Electrocardiograms

Question 1

What does an ECG measure? What graph does it produce?

Answer 1

Electrical potential difference in the heart, detected at skin surface
Voltage against time

Question 2

Why is the P wave the smallest?

Answer 2

Atrial chambers are smaller - smaller tissue –> smaller action potential

Question 3

Why can T be positive or negative?

Answer 3

Ventricular depolarisation less organised

Question 4

Describe each wave of the ECG

Answer 4

P - atrial depolarisation
QRS - ventricular depolarisation
T - ventricular repolarisation

Question 5

What does the P-Q interval show?

Answer 5

Pause in conduction as AP passes through AVN

Question 6

Why is atrial repolarisation not visible?

Answer 6

Small in amplitude and occurs during ventricular depolarisation - masked by QRS complex

Question 7

Cells which are not polarised (not undergone an AP) have what change outside the cell

Answer 7

Positive charged

Depolarised cells have negative charge outside the cell

Question 8

What are the 6 standard limb leads?

Answer 8

Bipolar leads I, II, III

Augmented unipolar leads AVR, AVL, AVF

Question 9

Why are 6 leads needed?

Answer 9

Read across heart in different planes - each graph different in size and direction

Question 10

Each lead has a positive electrode. How can AP movement be measured on a graph in relation to the positive electrode?

Answer 10

Movement of AP towards +ve electrode = upward/positive deflection
Movement of AP away from +ve electrode = negative deflection

Question 11

The signal measured on an ECG is a vector. Why is it classed as a vector?

Answer 11

Has 3 dimensional direction and magnitude (size of pd)

Question 12

What does the size of the deflection show?

Answer 12

Size in one lead compared to another indicates which lead is most parallel
Most in line vector gives the biggest deflection

Question 13

What is the mean electrical axis?

Answer 13

Overall vector for the cardiac cycle

Question 14

What diagram is used to to work out the MEA?

Answer 14

Bailey’s Hexaxial diagram

Shows all 6 leads crossing at 0

Question 15

What is the normal MEA in a dog and cat?

Answer 15

Dog = +40 to +100
Cat = 0 to +160

Question 16

What is an axis deviation?
What does this show?
What can this be due to?

Answer 16

MEA outside of normal range - can be left or right
Shows AP hasn’t followed normal direction
Abnormal heart position, chamber size or conduction pathway

Question 17

What are the 3 methods to estimate the MEA?

Answer 17

Isoelectric lead
Biggest QRS complex
Quadrant graphing method

Question 18

How do you use the biggest QRS complex to calculate the MEA?

Answer 18

Look for lead with biggest QRS complex
Note if it’s above or below baseline
If it’s above baseline read positive end, if below read negative end
MEA within 30 degrees of this

Question 19

How do you use the isoelectric lead to calculate MEA?

Answer 19

Look for lead where deviation above and below baseline are equal
Look 90 degrees to this lead and see what lead it is
If trace is mainly above the baseline, read positive end. If mainly below baseline, read negative end

Question 20

How do you use the quadrant graphing method?

Answer 20

Leads I and AVF divide circle into quarters
Look at the traces of these leads and see if they have +ve or -ve QRS complex
Draw arrows on hex axial diagram to show if positive or negative - this will show you the quarter the MEA is in
Compare the sizes of the QRS in these leads and the biggest one is closer to the MEA