The Mean Frontal Plane QRS Axis

Catherine BarmanSenior Chief Cardiac Clinical Physiologist

North Cheshire Hospitals NHS TrustMonday Jan 15th 2007

Background

• The spread of the cardiac impulse gives rise to the main deflections of the ECG.

• The total electrical activity at any one moment of time can be summated and represented by a single electrical force.

• This force has magnitude and direction and is termed the instantaneous vector.

Background continued

• All the vectors occurring in the ventricles during the production of the QRS can be averaged.

• The direction of the resulting vector of electrical spread is called the MEAN QRS AXIS.

Background continued

• It is customary to measure this only in the frontal plane.

• It is based on the orientation of the limb leads, I, II , III, aVR, aVL & aVF.

• The limb lead with the tallest R wave will be the closest to the QRS axis.

Theory

•E E

E

Muscle strip

QRS FRONTAL PLANE AXISHEXAXIAL REFERANCE SYSTEM

-90 -120º -60

AVR -150º -30 AVL (Cavity ) (High Lateral)

- 180º +180º 0 LEAD I

(Lateral)

+150º +30

+120 +60

LEAD III +90 LEAD II ( Inferior) AVF ( Inferior)

( Inferior) MAIN CARDIAC VECTOR

To find the mean QRS Axis

• 1. Find the limb lead on the ECG with the most equiphasic QRS complex.

• 2. Locate this lead on the hexaxial reference system.

• 3. Find the lead that is located at 90º to the most equiphasic lead.

• 4. Is this lead mostly positive or negative.

To find the mean QRS Axis

• 5. If this lead is positive stay with this lead, it is the mean QRS axis for this patient.

• 6. If this lead is negative the lead diametrically opposite to this lead is the mean QRS axis for this patient.

• This will give the mean frontal plane QRS axis to within 30º

15º Correction of QRS Axis

• To correct the mean frontal plane QRS axis to the nearest 15º in addition to steps 1-6

• 7. Look back at the original, most equiphasic, lead.

• 8. If this is slightly more positive then correct the main cardiac vector 15º towards that original lead.

15º Correction of QRS Axis

• 9. If this is slightly more negative then correct the main cardiac vector 15º away from that original lead.

• This will give the mean frontal plane QRS axis to within 15º

QRS FRONTAL PLANE AXISHEXAXIAL REFERANCE SYSTEM

-90 -120º -60

AVR -150º -30 AVL (Cavity ) (High Lateral)

- 180º +180º 0 LEAD I

(Lateral)

+150º +30

+120 +60

LEAD III +90 LEAD II ( Inferior) AVF ( Inferior)

( Inferior) MAIN CARDIAC VECTOR

Normal Range

• In the normal adult the mean frontal plane QRS axis lies between -30º and +90º

• Left axis deviation is present when the axis is less than -30º

• Right axis deviation is present when the axis is greater than +90º

QRS FRONTAL PLANE AXISHEXAXIAL REFERANCE SYSTEM

-90 -120º -60

AVR -150º -30 AVL (Cavity ) (High Lateral)

- 180º +180º 0 LEAD I

(Lateral)

+150º +30

+120 +60

LEAD III +90 LEAD II ( Inferior) AVF ( Inferior)

( Inferior) MAIN CARDIAC VECTOR

Left Axis Deviation

• Often due to block of the anterior division of the left bundle branch

• If this is also associated with RBBB it can develop into complete heart block

Left Axis Deviation.

Remember.

• Use Hexaxial System.QRS FRONTAL PLANE AXIS

HEXAXIAL REFERANCE SYSTEM

-90 -120º -60

AVR -150º -30 AVL (Cavity ) (High Lateral)

- 180º +180º 0 LEAD I

(Lateral)

+150º +30

+120 +60

LEAD III +90 LEAD II ( Inferior) AVF ( Inferior)

( Inferior) MAIN CARDIAC VECTOR

Right Axis Deviation

• Commonly accompanies RVH

• May be due to block of the posterior fascicle of the left bundle

Right Axis Deviation.

Remember.QRS FRONTAL PLANE AXIS

HEXAXIAL REFERANCE SYSTEM

-90 -120º -60

AVR -150º -30 AVL (Cavity ) (High Lateral)

- 180º +180º 0 LEAD I

(Lateral)

+150º +30

+120 +60

LEAD III +90 LEAD II ( Inferior) AVF ( Inferior)

( Inferior) MAIN CARDIAC VECTOR

What About These?