Comprehensive Guide to ECG Lead Systems and Their Clinical Importance

 Good morning.   I am Dr. A. Ahamed Basha, Professor of Physiology,  Chettinad Hospital and Research Institute. Today   I will be speaking about ECG lead system.  ECG, an electrocardiogram is a record of cardiac  

electrical activity and this electrical activity  is recorded, and the procedure of recording   is electrocardiography. It is a non-invasive  procedure. This electrical activity is detected   by placing an electrode over the body surface, and  that electrode picks up the electrical potential  

and it is transferred to the ECG machine,  electrocardiograph. ECG machine records   the electrical activity in a moving paper.  Electrodes: Electrode is a conductor material   which conducts electrical potential  over the body surface. So,  

whenever an electrode is placed over the body  surface, it conducts cardiac electrical potential   and then it transmits to the ECG machine.  Lead: A lead is 2 or more electrodes which   one end is connected to the body surface and the  other end is connected to the ECG machine.   

So, if there are 2 leads, the potential  difference between those 2 leads are picked   up from the body surface and it is conducted to  the ECG machine. In turn, ECG machine records the   electrical activity in a moving paper.  Willem Einthoven, inventor of practical  

electrocardiogram; he invented the practical  electrocardiogram and he was awarded a Nobel   Prize in the year 1924 for his discovery of  the mechanism of electrocardiogram.    So, what are the basics of ECG leads? Heart  generates its own electrical potential.  

That electrical potential is picked up by the  electrodes. We are placing the electrodes over   the body surface, so, whatever the electrical  potential is generated from the heart, it is   viewed by the electrodes in a 3-dimensional  manner. That means, if 2 electrodes are placed,  

the potential travels towards one direction.  If the depolarisation, that is, a spread of   depolarisation, if it is towards a positive end,  then we get a positive wave. If the spread of   depolarisation is towards a negative pole,  then we get a negative wave in the ECG. If  

the spread of the depolarisation wave is  perpendicular to the lead, then we get a biphasic   deflection. So, depending upon the conduction  of impulse towards the positive or negative,   the waves of ECG, either a positive wave  or negative wave or a biphasic wave.   

So, Standard lead system: The standard  lead system is called as 12 lead system.   It is classified into 2 categories: 1-limb  leads; other one is chest leads. Limb   leads indicates leads placed over the limbs. Chest  leads are the leads placed over the precordial  

surface, that is, on the chest area, over the  chest area. The limb leads again classified into   bipolar limb leads and unipolar limb leads.  We can see one by one, what is bipolar and what is   unipolar, in the subsequent slides. Unipolar chest  leads are placed over the precordial surface.  

So, these are the limb leads;   these are the areas to place the chest leads,  precordial area to place the chest leads.    d from the same electrode as L I, L  II and L III, but the difference is,  

it is these waves are augmented waves when  compared to the L I, L II and L III. So,   they are called as unipolar limb leads. Here  also, the right leg serves as an earth, earthing   purpose; it acts as a reference electrode.  Next, we move on to the unipolar chest leads.  

Unipolar chest leads name indicate these leads So, bipolar limb leads: Bipolar limb leads are   indicated as L I, L II and L III. Electrodes  that form these signals are located on the limbs.   Unipolar limb leads: In unipolar limb  leads, the electrodes are same as  

L I, L II and L III, but here, the electrodes,  that is, 2 end, 2 terminals are connected to a   common end with a resistance closer to 0, and the  other terminal is connected to the positive pole.   So, that is about the unipolar limb leads.  So, unipolar limb leads are indicated as aVR,  

aVL and aVF; a stands for augmented. Unipolar  chest leads: These leads, as I mentioned earlier   are placed over the precordial surface. They  are indicated as V1, V2, V3, V4, V5 and V6. So,   it is indicated from V1 to V6. Here, the Wilson  central terminal is used as a negative pole.   

Next we move on to the Einthoven's law. As I  mentioned earlier, Einthoven is the father of   practical electrocardiogram. He constructed  an imaginary triangle and assumed that   heart is at the centre of that triangle. So,  he placed heart as the centre of the triangle,  

and all the 3 ends of the triangle, he connected  the limb leads. If an ECG taken simultaneously   with the 3 limb leads at any given instant, the  potential of 1 lead is equal to the arithmetic sum   of the potential of other 2 leads.  In general, we used to say as lead II equal to  

lead I plus lead III. So, lead II is  equal to the potential difference of   lead I and potential difference of  lead III. This is called as Einthoven's   law, L II equal to L I plus L III.  So, bipolar limb leads: In bipolar limb leads, as  

I mentioned earlier, it is indicated as L I, L II  and L III. Now, we can see what is L I, from where   to where it is connected and what is the potential  difference. L I lead is connected between   left arm to the right arm, and L I lead  view the heart from the superior aspects.  

Next, L II, that is lead II is connected  between right arm and the left leg. This lead   views the heart from the right side; that is,  it views right side aspect of the heart.    Next, lead III. Lead III is connected  between left arm and left leg.  

It views the left side of the heart, that is,  left aspect of the heart. So, these 3 leads   view the heart from the 3 views; one from lead  I, superior aspect; lead II, right aspect;   and lead III, left aspect. Here, one more lead  is there, that is right leg. In the ECG, right  

leg lead serves as an earth for the reference  electrode, act as a reference electrode.    Next we move on to the unipolar  limb leads. In unipolar limb lead,   1 terminal is connected to the positive; that  is, 1 limb is connected to the positive and  

the other 2 limbs are connected through a high  resistance, making the potential close to 0. So,   what is the use of this? It increases the voltage  recorded. It increases about 50% of the voltage   recorded when compared to L I, L II and L III.  So, aVR: Here the positive limb is right arm.  

The aVL: The positive terminal is left arm. In  aVF, the positive terminal is left leg. So, here,   aVR, aVL and aVF are recordeare placed over  the precordial surface. It is indicated as   V1 to V6. V1: V1 is placed at the level of fourth  intercostal space nearer to the sternum. V1: Right  

fourth intercostal space nearer to the sternum.  V2: Exact opposite to the V1, but on the left hand   side. So, the location for V2 is left fourth  intercostal space nearer to the sternum.    Next, we move on to the V4. We can see V3 later.  V4 is placed at the fifth intercostal space,  

left hand side, at the level of the  midclavicular line. Mid clavicular line is an   imaginary line that falls from the centre of  the clavicle downwards towards thorax. So, V4   is placed at left fifth intercostal space  at the level of mid clavicular line.  

Now V3: V3 has to be placed just centre in between  V2 and V4. So, V3 is placed between V2 and V4.    Next, we move on to the V5. V5 is also placed at  the same plane as V4 but at the level of anterior   axillary line. Anterior axillary line is also  an imaginary line in the front of the axilla.  

So, same plane as V4 but little lateral at the   level of anterior axillary line is V5. Next lead  is V6. V6 is also in the same plane V4 and V5, but   at the level of mid-axillary line.  So, the summary of the leads are: L I, from  

left arm to the right arm, that is, the potential  difference between left arm to the right arm;   L II, from the right arm to the left  leg; L III, from the left arm to the   left leg. So, the potential differences between  these 2 leads are recorded as L I, L II and L II.  

The next unipolar limb leads are aVR for right  arm, aVL for left arm, aVF for left foot.    The precordial leads are V1 to V6. So,  these are the 12 lead system in a standard   ECG. We can ask, why these 12 leads are being  used? What is the importance? These 12 leads  

viewing the heart at a different angle.  Till now, we saw about various lead system; that   is, various leads, 12 leads, what are the names  of the 12 leads and where it has to be placed.   Now, we can get a question, why  there are 12 leads? What is its  

importance? The importance of 12 ECG leads are,  it records the electrical activity of the heart   from different angle and therefore they align with  the different anatomical areas of the heart.    For example, any 2 leads which view a particular  area or particular wall; for example, if any 2  

leads view the anatomical wall, then they are  said to be contiguous. In the 12 lead system,   there are groups of leads which view the  same wall of the heart; they are said to be   contiguous. Example, septal leads. In septal  leads, V1 and V2 are said to be contiguous.  

If any 2 leads view anterior wall, then  they are called as anterior leads.    Example: In the 12 lead system, V3 and V4 are said  to be anterior leads. Similarly, lead II, lead III   and aVF as inferior leads; lead I, aVL, V5 and  V6 are lateral leads. So, all the leads view the  

heart from a different angle. It gives a graphical  representation through ECG machine.    Till now we saw 12 lead systems in ECG  and their placement and their importance.   Now, we are moving on to the  additional lead placement.  

There are several additional lead placement  techniques, but here I am going to say only a   few important additional lead placements. That is,  the same electrodes are being placed in different   areas of the precordial surface and  we are going to learn one by one.   

So, right sided ECG electrode placement is  one of the additional lead placements. Here,   a complete set of right sided lead is obtained  by placing V1 to V6 in a mirror image position   on the right side of the chest; so,  which is the same precordial recording,  

but it is opposite to the standard ECG precordial  recording. So, what is the importance? It is more   important, especially in the case of diagnosis of  right ventricular myocardial infarction, where the   amplitude of ST elevation is higher when compared  to the normal precordial chest leads.   

Next additional lead is V4R ECG lead placement.  Here, use of right sided precordial lead   V4R is alone kept on the opposite side when  compared to the normal precordial ECG recording;   V4 is kept exactly opposite at  right side. It is helpful for the  

diagnosis of right ventricular involvement  in inferior myocardial infarction.    Next additional lead is posterior lead.  In posterior leads, they are indicated as   V7, V8 and V9. V7 is placed at the same level of  V6, but at the level of posterior axillary line.  

Next, V8: V8 is placed in the same level of V6  in the left hand side, but at the tip of scapula.   Next lead is V9, which is placed at  the level of V6 on the left side but   nearer the paraspinal region; that is, left  paraspinal region in the same horizontal plane as  

V6. So, what is the importance of these  posterior leads? The degree of ST elevation seen   in V7 to V9 is required to make the diagnosis  of posterior myocardial infarction.    Next additional lead placement is Lewis lead. This  lead is useful in detecting the atrial activity  

and its relationship to the ventricle activity.  The electrodes for Lewis lead are the same   limb lead electrode but the electrode placement.  Example: Right arm limb lead electrode placed   over the manubrium; left arm limb  lead electrode placed over the  

right fifth intercostal space; left leg electrode  placed over the right lower costal margin.    The recording of Lewis lead read in the lead I  of the ECG. It is important especially during the   atrial flutter waves; that is, for  the diagnosis of atrial flutter,  

this Lewis lead is highly beneficial.  Another additional lead system is 3-electrode   system. Here, in the 3-electrode system, 3  limb electrodes: right arm, left arm and left   leg are used and the monitor displays the bipolar  leads, that is, lead I, lead II and lead III.  

The next additional lead system is  5-electrode system. As the name indicates 5,   here it uses 5 electrodes, right arm, right  leg, left arm, left leg and any one chest   electrode, depending upon the requirement. So,  these are the additional lead system.   

The other additional leads are  esophageal lead. In esophageal lead,   a lead inserted into the esophagus approximately  35 centimetres from the external nostrils. This   ECG lead is beneficial especially in the diagnosis  of atrial flutter and Wolff-Parkinson-White  

syndrome. Next is intracardiac electrocardiogram.  Here, ECG along with the standard ECG electrodes,   some added electrodes are placed inside  the heart; that is, 2 to 4 intracardiac   electrodes are added via cardiac catheterisation.  A record of changes in the electrical potential  

of specific cardiac loci is done with the  help of intracardiac electrocardiogram.    This slide shows lead II ECG. That is,  in a recording of 12 lead ECG system,   if I take a lead II recording alone, it will  represent a graphical representation like this.  

If you see the graphical representation of  a lead II ECG, I can see some waves with   upward deflection positive waves and some  downward deflection negative waves. The waves   are labelled as P wave, Q wave, R, S, T and U.  Generally, this QRS is called as QRS complex.   

So, any ECG recorded from a 12 lead ECG system,  it contains P wave, QRS complex, T wave and   U wave, but in some cases, U wave may not be  visible, but still, U wave has to be considered.   So, with this, I am concluding this module. We  learnt about what are the ECG lead system.   

But this presentation is to be used  as an adjunct to other learning   methodology and not a substitute for  an appropriate clinical training and   qualification.  I would like to thank  

my institute, Chettinad Hospital and Research  Institute and Swayam NPTEL for giving me an   opportunity to present this session. The sources  of references are Guyton and Ganong. Thank you.