ECG theory and practice Flashcards
What is detected by an ECG?
The extracellular current of the myocytes on the skin surface
How many electrodes are used to perform a 12-lead ECG?
10
What is an ECG lead?
An imaginary line (Lead axis) between 2 or more electrodes, it is NOT the wire that connects the electrode to the recording device
Which electrode acts as a recording electrode?
The positive electrode
What is represented by an upward deflection on an ECG?
The movement of depolarisation towards the recording electrode or depolarisation away from the recording electrode
What is represented by a downward deflection on an ECG?
The movement of depolarisation away from the recording electrode
What is represented by an isoelectric line?
No wave of depolarisation
What are the 3 standard limb leads?
Lead I - RA (-ve) to LA (+ve)
Lead II - RA (-ve) to LL (+ve)
Lead III - LA (-ve) to LL (+ve)
Which electrode is placed on the right leg?
A neutral, earthed electrode
Which electrodes form Einthoven’s triangle?
Right Arm
Left Arm
Left Leg
What causes the appearance of the P wave in a Lead II ECG?
Atrial depolarisation spreads from the SA node, inferiorly and to the left, so it moves towards the recording electrode (Left Leg) and so shows an upward deflection
What causes the appearance of the Q wave in a Lead II ECG?
Left to right depolarisation of the intraventricular septum, which moves slightly away from the recording electrode (Left Leg) and so shows a slight downwards deflection
What causes the appearance of the R wave in a Lead II ECG?
Depolarisation of the main ventricular mass, which moves towards the recording electrode (Left Leg) and so shows a large upwards deflection
What causes the appearance of the S wave in a Lead II ECG?
Depolarisation of the ventricles at the base of the heart, moving slightly away from the recording electrode (Left Leg) and so shows a slight downwards deflection
What causes the appearance of the T wave in a Lead II ECG?
Ventricular depolarisation, which moves away from the recording electrode. This has an opposite effect to depolarisation and so shows an upwards deflection on the ECG
What is a normal QRS duration?
<0.1 seconds
What is the normal P wave duration?
0.08 - 0.1 seconds
What is the normal PR interval duration?
0.12 - 0.2 seconds
What is the normal QT interval duration?
0.36 - 0.44 seconds at 60bpm
What are the 3 types of leads?
Bipolar limb leads
Augmented limb voltage leads
Chest leads
What are the 3 types of augmented voltage leads?
aVR (Right arm +ve)
aVL (Left arm +ve)
aVF (Left Food +ve)
What are the 6 chest leads?
V1
V2
V3
V4
V5
V6
What leads give a coronal plane view of the heart?
I
II
III
aVR
aVL
aVF
What leads give a transverse plane view of the heart?
V1 - V6
In what direction do the limb leads and augmented voltage leads record?
What is the name given to the system comprising of the 3 bipolar limb leads and 3 augmented voltage leads?
Hexaxial reference system
Which leads are lateral leads?
Lead I and aVL
Which leads are inferior leads?
II, III and aVF
Where should the 6 chest leads be placed?
V1 = 4th intercostal space, immediately right of sternum
V2 = `4th intercostal space, immediately left of sternum
V3 = Mid-way between V2 and V4
V4 = 5th intercostal space at mid-clavicular line
V5 = Same horizontal level as V4 at anterior axillary line
V6 = Same horizontal level as V4 at mid-axillary line
What colour is the right arm electrode?
Red (R)
What colour is the left arm electrode?
Yellow (L)
What colour is the left leg electrode?
Green (F)
What colour is the right leg electrode?
Black (N)
To what level should an ECG be calibrated?
25mm/second on the time (x) axis
10mm/1mV on the amplitude (y) axis
How can you calculate heart rate from an ECG?
HR = 300 ÷ Number of big squares between R waves
What is the ECG rhythm strip?
A prolonged section of an ECG, usually from Lead II
What are some examples of conditions that may not show up on an ECG?
Myocardial infarction
Intermittent rhythm disturbances
Stable angina
What 7 questions should you ask yourself when looking at an ECG?
- Is electrical activity present?
- Is rhythm regular or irregular?
- What is the heart rate?
- Are P-waves present?
- What is the PR interval? (Average 3-5 small squares)
- Is each P-wave followed by a QRS complex?
- Is the QRS duration normal? (< 2.5 small squares)
