ECG Basics

Question 1

How do action potentials manifest on the skin?

Answer 1

Heart electrical activity leads to current flow within heart and surrounding tissues. Can measure potential differences between these sites and body surface, measured by ECG - electrocardiograph

Question 2

Dipole context of ECG

Answer 2

Different regions of heart depolarized, or polarized leading to difference in charge - dipole.

Question 3

What is a lead axis?

Answer 3

Imaginary line between two electrodes on a electric field generated by dipole. Measureable change is greatest when lead axis is parallel to dipole.

Question 4

What are the components of an electrical vector?

Answer 4

Represented by dipole (change in charge)
Consists of
Magnitude - determined by mass of cardiac muscle involved in generation
Direction - overall activity of heart, varies during cardiac cycle

Question 5

What is a lead?

Answer 5

Imaginary line between two electrodes - electrodes are what you place on limbs/chest.

Question 6

What is an electrode?

Answer 6

Site of measuring changes in electrical activity.
+ve electrode is the recording electrode.
Depolarization moving towards +ve electrode produces an upward deflection on ECG.

Question 7

Lead 1

Answer 7

Right arm to left arm, left arm is +ve electrode

Question 8

Lead 2

Answer 8

Right arm to left leg, left leg +ve electrode

Question 9

Lead 3

Answer 9

Left arm to left leg, left leg +ve electrode

Question 10

What are augmented limb leads?

Answer 10

Derived from same electrodes as limb leads but use halfway pts between limb leads as negative electrodes.

Question 11

aVL

Answer 11

Mid point lead 2 as negative electrode

Left arm as positive limb lead

Question 12

aVR

Answer 12

Mid point lead 3 negative electrode

Right arm positive electrode

Question 13

aVF

Answer 13

Mid point lead 1 negative electrode

Left foot positive electrode

Question 14

If a particular lead shows a wave to be isoelectric then an electrical projection should be present on…

Answer 14

The perpendicular lead

Remember circle of axes!

Question 15

Where do you place the chest leads?

Answer 15

1 and 2 are parasternal in the right and left 3rd intercostal space respectively.
4 is in the 4th intercostal space on the midclavicular line.
3 is placed between 2 and 4.
5 and 6 are lateral to 4, 5 is on the anterior axillar line whereas 6 is on the mid axillary line

Question 16

What is a structured approach to analysing an ECG?

Answer 16

1) Verify patient details
2) Check time and date of ECG
3) Check calibration of ECG paper (usually small square 0.04, large square 0.2 sec)
4) Determine axis
5) Work out rhythm
6) Look at individual leads for ST or T wave changes, or voltage criteria for disease.

Question 17

What 7 questions should ask when working out the rhythm?

Answer 17

Electrical activity?
Regular or irregular?
what is the HR?
P waves?
what is the PR interval?
Is each P wave followed by a QRS complex?
Is the QRS duration normal?

Question 18

Normal QRS duration?

Answer 18

Less than 0.1 sec/ half a large square normal ECG paper

Question 19

Normal P wave duration?

Answer 19

0.08-0.1 sec

Question 20

Normal PR interval?

Answer 20

0.12-0.20 sec

Question 21

How to work out HR during a regular rhythm?

Answer 21

300/number of large square R-R interval

Question 22

How to work out HR during a irregular rhythm?

Answer 22

Count number of QRS complexes in 30 large squares, and multiply by 10