Comprehensive Guide to Vector Electrocardiography and Axis Determination

Welcome to this course on electrocardiogram,  interpretation and application in clinical   practice. In this lecture series, today's talk  will be on vector electrocardiography. And myself,   Dr. Brinda, Associate Professor,  Department of Physiology,  

Chettinad Hospital and Research Institute. The learning objectives of today's talk will be   electrical circuits and recording of electrical  events, principles of factorial analysis of ECG,   determining the axis, axis  deviation, and dextrocardia.  

To start with the electrical circuits  and recording of electrical events.   As we already know electrocardiogram is the  record of the electrical events of the heart.   These electrical events of the heart are mainly  due to the depolarization and repolarization  

of the muscles of the heart, that is, the  atrium and the ventricles. So first, let us   know how this depolarization affects  the ECG on the, when we try to record it   as an electrode. So when an electrode is placed  

on the surface of the body, the event that  happens in the heart will be recorded as an ECG.   So the first event that will be happening in  the heart is the depolarization event. So when   a wave of depolarization passes through the  heart muscle. As you can see in this picture,  

considering a heart muscle, as the wave of  depolarization goes towards the heart muscle   and an positive electrode placed in the center  of the heart muscle will record the ECG.   How does it record? So when the wave  of depolarization passes before the  

positive electrode, it will be indicated as a  positive deflection on the ECG. As this wave   of depolarization now passes along, that is, it  goes near to the positive electrode that is, it   is nearer to the positive electrode now. So when  it reaches the positive electrode, the deflection  

will come back downwards again to the baseline. Next, when the depolarization goes or passes away   from the positive electrode, the deflection on  the ECG will move downward and forms a negative   wave. Hence, when the complete depolarization of  the muscle take place, it will be recorded as a  

biphasic wave on ECG. So the final  depolarizing wave as it moves   perpendicularly to the positive electrode  will produce a biphasic wave on the ECG.   This is the basis of recording of ECG due to the  effect of depolarization of the heart muscle.  

As I said in this previous slide, considering the  same fact in the heart, now considering the heart   with a wave of depolarization, the depolarization  usually passes from the atrium to the ventricles.   And it is always downwards slightly towards  the left side. And when an electrode is placed,  

for example, electrode A is placed towards  or on to the direction of the depolarization,   the depolarization will be a  positive deflection on the ECG.   That is depolarizing wave toward a surface  electrode that is A, electrode A will be  

recorded as a positive deflection. Similarly, when  an electrode B, which is away from the wave of   depolarization will produce a negative deflection  in the ECG and any depolarization wave which moves   perpendicularly to the electrode considering  electrode C will produce a biphasic wave.  

So the effects of repolarization are precisely  the opposite those of depolarization.   So now considering heart as a 3-dimensional organ,  we keep the 12 standard ECG leads that is the   6 limb leads and 6 precordial leads  which we have discussed previously  

will view the heart at different angles and that  is called as angle of orientation. Considering   the 6 limb leads, which will view the heart  in a vertical plane that is called as the   frontal plane as depicted in this figure. These leads show the electrical forces of  

the heart moving up and down or left and right  and is viewed as a circle in the frontal plane   and the angle of these lead is determined  by drawing a line from positive to negative   electrodes. These principles are very important in  understanding the vectorial analysis of ECG.  

So considering and to determine the  angle of orientation for all the leads,   that is the limb leads and chest leads, let us  see what will be the angle of orientation. First   for the three standard limb leads that is  the lead I. So in this figure, the lead I,  

the left arm is positive and the right  arm is negative. Hence the angle of   orientation is at 0 degrees. For lead II the legs are positive,   the right arm is negative, hence the angle  of orientation is +60 degree. For lead III,  

the legs are positive, the left arm is negative,  and the angle of orientation is +120 degree.   Coming to the augmented limb leads first  with the aVL, the left arm is positive,   the other limbs are negative, and the angle of  orientation is -30 degree. For the augmented  

limb lead aVR, the other limbs are negative  and the angle of orientation is -150 degree.   For the lead aVF the legs are  positive, the other limbs are negative,   and the angle of orientation is +90 degree. So this picture now shows all the six leads in the  

frontal plane with their angles of orientation,  and it is necessary to memorize these angles   to determine the axis of ECG. To reemphasize  you can see the lead I is at angle of 0 degree,   lead II at an angular +60 degree which means  it views the heart at an angle of 60 degree.  

And lead III at +120 degree. Lead aVF  views the heart at +90 degree angle.   aVL at -30 degree angle and aVR at  -150 degree angle and I reemphasize   it is necessary to memorize these angles  to determine the axis of an ECG.  

So this slide again gives the chart on the lead  angle of orientation of the different leads.   Lead aVR is at -150 degree which is  considered as a right sided lead and   inferior leads are lead II at +60 degree,  lead III at +120 degree and lead aVF  

at +90 degree. The left lateral leads are lead  I at +0 degree and lead aVL at -30 degree.   So far we have seen the limb leads. Coming on to  the chest leads or the precordial leads. We have   already seen in the previous lectures, regarding  the positioning of all these precordial leads.  

So there are 6 precordial leads V1 to V6 which are  placed over the ventricle. For example, lead V1 is   placed directly over the right ventricle. Lead  V2 and V3 over the interventricular septum.   Lead V4 focuses on the apex of the left  ventricle and lead V5 and V6 over the lateral  

left ventricle. Both these limb leads and the  precordial leads will form a group of leads which   has been emphasized in this table. The anterior  group of leads will include V2, V3 and V4.   The left lateral group of leads includes lead  I, aVL, V5 and V6. The inferior group of leads  

includes lead II, lead III and aVF and the  right ventricular leads include aVR and V1.   So based on this, let us go on to understand the  principles of vectorial analysis in ECG.   To understand what is a vector, I would like to  give an analogue of a football game. Considering  

the football ground as the goalkeeper strikes  the ball we can see that the ball can go on to   any direction, but the final area where the ball  has to go is into the opposite net. So similarly,   the vector is the sum of all the average  forces of this direction. Similarly, in ECG,  

the ECG electrode records only the average  current flow at any given moment.   So what is a vector? The average  electrical movement which is represented   by a single arrow is called as a vector and  the vector is recorded by the ECG electrodes.  

This vector has two things, one is the angle and  the length. The angle of the vector represents the   average direction of the current flow, and the  length of the vector represents the voltage.   This vector is translated as ECG wave patterns  when recorded by the 12 ECG leads.  

As the ECG reads, records this vector, it is being  represented as waveforms and we know the waves of   ECG, which include P wave, QRS complex and T wave.  Now how can we apply this vector analysis on these   waves of ECG. Coming to the P wave, we know  that P wave represents atrial depolarization.  

So the vector current flow for this  is from the atria that is which points   down from right to left slightly inferiorly. So any lead that views this wave of depolarization   as moving toward it will record as a positive  reflection on ECG paper. In this figure,  

you can see that this wave moves towards the lead  I. Hence, it is recorded as a positive deflection   on ECG paper. So the P wave will be a positive  wave on lead I in the ECG. In the frontal plane,   additionally, aVL also will  have a positive deflection.  

Inferior leads like aVR will show a negative  deflection because the wave is moving   against the lead aVR. So it is a negative  deflection on the ECG, whereas lead III will   produce a biphasic wave because the angle is  perpendicular to the position of the lead.  

Considering the horizontal plane, that is with  the chest leads, V1 to V6, the left lateral leads   V5 and V6 will produce a positive wave as the  wave is towards those leads and the V1 lead   will produce a biphasic wave because this lead is  perpendicular to the wave of depolarization in the  

atria. Hence, the normal range of P wave vector  will always lie between 0 to 70 degrees.   Next wave of ECG is the QRS complex, which  is first initiated by the septal Q wave,   which represents the depolarization of  the interventricular septum and it is  

the first to depolarize and usually  begins from left to right direction   and this is viewed as a tiny negative  deflection in leads such as lead I, aVL,   V5 and V6. The remainder of the  ventricles will depolarize next.  

Then that will dominate the remainder of the QRS  complex especially the R wave. And the vector   current will also flow leftwards as you can see in  this diagram. And this vector points from 0 to 90   degree and it is considered to be normal. Coming to the R wave of the QRS complex,  

in the frontal plane, the R wave is considered  as a positive deflection in lead I and lead II   as the wave of depolarization is going towards  these two lead, lead I and lead II. Whereas,   the S wave will be a deep negative deflection  which will be seen in lead aVR lying rightward.  

In the horizontal plane, lead V1 and V2,  we will record a deep S wave as the current   is moving away from them and lead V5 and V6  will have a positive deflection as the wave   of current is moving towards them and that is  considered to be tall positive R waves whereas  

lead V3 and V4 they are called as transition  zones and they are called a biphasic wave.   This pattern of increasing in the R wave amplitude  from right to left in the precordial leads is   called as R wave progression, where V1 is the  smallest R wave and V6 is the largest R wave.  

Coming to the T wave which represents ventricular  repolarization, it has a variability in its   appearance. That is repolarization usually  begins in the last area of the heart to have   been depolarized and usually travels backwards. Because both an approaching wave of depolarization  

and the receding wave of repolarization  generate a positive deflection on ECG   and the same electrodes that will record a  positive deflection during depolarization which is   tall R wave will also record a positive deflection  during repolarization a positive T wave. It is  

typical and normal to find positive T waves in  the same leads that have the tall R waves.   To summarize the vector aspects of  electrocardiography, the P wave is   usually small and it is positive in left lateral  inferior leads. It is biphasic in lead III and V1  

and the normal range of P wave vector is 0 to 70  degrees. With the QRS complex it is usually large,   with tall R waves in left lateral and inferior  leads with R wave progression seen from V1 to V5,   and a small Q wave is seen as in one or  several of the left lateral leads.  

The vector for QRS complex always points  from 0 to 90 degree. This is very important   while we study the axis determination.  The T wave is variable with positive   in all leads, along with the R wave. Coming to the next concept that is  

determining the axis in a ECG. To determine the axis in an ECG,   we need to know what is the main electrical  axis. In ventricular depolarization, which is   represented as QRS complex in ECG, the first  vector, here you can see number 1 represents  

the septal depolarization and the successor  vector 2, 3, 4, 5 and till 8 represents the   progressive depolarization of the ventricles. You can see these vector will swing leftward   because the electrical activity of the left  ventricle is of larger and predominates the ECG.  

The average of all these vectors 1 to 8 at one  instant that is called as mean vector and the   direction of this mean vector is called as  the mean electrical axis and this is very   important in determining the axis of ECG. So now how to determine the axis of an ECG?  

The axis of an ECG is mostly determined by the  QRS complex wave. The mean QRS vector as we see   points leftward and inferiorly and it represents  the average direction of current flow.   We already seen the normal direction  of the QRS mean vector is between 0 and  

+90 degree that is from here, 0 to +90 degree.  So with this falls the normal QRS axis.   How to determine the axis based on this QRS  complex? Three methods are identified for axis   determination. First is the quadrant  method, second three lead analysis  

and third is the isoelectric lead analysis. Let  us see in detail about each of these methods.   First is the quadrant method of axis  determination. This is the most efficient   way to estimate the axis of an ECG. And  because it is using lead I and lead aVF.  

And in these leads, we examine the QRS complex  and determine whether it is positive. That is,   it is an upward deflection or a biphasic  deflection or a negative deflection.   So based on these, we will  determine the axis of an ECG.  

So when there is a positive QRS  complex in an ECG in lead I,   that puts the axis in the same direction as  that of the lead I that is at 0 degrees and   positive QRS complex in lead aVF will align the  axis with the lead aVF that is at +90 degree.  

So combining both these colored areas, the  quadrant of overlap. This is the quadrant of   overlap, which determines the axis of ECG and it  is normally between 0 to +90 degree. So if lead I   and lead aVF have positive QRS complex, then  the axis always lies between 0 and +90 degree  

and that is considered to be normal axis. The next step in determining the axis of an ECG is   by the three lead analysis method, which includes  three leads that is lead I, lead II and lead aVF.   In lead I, a positive QRS complex puts  the axis in the same direction as lead I.  

A positive QRS complex in lead II aligns it  with the direction of lead II at +60 degree   and lead aVF we have already seen. So when we combine all these areas,   the area of overlap will determine the axis as  before and if both lead I and lead II are positive  

the axis will lie between -30 and +90 degree  and that is considered to be normal. So it is   from here -30 degree to +90 degree that  is considered to be a normal axis.   The next method to identify is by means of an  isoelectric lead analysis. It is a more precise  

method of estimation of the axis. If  the QRS is positive at any given lead,   the axis will be in the same direction of the  lead. If the QRS is negative at any given lead,   the axis will be in the opposite direction.  This concept we have already seen before,  

I am trying to reemphasize again. If the QRS is biphasic, at any given lead,   the axis will be 90 degree or perpendicular to  that of the lead. So in this method, there are   three steps in determining the axis. Step 1,  you have to find the biphasic lead of the ECG.  

Step 2, you have to find the positively,  that is lead with the tallest R wave. Step 3,   you have to calculate the axis that is, the QRS  axis is plus or minus 90 degree perpendicular   to the biphasic lead, and the direction is always  pointing towards that of the positive leads.  

So based on this method, let us try  to solve this example to understand   the better way of determining the axis of an  ECG. So considering this ECG strip, how do you   determine the axis of an ECG by using the three  method. The first method is the quadrant method,  

where we use lead I and lead aVF. So let us  see what is the QRS complex in lead I.   So it is considered to be positive. How about lead  aVF? Here also the QRS complex is positive. So   coming to the next method, that is  the three lead analysis. In addition,  

we have to look at lead II also. So lead II  also gives a positive QRS complex. So since it   is positive in all three leads, the axis of the  quadrant usually lies between 0 and +90 degree   and it is considered to be a normal axis. Coming to the third method to have a precise  

estimation of axis by means of the isoelectric or  biphasic lead analysis method. In that the step 1   is to find the biphasic wave. In this ECG strip,  where do we find the biphasic wave? You can see it   is at the aVL. And step 2, you have to find the  positive leads. We already know there are three  

positive leads, lead I lead II and lead aVF. Step 3, the QRS axis it is at plus or minus   90 degree to that of the biphasic lead. So the  biphasic lead is aVL. So aVL is at -30 degree. So   this should be plus or minus 90 degree to -30. It  should be equivalent to +60 degree or -120 degree.  

The final step in analysis is, it is the direction  is always towards the positive electrode. That is,   the positive electrodes here are I, II and aVF. So the direction towards the positive electrode.   Hence, the QRS axis is considered to be  normal, that is at +60 degree and that is  

considered to be a normal QRS axis. So considering this, let us go to some   deviation aspects that is axis deviation. The normal QRS axis again I emphasize is between   0 and 90 degree with a positive QRS complex  in lead I and aVF. If the axis lies between  

90 and 180 degree that is called as a right axis  deviation here. And here how the ECG will appear,   lead I will be negative and lead aVF will be  positive. And if the axis lies between 0 and 90   degree that is called as a left axis deviation. And how does the ECG appear in lead I,  

it will be positive QRS complex and lead  aVF will have negative QRS complex.   There is another type of deviation called as  extreme right axis deviation. It happens rarely,   where the axis becomes totally disoriented  and lies between -90 and +180 degree. That  

is called as extreme axis right axis deviation.  Here the QRS complex will be negative in both   the leads that is lead I and lead aVF. Also the P wave has its contribution towards the   axis. The normal P wave lies approximately between  0 and 70 degree and the T wave axis is usually  

variable, and it approximates with the QRS axis  lying to about 50 to 60 degree of the QRS axis.   Why do we have to study the concept of axis? It is very important in clinical application   in diagnosing hypertrophy of the heart  and enlargement of the heart, especially  

right and left ventricular hypertrophy  and atrial enlargement.   So this is a summary slide which says about the  axis of the heart, most of which has been covered   in the previous slide. So axis refers to the  direction of the mean electrical vector and it  

represents the average direction of the current  flow. Three methods of determining the axis,   quadrant method, three lead analysis and  isoelectric lead analysis. The most precise   method is the isoelectric lead analysis and  you can memorize this table to understand  

what type of deviation could be possible in  relation to the lead I and lead aVF.   Now let us solve this graph of ECG to determine  the axis. So on this ECG comment on the QRS axis.   Tell if it is normal or is there any deviation?  So what type of method do we choose?  

We will do all the three methods to solve this.  First quadrant method, lead I and lead aVF. So   lead I, it is a negative wave and lead aVF it is a  positive wave. So when lead I is negative and lead   aVF is positive, we have seen in earlier that it  could be a right axis deviation. The next method  

three lead analysis, we would include lead  II here. In lead II it is a biphasic wave.   Hence, in right axis deviation, the quadrant  could be between +90 degree or +180 degree.   Coming to a precise estimation of axis  with isoelectric lead analysis method,  

step 1 we need to find the biphasic wave,  which we already know it is lead II. Step 2,   we will find the positive lead that is lead aVF  which will denote the direction of the axis.   Step 3, we have to take the QRS axis plus  or minus 90 to that of the biphasic wave.  

So here the lead II is biphasic. So it is +60  degree which is biphasic. So it should be plus   or minus 90 to 60, which will be equal to +150  degree or -30 degrees. So it should be either from   -130 or to +150 degree. But we already know  that the direction is always towards the  

positive lead. That is the positive  lead here is aVF, so the direction is   downward here with a right axis deviation. Hence, the correct axis for this ECG is at   +150 degree with right axis deviation. Based on this, I would like you to solve this  

ECG and practice on your own using the three  method of determination of the axis.   To complete the session, a few  points on dextrocardia.   Dextrocardia is defined as a condition where the  location of the heart is in the right hemithorax  

with apex pointing towards the right. We already  know that the apex is always pointing towards   leftwards. This is an abnormal condition where the  apex points towards the right. There are lot of   investigations that can be done to determine  this. A chest X-ray could show a mirror  

image of the heart on the right side. ECG is very classical,   it will show a right axis deviation and all  the complexes in the ECG will be inverted,   predominantly negative P wave QRS complex and  T wave in lead I and a positive QRS in lead aVR  

and there will be low voltage  V3 to V6 leads and reverse R   wave progression in the precordial leads. An x-ray abdomen can also be done which could show   fundal gas shadow on the right side and the left  dome of the diaphragm is at a higher level.  

Thank you for your patient  listening and a happy learning.