Cardiac Axis

Question 1

What is Cardiac Axis?

Answer 1

“Cardiac axis represents the sum
of depolarisation vectors
generated by individual cardiac
myocytes”

“Cardiac axis represents the
overall direction of electrical
activity”

Question 2

each individual myocyte has its own vector, what is a vector?

Answer 2

Vector represents both magnitude and direction of the action potential (AP).

Question 3

what can cause Cardiac Axis to change?

Answer 3

There are many factors that can influence the axis of the
ECG, but broadly speaking there are several mechanisms
which influence the Axis:

1. Size of myocardium (amount of myocytes)
2. Death of myocardium (e.g. due to infarction)
3. “Mechanical shift” (physical movement of the heart)
4. Conduction defects (e.g. LBBB)

Question 4

Why do we measure Cardiac Axis?

Answer 4

• If the axis has deviated, it can
  indicate underlying pathology (some
  of which, may be life threatening)
• It can be useful when measuring the
  progression of chronic conditions
  such as heart failure
• Can help steer your provisional
  diagnosis

Question 5

Causes of left Axis Deviation?

Answer 5

• can be normal variance
• Left Ventricular Hypertrophy
• Conduction defects (LBBB)
• Inferior wall MI
• Pre-excitation syndromes , such as Wolf-parkinson-white syndrome.
• Congenital Cardiac defect
• Hyperkalemia
• mechanical shift (pregnancy, tumor etc)
• Paced rhythm

Question 6

Causes of right Axis deviation?

Answer 6

• Normal variation
  (children and young
  adults)
• Limb lead reversal

-Right ventricular hypertrophy

-Conduction defects (RBBB)

-Lateral wall MI

-Preexcitation syndromes (WPW)

-Congenital Cardiac Defect

-Dextrocardia

-Pneumothorax

-Mechanical shift (e.g. emphysema)

-Conditions leading to right ventricular strain (pulmonary embolism, chronic
lung disease).

Question 7

What is normal QRS axis?

Answer 7

-30 degrees - + 90 degrees

Question 8

What is Left Axis Deviation?

Answer 8

Left Axis Deviation -90 to -30 (degrees)

Question 9

What is right Axis Deviation?

Answer 9

Right Axis Deviation +90 to 180 (degrees)

Question 10

what is extreme axis deviation?

Answer 10

Extreme Axis Deviation -90 to 180

Question 11

what does Equiphasic mean?

Answer 11

The isoelectric (equiphasic) lead is the frontal lead with zero net amplitude. (90 degrees)

Question 12

How do you measure the Axis if the most equiphasic lead is more positive?

Answer 12

move 15 degrees towards the lead (+15 degrees)

Question 13

How do you measure the Axis if the most equiphasic lead is more negative?

Answer 13

move 15 degrees away from the lead (-15 degrees)

Question 14

How do you measure Cardiac Axis?

Answer 14

tep 1: Find the isoelectric lead. The isoelectric (equiphasic) lead is the frontal lead with zero net amplitude. This can be either:

A biphasic QRS where R wave height = Q or S wave depth.
A flat-line QRS with no discernible features.
Step 2: Find the positive leads.

Look for the leads with the tallest R waves (or largest R/S ratios)
Step 3: Calculate the QRS axis.

The QRS axis is at 90° to the isoelectric lead, pointing in the direction of the positive leads.