Cardiac axis

Question 1

What is cardiac axis

Answer 1

The sum of the depolarisation vectors generated by individual cardiac myocytes.

Question 2

Causes of cardiac axis change

Answer 2

Size of myocardium (athletes, HCM)
Death of myocardium (MI)
Mechanical shift (birth defect, pregnancy, ascites, tumours, PE)
Conduction defects (LBBB, LQTS, baraguda)

Question 3

Why measure QRS axis

Answer 3

Can indicate underlying pathophysiology, can be useful for measuring progression of chronic conditions, can help steer provisional diagnosis.

Question 4

Positive QRS

Answer 4

R wave is more dominant than S wave meaning overall vector of electricity is towards the lead.

Question 5

Negative QRS

Answer 5

S more dominant than R meaning vector of electricity is away from the lead.

Question 6

Isoelectronic (equiphasic) QRS

Answer 6

Equal R and S waves mean vector is parallel to lead.

Question 7

Left axis deviation causes

Answer 7

Can be normal variant
Left ventricular hypertrophy
Conduction defects
Congenital heart defects
Inferior wall MI
preexcitation syndromes
Paced rhythms

Question 8

Right axis deviation causes

Answer 8

Right ventricular hypertrophy
limb lead reversal
Conduction defects
Lateral wall MI
Dextrocardia
Pneumothorax
Right ventricular strain conditions

Question 9

Method 1 (thumbs)

Answer 9

looking at leads 1 and aVF
Positive lead 1 indicates axis of -90 to 90
Positive lead aVF indicates axis of 0 to 180
negative of both leads looks at the opposite angles.
See if lead 1 and aVf are positive or negative and the quarter they have in common on hexaxial reference system gives you the axis.

Question 10

Method 2 (limb leads only)

Answer 10

Find isoelectronic limb lead.
Using hexaxial reference system, find lead which is 90 degrees from isoelectronic lead.
If this lead is positive, its value on hexaxial reference gives the cardiac axis.
If this lead is negative, trace back the line and this angle (opposite side) gives you the cardiac axis.

Question 11

No isoelectronic lead

Answer 11

Look for the ‘most’ isoelectronic QRS in limb leads.
Is this more negative or positive (count)
Using lead which is 90 degrees from most isoelectronic.
If ‘most’ isoelectronic line is more positive, cardiac axis is 15 degrees closer to that line from 90 degree lead. If ‘most’ isoelectronic line is more negative, cardiac axis is 15 degrees further away from 90 degree lead.