Intro to ECGs Flashcards
What is an ECG?
An electrical tracing of the heart showing the hearts generation of electrical impulses which travel in different directions simultaneously - measuring these gives us an idea of the condition of the heart
What are the two types of cardiac muscle cells?
myocardial conduction cells (1%)
myocardial contractile cells (99%)
Function of myocardial conduction cells
create and propagate electrical impulses
Function of myocardial contractile cells
contract to propel blood and have an extended refractory period to allow for complete contraction
4 characteristics of cardiac cells
Automaticity: ability to spontaneously create and impulse by themselves
Excitability: ability to respond to impulses
Conductivity: ability to transmit
Contractility: how well the cell contracts after receiving a stimulus
How quickly does each part of the electrical pathway fire electrical impulses?
SA node = 60-100bpm
AV node = 40-60bpm
AV bundle = 30-40bpm
Bundle branches = 20-30bpm
Purkinje = 15-20bpm
What do electrodes do?
measure the direction and amplitude of the heart’s electrical impulses
different electrodes provide different views of the heart - can work out which part of the heart is not working properly
What is a lead?
view of the heart’s activity between two electrodes
Electrode placement (5 points)
patient should sit still
shave hair
keep electrical devices away
wiggle the green lead if Zoll isn’t picking up the ECG
ask patient not to talk
Which order do the electrodes go on the patient
RIDE YOUR GREEN BIKE
RED = RA
YELLOW = LA
GREEN = LF
BLACK = RF
Lead I
left arm to right arm
Lead II
left leg to right arm
Lead III
left leg to left arm
in the context of ECGs, what does augmented mean?
the ECG machine makes the electrical impulses bigger
What view of the heart does the AVL lead show?
left arm to centre of chest
What view of the heart does the AVR lead show?
right arm to centre of chest
What view of the heart does the AVF lead show?
left leg to centre of chest
How many seconds and voltages does one tiny square on ECG paper represent
0.04s and 0.01mV
How many seconds and voltages does one big square on ECG paper represent
0.2 seconds and 0.5mV
How many seconds do 5 large boxes represent?
1 second
If an action potential moves towards a positive electrode what type of deflection is created?
positive
What does the P wave represent?
atrial contraction
What does the PR interval represent?
impulse to AV node is slowed
What does the T wave represent?
repolarisation
What does the ST segment represent?
period after ventricular contraction
What does the QRS complex represent?
ventricular contraction
What does the QT interval represent?
depolarisation and repolarisation of the ventricles
Why do we not see atrial repolarisation on an ECG?
Happens at the same time as ventricular but doesn’t gain enough amplitude to be seen on an ECG
what type of tissue conducts action potentials
nerve
What change inside of nerve cells does an ECG pick up?
the inside of cells is negative, but when an action potential starts it becomes negative due to the movement of ions
if the action potential travels towards the green electrode what type of deflection is created
positive
isoelectric line
baseline of the ECG tracing
Why is there negative deflection before the QRS complex?
the left bundle branch is insulated further down than the right bundle branch. This means a wave of positive energy travels up the septum to a negative electrode = negative deflection
Why does ventricular repolarisation cause positive deflection?
because the ventricles repolarise from the outside inwards = negative energy towards negative electrode = positive deflection
10 step approach to ECGs
- is it regular
- what is the rate
- are there p waves
- are all p waves the same
- is there a p wave before every QRS
- is there a QRS after every p wave
- is the PR interval normal (3-5 small)
- is the QRS normal (1-3 small)
- is the ST segment isoelectric
- is the t wave normal
10 step approach: Are there P waves (step 3)
1.5 - 3 small boxes
negative in aVR
10 step approach: is the QRS complex normal (step 8)
1-3 small squares
more than 0.12 seconds = abhorrent conduction/branch blocks
less than 0.4 seconds = supraventricular origins / tachycardias
10 step approach: is the ST segment isoelectric (step 9)
depression = damage
elevation = MI (can be NSTEMI)
10 step approach: is the T wave normal (step 10)
inverted = signifies ischaemia
inverted in aVR
Upright in leads I, II and V3-6
Where do sinus rhythms originate
SA node
