ECG

Question 1

What is a cardiac vector and how is it used to model the electrical activity of the heart?

Answer 1

vector
- represents mag and dir of action potential generated by one myocyte

modelling the heart
- electrical activity = net equivalent vector related to current dipole located at point inside heart boundary

Question 2

Describe the basic leads used in ECG and how it can be used to observe cardiac electrical output

Answer 2

Basic leads
- frontal plane electrodes
- forms einthoven’s triangle
(Lead 1: 0 degrees)
(Lead 2: 60 degrees)
(Lead 3: 180 degrees)

• used to measure the difference in electrical activity between each other (ex. diff between Lead 1 and lead 3)
  (Lead 1 - Lead 2 + lead 3 = 0)
• looks at the same electrical activity from different angles –> able to guess characteristics (shape / size) of cardiac electrical output

Question 3

How are unipolar leads different from basic leads?

Answer 3

Placement:
- same as basic leads (frontal plane electrodes)

What makes them different?
- they have a central reference (Wilson’s Central Terminal)
–> measures the mean (RA, LA, LL)

ex. VL = LA - mean(RA, LA, LL)

Can be augmented:
- augmented (same direction, bigger amplitude than basic leads)
- does not need a WCT

ex. aVL = LA - mean (RA, LA)
ex. aVL = (3/2)VL

Question 4

What are 12-lead ECGs?

Answer 4

In addition to Basic and unipolar leads, 12-lead ECGs contain

transverse-plane electrodes (precordial leads)
- measures electrical activity in the frontal plane at different angles (0, 30, 60, 75, 80, 100)

Question 5

Describe the 2 cardiac axis interpertation methods based on QRS axis

Answer 5

QRS axis
- normal = 45 degree angle pointing in the 3rd quadrant

1. Quadrant method
   - compare the pos and neg wave forms from Lead 1 and Lead aVF
   - if Lead I is more neg (move CW)
   - if Lead aVG is more neg (move CCW)
   (if both are neg, vector pts in 2nd quadrant)
2. Isoelectric lead method
   i. find isoelectric lead (pos and neg magnitudes are equal)
   ii. find lead normal to isoelectric lead (on QRS axis)
   iii. determine if normal lead is neg or pos

Question 6

What are the 6 important functional blocks of the electrocardiograph?

Answer 6

1. Amplifer protection circuit
   - protects ECG circuit against high voltages at input
2. Lead selector
   - determines which electrode is needed for each lead
   - ensures connection to rest of circuit
   (manual –> operator)
   (automatic –> microcomputer)
3. Autocalibration
   - 1 mV calibration signal for each channel
4. Preamplifier
   - Needs to have high input impedence and high common-mode rejection ration
   - includes gain control switch
5. Isolation circuit
   - barrier to passage of power line current from patient to amplifer (prevents patient electrocution)
6. Driver amplifier
   - amplifes to level suitable for recorder
   - removes offset voltages
   - bandpass filtering