ECG - Cardiac Axis Flashcards
Draw the angles of the frontal leads relative to lead I
What type of axis deviation occurs in left anterior fascicular block?
Left axis deviation
Why does Left axis deviation occur in Left anterior fascicular block?
If LAF is blocked, chamber depolarisaiton occurs through the posterior fascicle, therefore depolarising anterior fascicular fibres in a retrograde fashion. This leads to a reversal of the vector of depolarisation, leading to an overal vector change to a left axis deviaiton
Why do you get right axis deviation in left posterior fasicular block?
If LPF is blocked, chamber depolarisaiton occurs through the anterior fascicle, therefore depolarising posterior fascicular fibres in a retrograde fashion. This leads to a reversal of the vector of depolarisation, leading to an overal vector change to a right axis deviaiton
What leads would you look at to tell the axis of the heart?
Look at leads I, II and III, and the limb leads aVL, aVF, and aVR
Will the QRS in lead II be positive or negative in Left axis deviation?
Negative
If the cardiac axis was orientated towards aVL, would the QRS complex be positive, negative or isoelectric in lead II?
Often looks isoelectric - due to vector being perpendicular to lead II
Will the QRS complex in lead II and III be positive, negative or isoelectric in left axis deviation?
Both overall negative, more strongly negative in lead III
What is the most common cause of left axis deviation?
Left anterior fascicular block
Why are QRS complexes overall negative in leads II and III in left anterior fascicular block?
Due to retrograde depolarisation of anterior fascicle from posterior fascicle
If the depolarisation vector was straight down aVF, would the QRS complex be positive, negative or isoelectric in lead I?
Isoelectric
Is the QRS complex in lead one isoelectric, positive or negative overall in right axis deviation?
Negative overall
Why are QRS complexes in positive in leads II and III in right axis deviation?
Due to retrograde depolarisation of the left posterior fascicle
What is the left anterior fascicle supplied by?
Septal branches of the left anterior descending artery
What can cause left axis deviation?
Disruption of left anterior fascicle
- LAD ischaemia/infarction
- LVH
- Cardiomyopathy
Large right sided infarct
Why can a left lateral infarct lead to right axis deviation?
Knocks out some of the LV depolarisation vectors, leading to an increase in RV depolarisation vectors relative to the LV depolarisation vectors. This pulls overal depolarisation vector to the right, causing right axis deviation
How would you determine p-wave axis?
If p-wave is positive in lead, it is moving towards that lead in some capacity (within 90 degrees of the lead vector). If it is negative, the p-wave is moving away from the lead (outiwth 90 degrees of the lead). This can show which direction the atria are depolarising
What might the following indicate?
P-wave axis is travelling upwards, away from inferior leads. This could be caused by atrial ecoptic tachycardia - focus within the atria
How can you determine the degree of axis deviation based on the QRS amplitude?
Count the number of small squares of the QRS complex in leads I and aVF above and below isoelectric line, and add them together to get the net deflection
Once deflection in each lead has been calculated, plot on skematic of lead axes and determine the axis
