Lecture 4: The Heart Axis

Question 1

Explain the function of one lead

Answer 1

• 1 lead= set of 2 electrode

- >can record a voltage shift as the waves of electrical activity sweep over the heart

Question 2

Explain the problem of 1 lead

Answer 2

• if a wave is travelling in the direction of (parallel to) the lead= strong signal
• if perpendicular to the lead= no signal->as the voltage is not being changed in favour of the positive or negative electrode

Question 3

Explain the solution to having 1 lead

Answer 3

• Solution to lead 1 problem: Einthoven’s triangle
• With a triangle- any wave will be moving parallel at least to one lead within 60° + be moving perpendicular too
• Ground electrode = machine can establish where “zero”
• > data given in reference to that baseline

Question 4

Explain how different directions of depolarisation can cause traces

Answer 4

1. Wave of depolarisation travelling from neg. to pos. lead =positive trace
2. Wave of repolarisation travelling from neg. to positive lead= negative trace
3. Wave of depolarisation from pos. to neg lead= negative trace

Question 5

State how ECG traces can vary depending on the lead

Answer 5

• A wave of APs can give higher or lower recordings depending on which lead you are considering
• strongest in lead II and weakest in lead III

Question 6

Why is lead II used for ECG

Answer 6

• Lead II used for ECG= shows the strongest response to most of the information that are being monitored in a healthy heart

Question 7

Explain the trace of the QRS complex

Answer 7

A. trace goes down-> direction of the wave of depolarisation is slightly away from the positive electrode
->vector= magnitude- small mass + direction -perpendicular>parallel
B. ventricle is being depolarise- big mass
-ventricular muscle closest chamber- starts to squeeze first
= AP wave travels down the tissue= big pos. trace
C. ventricle is still depolarising
- wave has travelled up= less pos. trace
D. Wave travelling up ventricle towards atria= small mass left
- away from the pos. electrode= neg. trace
- Top of ventricle would be first to depolarise but blocked by:
->cardiac skeleton insulates
-> conduction faster via purkinje fibres

Question 8

what is the mean electrical vector activity used for

Answer 8

• mean electrical vector- general description of what’s going on in heart=> used as standard
• all red arrows at stages of QRS complex taken mean of= mean electrical axis/ heart axis

Question 9

State the range for normal activity

Answer 9

• Normal range= 45°-65°

Question 10

List the different leads used for different uses

Answer 10

• Lead I, II, III-> give 2D info
• Lead I, II, III + 3 auxiliary leads: aVR, aVL, and aVF- used for clinical use
• Lead I, II, III + 3 auxiliary leads: aVR, aVL, and aVF + 6 chest electrodes that cover the front to back direction= gives 3D info

Question 11

Explain how the mean heart axis can be calculated

Answer 11

-	Mean heart axis calculated using lead I + aVF 
E.g cardiac axis (θ):
= tan-1 (opp/adj)
= tan-1 (5/8)
= 32◦

Question 12

Describe by drawing and explain how conditions can be diagnosed using the ECG

Answer 12

Electrodes for limb on the wrong limbs
= inverted reading

Flatline= asystole
could be due to faulty equipment – check the connections
- Asystole ≠fibrillation
-> defibrillator should not be used
Preferred method of restart is CPR and/or injection with vasopressin (or epinephrine)

Ventricular fibrillation
AP wave keeps circling on itself
cardiomyocytes are repeatedly contracting out of phase
Defibrillator should be used-> depolarising all cardiac cells at once
= Hope for (reset) myogenic (pacemaker) cells will then create a fresh signal that will get things beating in time