The Mean Frontal Plane QRS Axis Catherine Barman Senior Chief Cardiac Clinical Physiologist North Cheshire Hospitals NHS Trust Monday Jan 15 th 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

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 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

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

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.

Right Axis Deviation Commonly accompanies RVH May be due to block of the posterior fascicle of the left bundle

Right Axis Deviation.

Remember.

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