Cardio Block 5: More ECG

Which electrode in a 12 lead ECG is ground?

• right leg

What plane to leg leads look at the electrical activity of the heart in?

• frontal plane

What plane to chest leads look at the electrical activity of the heart in?

• horizontal perspective; anterior, septal and lateral and posterior parts of the heart

What terminal do chest leads use as a reference point?

• Wilson’s central terminal

What is present at the bottom of an ECG?

• rhythm strip

• lead 2 - best parallel with the electrical activity of the heart

What is the normal QRS duration on an ECG?

2 ½ small squares

What exact fibres does the S wave represent?

• depolarisation of the Purkinje fibres

Where does the PR interval start and end?

• starts at beginning of P wave and ends at start of the QRS complex

What does the PR interval represent?

• the time taken for the electrical impulse to be conducted through the atrioventricular node

What does the QT interval represent?

• the time for ventricular depolarisation and repolarisation to occur (systole duration)

When does the QT interval start and end?

Starts at the beginning of the Q wave and ends at the start of the T wave.

The duration of the PR and QT interval are dependent on what?

• on heart rate

• increase in HR = shorter intervals and vice versa

The PR segment should look…

• flat (isoelectric) under normal circumstances

What is another method of calculating heart rate from an ECG?

• look at rhythm strip

• count number of QRS complexes that the rhythm strip shows in 10 seconds

• multiply by 6

Which limb leads and used to determine the electrical axis of the heart (overall direction of ventricular depolarisation)?

• Lead 1

• Lead aVF

If both Lead I and Lead aVF are positive what does the show about the electrical axis of the heart?

• lower left

• normal axis

• -30 and +90

If Lead I is positive and Lead aVF is negative what does the show about the electrical axis of the heart?

• upper left axis deviation

• 0 to -90 degrees

If Lead I is negative and Lead aVF is positive?

• lower right deviation

• + 90 to 180 degrees

If Lead I is negative and Lead aVF is negative?

• -90 to 180 degrees

• extreme axis deviation

• upper right

Give a limitation of the QRS method?

• the QRS axis can be up to -30 degrees in the upper left quadrant and it is still normal heart activity

What can be used as a way around the QRS axis limitation?

• use Lead II as well

• if lead II QRS is positive the axis is generally normal

• if negative there is a left axis deviation

If you had a problem with your right coronary artery what leads would it affect?

V1

V2

V3

aVF

If you had a problem with your circumflex artery what leads would it affect?

Lead I

aVL

V5

V6