Lecture 5: ECG 1

Question 1

What does ECG measurement depend on?

Answer 1

1) Magnitude of the charge (True size of the dipole)
2) Distance between dipole and recording electrode
3) Orientation of the dipole and recording electrodes

Question 2

Describe what is actually measured on an ECG? What does the waveform represent

Answer 2

The influx of positive ions into the cell leaving the outside relatively negative. (the dipole this generates in the ECF)

Depending on the recording set up i.e if the recording electrodes have poles then this will influence the direction of wave inflexion.

Question 3

Describe QRS on a lead 2:

Answer 3

Q = Left bundle depolarisation of the interventricular septum i.e left to right
R = Depolarisation of the ventricles from endo to epi cardium direction
S = Depolarisation through the ventricles back towards the fibrous skeleton, b/c direction we see negative deflection.

Question 4

Describe leads on eithovens triangle;

Answer 4

Lead 1 = RA -> LA (+ve electrode)
Lead 2 = RA -> LL (+ve electrode)
Lead 3 = LL -> LA (-ve electrode)

1 + 3 = 2 in eithovens triangle

Question 5

Describe the augmented leads;

Answer 5

Vr = +ve
Vl = +ve
Vf = +ve

• ve points are midpoints of the leads
  i. e refer to the diagram

Question 6

What happens with an obese ECG?

Answer 6

Smaller values because fat is insulating

Question 7

What is the of axis deviation considered normal?

Answer 7

-30 -> +110 degrees

Question 8

What is characteristic of wolf parkinson white syndrome?

Answer 8

Pre-excitation:
Short PR
Wide QRS
Delta wave

Because of accessory pathway

Post excitation:
Re-entrant SVT narrow or wide complex

Look at notes

Question 9

Whats unique about the ST segment?

Answer 9

• Its isoelectric
• Reflects all ventricular myocytes going through Ca plateau
• No moving wavefront

Question 10

What does the QT interval reflect?

Answer 10

Ventricular action potential duration i.e if prolonged then might have asynchronous electrical spread

Question 11

Write some notes on healthy T wave?

Answer 11

• Epi to Endo i.e Endo have longer AP

Question 12

How does a unipolar system work?

Answer 12

Potential of a single electrode relative to the reference central terminal of wilson.

Central terminal of wilson achieved by connecting all three leads together through a resistor = constant value which we assign as 0.

Question 13

How would you use eindhovens triangle to work out the mean QRS vector?

Answer 13

Max deflection - min deflection i.e for each leads QRS