13. Understanding the ECG

Question 1

What are ECGs used to distinguish?

Answer 1

• Tachyarrhythmia/Bradyarrhythmia
• Myocardial infarction/ischaemia
• Cardiomyopathy
• Assessment of pacing
• Electrolyte disturbances

Question 2

Which limb is the neutral/ground electrode for the limb leads?

Answer 2

Right leg

Question 3

Which limb leads have upward deflections for P and QRS waves?

Answer 3

Leads I and II

Question 4

How are augmented leads derived?

Answer 4

• Derived from the limb leads
• Use the average voltage of any 2 points on the skin as a negative pole
• Reading from the third

Question 5

How is the aVL, aVF and aVR calculated?

Answer 5

• aVR - average between the left arm and left leg, read from the right arm
• aVL - average between the left leg and right arm, read from the left arm
• aVF - average between the right arm and left arm, read from the left leg

Question 6

How high is 1 mV on an ECG?

Answer 6

10mm

Question 7

Where is the left, QRS and right axis, with reference to Lead I (0°)?

Answer 7

• Left axis (-90° to -30°)
• QRS (-30° to +90°) - main direction of ventricular depolarisation
• Right axis (+90° to 180°)

Question 8

How do you calculate the QRS axis if you are given Lead I and aVF, where aVF QRS is negative?

Answer 8

• Lead I at 0° (adjacent) e.g. +12
• aVF usually points down, but negative points up (opposite) e.g. -14
• Tanθ = 14/12
• θ = 49°
• QRS axis = -49° (left axis deviation)

Question 9

In which direction does Lead I point if its QRS is negative, and what is the final QRS axis deviation with a positive aVF?

Answer 9

• Left arm to right arm

* Right axis deviation

Question 10

Which plane do chest leads cut into?

Answer 10

Horizontal plane

Question 11

If the chest leads are positive poles, what is the negative pole?

Answer 11

• Wilson’s Central Terminal
• A pole composed of the right arm, left arm and left leg
• Average potential derived from the limb leads

Question 12

What do the different chest leads provide info on?

Answer 12

• V1 and V2 - right ventricle
• V3 and V4 - sternum
• V5 and V6 - left ventricle