What is Cardiac Axis ? How to determine cardiac axis in ECG/EKG by thumb rule ?

 Cardiac Axis 


The average direction of spread of the Depolarization wave through the Ventricles as seen from the front is called "cardiac Axis". 

                                  Or 

Cardiac axis describe the overall direction of electrical spread within the heart.


NOTE -  It is usual to decide whether this axis is in a normal direction or not.


Lead aVR and lead II look at the heart from opposite directions. when seen from the front,
The depolarization wave normally spreads through the ventricles from 11 O 'clock' to 5 O 'clock', so the deflection in lead aVR are normally mainly downward -ve and in lead II mainly upward +ve. 


Cardiac Axis





Normal Axis


A normal 11 O 'clock - 5 O' clock axis means that the Depolarizing wave is spreading towards lead I, II and III,
                 and is therefore associated with a predominantly upward deflection in all these leads.


NOTE -  The deflection will be greater in lead II than in lead I and than in lead III. 

Cardiac Normal Axis



Right Axis Deviation 


If the right ventricle become hypertrophied, it has more effect on the QRS complex than the left ventricle and the average depolarization wave the axis will swing towards the right.
                     as a result, the deflection in lead I becomes -ve ( predominantly downward ), because depolarization is spreading away from it,
and the deflection in lead III becomes more +ve ( predominantly upward ), because depolarization is spreading towards it, this called right axis deviation.  


Right Axis Deviation


Left Axis Deviation 


When the left ventricle becomes hypertrophied, it exerts more influence on the QRS complex than the right ventricles. hence the axis may swing to the left,
                    as a result the QRS complex becomes predominantly -ve in lead III, this is called left axis deviation.

NOTE -  Left Axis Deviation  is not significant until the QRS complex deflection is also predominantly negative in lead II.
             although left axis deviation can be due to excess influence of an enlarged left ventricle, in fact this axis change is usually due to a conduction defect rather than to increased bulk of the left ventricle muscle.


Left Axis Deviation


CARDIAC AXIS THUMB RULE





The axis is sometime measured in degree, though this is not clinically particularly useful.

Lead I is taken as looking at the heart from 0°
Lead II from +60°; and Lead III from +120°. 
Lead aVF from +90°; Lead aVL from -30°; and Lead aVR from -150°.

  • Normal Axis  :-  QRS axis between -30° to +90°
  • Left Axis Deviation  :-  QRS axis less than -30°
  • Right Axis Deviation :-  QRS axis greater than +90°
  • Extreme Axis Deviation :- QRS axis between -90 to 180


Cardiac Axis




Why Worry About The Cardiac Axis ?


Right and Left axis deviation in themselves are seldom significant - 

  • Minor degree occur in tall, thin individuals and in short, fat individual, respectively.
  • The presence of axis deviation should alert you to look for other signs of right and left ventricular hypertrophy.
  • A change in axis to the right may indicate a pulmonary embolus. 
  • A change in axis to the left may indicate a conduction block. 




Example 1.

Right Axis Deviation

According to thumb rule :-
  • ECG show Right Axis Deviation
  • Negative QRS Complex in Lead I
  • Positive QRS Complex in Lead II and aVF. 


Example 2.

Left Axis Deviation

According to thumb rule:-
  • ECG show Left Axis Deviation
  • Positive QRS Complex in Lead I
  • Negative QRS Complex in Lead II and Lead aVF.


Example 3.


Normal Cardiac Axis

According to thumb rule:-
  • ECG show Normal Cardiac Axis
  • Positive QRS Complex in Lead I, II and aVF.




ECG Interpretation (How to read an ECG or EKG ?)


Electrocardiography Basics ( characteristics of P wave QRS complex T wave U wave ST segment PR interval )


Human Circulatory System / Blood Vascular System








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