have by ventricular hypertrophy that both the left and the right ventricle a hypertrophic these were the situations where knowledge of the electrical axis will provide you with valuable information what do I mean when I say valuable information well I'm talking about information that will actually help you to refine your clinical diagnosis and make it better so we can now include this new information into our cookbook let's see what has changed so here's what has changed in the cookbook we now can add item number five to our cookbook approach and that's the assessment of cure s axes you should use the step that you have now learned in order to assess the cardiac axis using lead one two and ABF and four now you should just look for the presence of left axis deviation or right axis deviation the other two situations are normal axis and the northwest axis will not add a lot of additional information to your clinical judgment so what's up next well I'd like you to watch the second video in this chapter on atrial enlargement and low voltage and after that there are some juicy cases and exercises waiting for you so take care and I'll see you later

Channel: Medmastery

Views: 207,189

Keywords: EKG, ECG, electrocardiography, clinical medicine, clinical skills, USMLE, cardiology, internal medicine, medical students

Id: WjTyiaTJs9k

Length: 15min 21sec (921 seconds)

Published: Tue Jun 28 2016