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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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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