Cardiac axis Flashcards
What is cardiac axis
-represents the overall direction of electrical activity
-in healthy people would be from right shoulder to left hip
What causes axis to change
-size of myocardium/ amount of myocytes
-death of myocardium due to infarction
-‘mechanical shift’- physical movement of the heart
-conduction defects eg. LBBB
Causes of left axis deviation
-normal variance
-left ventricular hypertrophy
-conduction defects eg. LBBB
-inferior wall MI
-preexcitation syndromes eg. wolf-parkinson white syndrome
-congenital cardiac defects
-hyperkalaemia
-mechanical shift eg. pregnancy, abdo tumour
-paced rhythm
Causes right axis deviation
-normal variation (children and young adults)
-limb lead reversal
-right ventricular hypertrophy
-conduction defects eg. RBBB
-lateral wall MI
-preexcitation syndromes eg. WPW syndrome
-congenital cardiac defects
-dextracardia (heart on right not left of chest
-pneumothorax
-mechanical shift eg. emphysema
-conditions leading to R ventricular strain eg. PE
Normal QRS axis
- -30 degress to +90 degrees
-left axis deviation= -90degrees to -30degrees
-right axis deviation= +90 degrees to +180
-extreme axis deviation= -90degrees to +180
Simple method calculating QRS axis
-if positive QRS in lead 1 and aVF, then there’s normal QRS axis as must be between 0 and +90 degrees
-if QRS positive in 1 and negative in aVF= left axis deviation
-if QRS negative in 1 and positibe in aVF= RAD
-if negative in both= extreme deviation
Harder but more accurate method calculate QRS axis
-find isoelectric limb lead= where R and S wave are same height
-find lead 90 degrees to the isoelectric lead
-if QRS positive, that is the axis
-if QRS negative, the axis is opposite to that lead
What if no equiphasic lead
-choose most equiphasic lead
-if more negative, then add 15 degrees to axis furthest from equiphasic lead
-if more positive add 15 degrees towards the equiphasic lead
