Route of electrical discharge in the heart:
SA node -> AV node -> Bundle of His -> R/L bundle branch -> Purkinje fibres
What part of the heart rhythm is associated with the P wave?
Depolarisation of the atria
What part of the heart rhythm is associated with the QRS wave?
Depolarisation of the ventricles
What part of the heart rhythm is associated with the T wave?
REpolarisation of the ventricles
What is the standard ECG calibration (speed)?
25mm/s
If the machine is calibrated at 25mm/s, and a large square is 5mm long, how much time does a large square correspond to?
0.2 seconds = 200 milliseconds
If the machine is calibrated at 25mm/s, and a small square is 5mm long, how much time does a small square correspond to?
0.04 seconds = 40 milliseconds
- How many large squares are there in one second?
- How many large squares are there in one minute?
- How can you use this to calculate HR?
- 5
- 300 (5x60)
- Divide 300 by the number of large squares between successive R waves
E.g. 3 large squares between R waves –> 300/3 i.e. HR=100bpm
- What is the PR interval?
- How is it measured on the ECG strip (i.e. start and end)?
- How long is it usually:
i. In squares?
ii. In milliseconds?
- time taken for excitation to spread from the SA node into the ventricular muscle
- From beginning of P wave to beginning of QRS (think PR should be called PQ interval)
- i. 3-5 small squares
ii. 120ms - 200ms
- What is the QRS interval?
- How is it measured on the ECG strip (i.e. start and end)?
- How long is it usually:
i. In squares?
ii. In milliseconds?
- How long it takes for excitation to spread through ventricles
- From start of Q wave to end of S wave
3.
i. No more than 3 squares
ii. 120ms
How long is a QT interval?
Less than 450ms
What is the calibration (mV/cm) of an ECG machine?
1 mV (millivolt) should move the stylus 1 cm vertically
- Calibration signal of 1mV:1cm should be included in every ECG
- How many electrodes are there (and where do they go?)
- Mnemonic for limb electrodes
- Where to place electrodes
- There are 10 electrodes (4 on limbs and 6 on chest)
- Starting on R arm -> Riding Your Green Bike (Red, Yellow, Green, Black)
3.
» V1 - 4th intercostal space (ICS) at the right border of the sternum
» V2 - 4th ICS at the left border of the sternum
» V3 - Midway between placement of V2 and V4
» V4 - 5th ICS at the midclavicular line
» V5 - Anterior axillary line on the same horizontal level as V4
» V6 - Mid-axillary line on the same horizontal level as V4 and V5
What plane do the limb leads assess?
Vertical place
What plane do the chest leads assess?
Horizontal place
Which LIMB leads look at the following parts of the heart:
1. Left lateral surface
2. Inferior surface
3. No lead contribution
- aVL, I, II (-30º, 0º, +60º respectively)
- aVF, III (+90º, +120º respectively)*
- aVR (-150º)
Link to image exemplifying this –> https://www.google.com/search?q=ecg+leads&oq=ecg+leads&aqs=chrome..69i57j0i512l6j69i61.1744j0j7&sourceid=chrome&ie=UTF-8#imgrc=1_l2DdllOKtBOM
*on other diagrams, II, III and aVF are considered inferior leads
Which CHEST leads look at the following parts of the heart:
1. Right ventricle
2. Ventricular septum
3. Anterior and lateral walls of Left ventricle
- V1 + V2
- V3 + V4
- V5 + V6
What is the normal range of the cardiac axis?
From -30º (aVL) to +90º (aVF)
What are the most common causes of R axis deviation?
RV Hypertrophy
What are the most common causes of R axis deviation?
RV Hypertrophy
What is the most common cause of L axis deviation?
Conduction defects –> L anterior fascicular block
LVH
