The Cardiac Axis

Cardiac Axis
This is the average spread of depolarisation through the ventricles, when seen from the front.

You need only look at leads I and II for this, as the normal cardiac axis is considered to be anywhere between -30 degrees and +90 degrees.
  

Positive Lead I + Positive Lead II = Normal Cardiac Axis
Positive Lead I + Negative Lead II = Left Axis Deviation
Negative Lead I + Positive Lead II = Right Axis Deviation

Left axis deviation can imply

1. Left Anterior Hemiblock
2. Inferior MI
3. Left Ventricular Hypertrophy (though it is not diagnosed this way)
4. Normal finding in short fat adults

and less common -

• artificial cardiac pacing
• emphysema
• hyperkalaemia
• Wolff-Parkinson-White syndrome - right sided accessory pathway
• tricuspid atresia
• ostium primum ASD
• injection of contrast into left coronary artery

Right axis deviation can imply

1. Left Posterior Hemiblock
2. Anterolateral MI
3. Right Ventricular Hypertrophy
4. Normal finding in children and tall thin adults 

 and anything causing a right-sided strain eg pulmonary disorders -

• chronic lung disease even without pulmonary hypertension
• pulmonary embolus
• Wolff-Parkinson-White syndrome - left sided accessory pathway
• atrial septal defect
• ventricular septal defect