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Basics of 12 Lead ECG's
Vector
There are three basic "laws" ofThere are three basic "laws" ofElectrocardiography.Electrocardiography.
These "laws" have to do with the direction of flow ofelectrical conduction in the heart or what we termas vector.
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1. Movement of the electrical impulsetowards the positive electrode will result in a
positive deflection on the ECG.
- + =
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Vector
2. Movement of the electrical impulse toward thenegative electrode will result in a negative
deflection on the ECG.
- + =
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Vector
3. Movement of an electrical impulse perpendicular to aline between the positive and negative electrodes
results in a bi-phasic (part upright and partdownward) deflection on the ECG.
- + =
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VectorBy understanding this concept of vector and knowing the"normal" vector of each of the 12 leads of the 12 lead ECGit becomes quite easy to identify problems in the directionof depolarization. It is also from this knowledge that one
identifies the Mean QRS Axis. Using the six limb leads weidentify the axis in relation to what is known as the frontalplane. Two major factors help to determine this mean QRS
Axis, they are:
1. The anatomical position of the heart2. The direction of ventricular depolarization.
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Determining AXISTechnique #1: Two Lead Method or Quadrant Method
This method uses just two leads of the six limb leads and workswell in critical situations to get a general idea of the Mean
Axis of the QRS. We will look at Lead 1 and at Lead AVF forthis technique.
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Determining AXIS1. Leads 1 and AVF divide
the thorax into quadrants,
(Left, Normal, Right, No Man's)
2. If leads 1 and AVF are bothupright then the Axis isnormal.
3. If lead 1 is upright and leadAVF is downward the Axis isLeft.
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Determining AXIS
4. If lead AVF is upright and lead 1is downward then the Axis is Right
5. If both leads are downward thenthe Axis is extreme Right Shoulder
and most often is Vent. Tachy
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Determining AXISMethod 2 : Three Lead technique
This method uses the three limb leads I, II, IIIto determine the axis of the QRS.
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LEAD 1 LEAD 2 LEAD 3
Normal UPRIGHT UPRIGHT UPRIGHT
Physiological
Left AxisUPRIGHT
UPRIGHT /
BIPHASICNEGATIVE
PathologicalLeft Axis
UPRIGHT NEGATIVE NEGATIVE
Right Axis NEGATIVE
UPRIGHT
BIPHASIC
NEGATIVE
UPRIGHT
Extreme Right
AxisNEGATIVE NEGATIVE NEGATIVE
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Determining AXIS
Method # 3: Using the Hexaxial Diagram to findMethod # 3: Using the Hexaxial Diagram to findthe degree of Axis Deviationthe degree of Axis Deviation
This technique is a little hard to learn
but it is also the most accurate( + or 100 to 150)
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Determining AXIS
To determine theaxis a scale mustbe used such asthe one at theleft.
-90-60
-30
0
aVL
I
30
60
aVR
II
90
120III
150
180
-150
-120
aVF
Marked RAD
LAD
RAD
Normal Axis
-30 to +100-30 to +120
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Determining AXISFirstyou must find the
mostequiphasic QRScomplex in the six limbleads. This is done byadding and subtractingthe number of squareseach QRS complex hasabove the baseline andbelow the baseline ineach lead.
-90-60
-30
0
aVL
I
30
60
aVR
II
90
120III
150
180
-150
-120
aVF
Marked RAD
LAD
RAD
Normal Axis
-30 to +100-30 to +120
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Determining AXISThe axis will be
perpendicular (right
angle) to this lead.
If the perpendicularlead has a positive QRS,
orient the axis to the
positive pole of this lead.
If it is negative thenorient it to the negative
pole.
-90-60
-30
0
aVL
I
30
60
aVR
II
90
120III
150
180
-150
-120
aVF
Marked RAD
LAD
RAD
Normal Axis
-30 to +100-30 to +120
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Causes of Abnormal AxisLeft Axis Deviation = An axis betweenLeft Axis Deviation = An axis between --30 and30 and --9090
May be normal in the elderly and very obese
Due to high diaphragm during pregnancy, ascites, or ABDtumors
Left Anterior Hemiblock
Left Bundle Branch Block
WPW Syndrome
Congenital Lesions Right Vent. Pacer or ectopic rhythms
Emphysema
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Basics of 12 Lead ECG's
Causes of Abnormal AxisRight Axis Deviation = An axis between +120 andRight Axis Deviation = An axis between +120 and
+180+180
Can be normal variant
Right Vent. Hypertrophy
Right Bundle Branch Block
Left Posterior Hemiblock
Left Ventricular ectopic rhythms Some right Vent. Ectopic rhythms
WPW Syndrome
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Causes of Abnormal Axis
Extreme Right Axis Deviation = An axisExtreme Right Axis Deviation = An axisbetweenbetween --90 and +18090 and +180
Ventricular ectopic rhythms
Right Vent. Paced rhythms
Less commonly seen in patients withcardiomyopathies and in those with multiple MI's