Introduction Flashcards
In sinus rhythm, where are the acton potentials generated
SA node (pacemaker)
Describe where the action potential travels after being generated in the SA node
SA node > rapidly through the atria (atrial contraction) > relatively slowly through the AV node > very rapidly through the bundle of His and left and right bundle branches > rapidly through the Purkinje fibres (ventricular contraction)
True or false - an ECG lead is the wire that connects the electrode to the recording device
FALSE - it is the imaginary line, the lead axis, between two (or more) electrodes
In a lead one electrode acts as the recording (positive) electrode. (Think of it as the ‘seeing electrode’).
When depolarization moves towards the recording electrode, it generates a(n) UPWARD/ DOWNWARD deflection on the ECG
upward
(and when depolarization moves away from the recording electrode, it generates a downward deflection on the ECG
What happens if there is no movement towards or away from the recording electrode
there is no deflection on the ECG (it is isopotential)
Describe what the different leads are called in a 12 lead ECG
Three standard limb leads (I, II and III)
Three augmented voltage (aV) leads [aVR (right), aVL (left) and aVF (foot)]
Six chest leads (V1 – V6) (aka precordial leads)
Collectively, the 12 leads provide a comprehensive ‘picture’ of the heart in different directions and planes
Which leads represent the
- vertical
- horiztonal
Direction
Vertical: I,II, III, aVR, aVL and aVF
Horiztonal: V1-V6
Which lead is often used as the rhythm strip?
II (one of the standard limb leads)
The three limb leads for a triangle called what?
Einthoven’s triangle (equilateral triangle where the heart is at the centre)
Draw Einthoven’s triangle
look it up (inc which end it -ve and which is +ve)
Depolarization moving towards the recording (+ve) electrode gives an _____ deflection on the ECG, whereas away from it causes a _____ deflection
1) upward
2) downward
What is the p wave and how long should it be?
depolarisation of atrial muscle (<0.12s)
What is the QRS complex and how long should it be?
Ventricular activation/ depolarisation (≤0.1s)E
What is the t wave?
Ventricular repolarisation
state which each aspect of the QRS complex represents
Q wave: depolarisation of the IV septum
R wave: depolarisation of the main free walls of the ventricles
S wave: depolarisation of ventricles at the base of the heart
What is the PR interval and how long should it last?
Start of the P wave -> start of the QRS complex
Reflects the time for the SA node impulse to reach the ventricles, largely caused by AVN delay (0.12 – 0.2s)
what is the ST segment?
end of the QRS complex - start of the T wave.
Normally isoelectric
Ventricular systole
What is the QT interval and how long should it be?
Start of the QRS complex - end of the T wave.
Ventricular depolarization and repolarization, (0.44s in males, 0.46s in females)
Prolongation predisposes to disturbances of cardiac rhythm – drugs may be causative
What are aVL, aVR and aVF called?
augmented limb leads (unipolar)
name the lateral leads
I and aVL - each has the recording electrode on the left arm and views the heart from the left
name the inferior leads
II, III and aVF - each has the recording electrode on the left foot and views the heart from an inferior direction
state which view of the heat the precordial leads have
V1 and V2: septal (coming from the right ‘looking’ at the IV septum)
V3 and V4: anterior
V5 and V6: lateral (left ventricle)
describe the placement of the precordial leads
V1: 4th IC space, immediately right of the sternum V2: same on left V3: between V2 and 4 V4: 5th IC space, midclavicular line V5: 5th IC space, anterior axillary line V6: 5th IC space, mid axillary line
normal ECG calibration
25mm/sec
how to calculate HR from the rhythm strip?
300/ no. of large squares between R waves
OR
No. of R waves x 6
in normal calibration, how many squares is 1 second, 0.2s, 0.04s
5 large squares = 1 s
1 large square = 0.2s
1 small square = 0.04s
6 key steps when analysing an ECG
1) Patient details: Name and date of birth
2) Date + time taken
3) Calibration of the ECG paper
4) axis
5) rate and rhythm
6) individual lead inspection for voltage criteria changes or any ST/ T-wave changes
for step 5 of analysing an ECG (work out the rate and rhythm) what 7 questions should you ask?
- 1 Is electrical activity present?
- 2 Is the rhythm regular or irregular?
- 3 What is the heart rate?
- 4 P-waves present?
- 5 What is the PR interval?
- 6 Is each P-Wave followed by a QRS complex?
- 7 Is the QRS duration normal?
