2.1 Electrocardiogram General Principles

Question 1

Can you name the parts of an ECG?

Answer 1

3 waves

2 segments

2 intervals

1 complex

Summary

1. P wave - depolarisation atria
2. QRS complex - depolarisation ventricles + repolarisation atria
3. T wave - repolarisation ventricle
4. U wave - repolarisation Purkinje fibres (seen in some patients, and often in hypokalemia

Question 2

Trace the pathway of depolarisation through the heart’s electrical conduction pathway.

Answer 2

SA node –> Atria –> AV node –> Purkinje fibres –> Ventricular septum –> Base of heart

Depolarisation is followed by contraction.

Question 3

Explain the direction of depolarisation and how it is recorded on the ECG.

Answer 3

An upstroke is recorded for an approaching depolarisation, a downstroke is recorded for a depolarisation that moves away.

Hence, the waveform depends on the location of the electrode.

• E.g. Lead II measures an upstroke as the depolarisation moves down the septum (towards the L arm) and a downstroke as the wave of depolarisationd doubles back on itself to move up through the ventricles to the base of the heart.

Question 4

What are the categories of the 12 leads of the 12-lead ECG?

Answer 4

1. Leads I-III: Bipolar limb leads
2. Leads aVR, aVL, aVF: Unipolar limb leads
3. V1-V6: Precordial leads

The bipolar and unipolar limb leads measure depolarisation on the frontal plane, whereas the precordial leads measure the depolarisation along the transverse plane.

Question 5

Explain Einthoven’s Triangle and the Axial Reference System.

Answer 5

Einthoven’s triangle is the hypothetical triangle created by the leads I, II and III.

If they are broken apart and superimposed on the heart to show the points of a compass, they form the axial reference system.

In this system, Lead II demonstrates the most deflection as a depolarisation travels at +60 degrees.

Question 6

Explain the what Leads I, II and III detect.

Answer 6

Einthoven’s Original Leads (Designated by Compass Points) - the 3 Bipolar Limb Leads.

• Lead I - Looks at the depolarisation travelling towards the LA. 0 degrees.
• Lead II - Looks at depolarisation travelling towards LL from RA. +60 degrees.
• Lead III - Looks at depolarisation travelling towards LL from LA. +120 degrees.

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Question 7

Explain the direction of depolarisation that leads aVR, aVL and aVF detect.

Answer 7

1. aVR - towards the R arm (+210)
2. aVL - towards the L arm (-30)
3. aVF - towards the L foot (+90)

Question 8

Explain what the precordial leads V1-V6 measure.

Answer 8

Leads V1-V6 measure the depolarisation of the heart at six different points on the transverse plane.

They are located from the R sternal border all the way around to the axilla, creating readings from the anterior (V1-V2) to lateral (V5-V6) aspects of the heart.

Question 9

List the degrees for each lead.

Answer 9