When you look at a Full EKG (that page with a bunch of different squiggly lines on it) you notice that each lead's squiggly line is shapped differently. This is because they are examining the heart from a different angle.
Imagine you are sitting at home reading this blog article about electro-chemical activity in the heart and your spouse or lover comes in to tell you s/he bought you a new car!
You are supprised and excited the first question you ask is, 'What kind of a car is it?'
Your loved one says excitedly, 'go see'
It's sitting in the driveway right now. You run out the front door and you see the outline of the car from the front: the windshield and the headlights. From here you will probably be able to tell if the car is a passenger sudan or an SUV/Truck. But to make sure you need to examine the car from the side.
In awe, your eyes are fixed on the car as you walk diagonally towards the side of the car. Now you can see the outline of the car from the side: the wheels and the sidedoors. From here you will probably be able to identify the car's features and further diffentiate the car type: For example, if it has an open truck bed, hatchback etc.
But if you still aren't sure of the make or model. You continue to walk towards the back of the car. Now you can see the rear tires, license plate, and the make/model.
You identify the car as the one you've wanted since you turned 16 (but hadn't been able to buy) and you turn to your loved one and say: 'What the flip were you thinking?! We can't afford this!'
The same principles involved in you identifying the new car apply to identifying the patterns of electrical activity in the heart on a Full 12-Lead EKG.
Electrical activity in a heathy heart starts in the atria (with the SA node in the upper righthand corner) and decends downward and to the left to the apex of the heart (I had to draw an arrow on my chest to visualize it).
A electrode sensor placed on the left side of the body will detect this electrical movement as an upward spike.
A sensor placed on the right side of the body will detect this electrical movement as a downward spike.
But it gets more specific then simply looking at the heart from either the right or the left side. Cardiologists (heart doctors) often need to pinpoint an ischemic area, or EKG technicians need to identify a specific kind of heart conduction abnormality that may not be visible from a simple side view.
This is why several different Limb and Chest Leads were developed.
Limb Lead Sensors-
Along with the diagonal (right upper to left lower) arrow on my chest, I also drew on my arms. RA, LA, RL, LL. These represent the locations on my limbs where electrode sensors could be placed if I were to have my heart examined via EKG-Limb Leads.
Right Leg Sensor-
Because the electrical activity moves from upper right to lower left, though, the right leg doesn't sense much of it (cuz it's traveling in towards the left leg). This means that the right leg electrode (RL) is not usually used. And when it is, it's usually used as a ground (negative) sensor.
Left Leg Sensor-
This location is used as a positive electrode sensor in leads II and III and AVF. It is used as a negative electrode sensor in leads AVR (along with the negative sensor in the left arm) and AVL (along with the negative sensor in the right arm).
Right Arm Sensor-
This location is used as a negative electrode sensor in leads I and II, as well as in AVF (along with the right arm(, and AVL (along with the left leg). It is used as a positive electrode sensor in the AVR lead.
Left Arm Sensor-
This sensor is used as a positive electrode sensor in lead I, and as a negative (ground) sensor in lead III. It is also used as a negative sensor in AVF (along with the right arm) and AVR (along with the left leg), and it as used as the positive sensor in AVL.
(Whew! It's a good thing the machine is in charge of determining the polarity of the sensors. I just want to know what the squiggly lines mean)
Bipolar Limb Leads-
Lead I- Because this lead is taken from the right arm to the left arm (with the right arm being negative and the left arm being positive) it captures the movement of the heart's depolarization wave from the left side of the heart. PQRST complexes appear roughly in their characteristic forms as described in Part 1.
Lead II- Because this lead is taken from the right arm to the left foot (with the right arm being negative and the left foot being positive), it captures the movement of the heart's depolarization wave from roughly the front/right angle of the heart (or maybe its the bottom, hard to tell from a two dementional picture). From this lead, the P Wave and the T wave of the PQRST complex appear to have a higher ampletude then compared to Lead I. The QRS complex also has a slightly different shape; although still tall and narrow, the R wave has a higher ampletude and the S wave has a lower negative ampletude.
Lead III-
This lead from the left arm to the left foot (with the left arm being a negative electrode sensor and the left leg being a positive electrode sensor). It captures electrochemical 'movement' of depolarizing cells from the lower right side of the heart. When the PQRST complex is seen from this lead, the P Wave is even taller then when it was seen from lead II, the T wave is of a lower ampletude, and the QRS complex is similar to the QRS complex as seen in lead II.
Unipolar Limb Leads-
AVF-
This lead runs between the two negative (ground) electrode sensors in the right and left arms to the positive sensor electrode in the right foot. It captures the electrical wave of depolarization from the front/center of the heart. This view appears similar to those in leads II (because it is right next to it, in between leads II and III).
AVR-
This lead runs between two negative electrode sensors in the left arm and left leg to a positive electrode sensor in the right arm. It captures the movement of depolarization energy from the upper righthand corner of the heart. When seen from this view, the PQRST complex has inverted P and T waves. The QRS complex has a prominent Q wave, and an R wave.that is of a smaller ampletude than leads II or III.
AVL-
This lead runs between two negative elecrode sensors, one in the right arm and one in the left leg, and a positive sensor in the left arm. It captures the wave of depolarization through the heart from the upper left corner. The PQRST complex has an inverted P wave and an upright T wave, the QRS complex is of a smaller ampletude (compared to leads I, III, AVF) and the R wave is more prominent than the S wave.
Chest Leads-
They are placed in six different positions on the chest (From right to left, numbered V1-V6).
Unlike the bipolar limb leads, the chest leads only have positive electrode sensors placed in six different positions on the chest. These positive electrode sensors make a positive deflction on the EKG squiggly line.
This means that the leads placed on the right side of the heart (V1 and V2) will have QRS complexes with prominent S waves and diminished Q waves. As the placement of subsequent electrodes moves to the left side of the chest (where the left ventricle and apex of the heart are) the QRS complexes will look more tall and peeked (because the depolarization wave is moving toward them!)
I can now sense the pattern that, because the left ventricle is responsible for cardiac output, the "classic" view of the PQRST complex from an EKG/telly box is taken from the leads that view the heart from this side. Good to know!
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