10. Electrocardiography 1

Question 1

Electrodes

Answer 1

Conductive material in contact with skin

Question 2

Cables/ Wires

Answer 2

Attached to electrodes

Question 3

Leads

Answer 3

Perspective of electrical activity of the heart from a given direction

Question 4

Isoelectric line

Answer 4

Represents no net change in voltage. i.e. vectors are perpendicular to the lead.

Question 5

Width of the deflection of a cardiac vector

Answer 5

denotes the ‘duration’ of the event

Question 6

Steepness of the line

Answer 6

denotes the ‘velocity’ of action potential

Question 7

Downward deflections

Answer 7

towards the anode (-)

Question 8

Upward deflections

Answer 8

towards the cathode (+)

Question 9

What is each wave composed of?

Answer 9

both the up- and downstrokes (sometimes waves overlap)

Question 10

What does P represent on an ECG?

Answer 10

Electrical signal that stimulates contraction of the atria

Atrial systole

Question 11

What does QRS represent on an ECG?

Answer 11

Electrical signal that stimulates contraction of the ventricles
Ventricular systole

Question 12

What does T represent on an ECG?

Answer 12

Electrical signal that signifies relaxation of the ventricles

Question 13

Describe the conduction system

Answer 13

SAN spontaneously depolarises
Nodal branches can rapidly conduct impulses into different parts of the atria and to the AVN
AVN slows signal down to facilitate mechanical events of heart (allows for filling)
Conducts impulse into bundle of his, which bifurcates into left and right bundle branches
The bundle of his and bundle branches are insulated so impulse can’t leave: Takes impulse to bottom of heart as heart contracts bottom up
Some branches come off left bundle: 1st part of heart to depolarise is ventricular septum
Fibres propagate into apex and up the sides

Question 14

Which lead is considered most useful? Why?

Answer 14

Lead II
As is negative electrode (RA) to positive electrode (LL)
Same direction as heart polarity (negative to positive, top to bottom)

Question 15

Describe each 5X5 square on an ECG

Answer 15

Total square: 0.2s (width) X 0.5mV (height)

Small squares: 0.04s X 0.1 mV

Question 16

Electrode placement: The rule of L’s

Answer 16

Lead I (1 L): Right Arm to Left Arm 
Lead II (2 L’s): Right Arm to Left Leg
Lead III (3 L’s): Left Arm to Left Leg

Question 17

How are leads read?

Answer 17

Drawn as a triangle and reading left to right and top to bottom
the 1st electrode of each pair you reach is the anode (-ve)

Question 18

How many cables and leads are there?

Answer 18

10 cables
12 leads (6 limb, 6 chest)

Question 19

How do you find amplitude of a lead?

Answer 19

Count squares for positive deflection
Count squares for negative deflection
Calculate net deflection by subtracting one from the other

Question 20

How can cardiac output be measured?

Answer 20

Using electrocardiography

Question 21

Where are the 6 limb electrodes placed?

Answer 21

RA
LA
LL
Einthovens triangle

Question 22

What do all leads have?

Answer 22

A fixed cathode (+ve)

Question 23

Which leads have a fixed anode? Which leads have a virtual anode?

Answer 23

Fixed anode: I, II, III

Virtual anode: aVR, aVL, aVF, V1-V6

Question 24

What does the cardiac axis provide? What is the norma range for this?

Answer 24

Net vector of the R-wave, which is usually the largest electrical event as it is the biggest amount of muscle Normal range is -30 to +90 degrees

Question 25

How do you calculate the cardiac axis?

Answer 25

Calculate amplitude of Lead II and aVL (these are 90 degrees apart when in the coronal plane)
Draw aVL line perpendicular to other end of lead II line to create a right angle triangle, label with amplitudes
Cardiac axis = 3rd line of triangle, we want it in relation to 0
Use tan(x)= (aVL/ lead II)
60-x = Cardiac axis
(lead II is 60 degrees)

Question 26

How many bipolar leads are there? What does this mean?

Answer 26

3 Bipolar

Bipolar: have a physical wire acting as the electrode