Understanding ECGs Flashcards
what type of activity does an ECG show?
electrical activity (not mechanical ie wont see valves closing etc)
what is the name of the recording of potential differences in the heart?
electrocardiogram
what is the name for the separation of charge one after the other?
(depolarisation and then hyperpolarisation)
dipole
what do electrodes record?
the difference in potential between two points
when will the net potential measured by the electrodes be 0mV?
when the lead (ie imaginary line between 2 electrodes) is perpendicular to the dipole
what is the leading charge and what is the lagging charge of the cardiac dipole?
leading charge: positive
lagging chard: negative
what limb lead is the most parallel to the cardiac dipole?
lead II
how does a lead look at the moving charge?
looks from the positive electrode to the wave coming towards it from the negative electrode
what direction does the depolarisation have to move in order for an upwards deflection?
depolarisation needs to be moving towards positive electrode
ie positive electrode is looking at wave coming towards it
what are PR, ST and TP intervals flat on an ECG?
because there is no moving wave
ie no change in potential at that time
how are augmented limb leads formed?
1 of the limb electrodes acts as the positive recording electrode and the other 2 are coupled together as teh negative reference electrode
what limb electrodes make up aVR?
positive electrode: RA
negative electrode: LA+LL
what limb electrodes makes up aVL?
positive electrode: LA
negative electrode: LL+RA
what limb electrodes make up aVF?
positive electrode: LL
negative electrode: LA+RA
why is the aVR recordings downwardly deflecting?
because the wave is manly moving away from the positive electrode
why is the limb lead II particularly good for looking at electrical activity?
because the wave is parallel to the lead axis and so has a great net potential difference so has well resolved waves
why is limb III not too good for looking at electrical activity?
because the wave is almost perpendicular to the lead axis so has a very small net potential difference so has poorly resolved waves
how many chest electrodes are there?
6
where is chest electrode 1 and what lead does it form?
1: 4th intercostal space, R of sternum
lead = V1
where is chest electrode 2 and what lead does it form?
2: 4th intercostal space, L of sternum
lead = V2
where is the chest electrode 3 and what lead does it form?
3: between 2 and 4
lead = V3
where is the chest electrode 4 and what lead does it form?
4: 5th intercostal space midclavicular line
lead = V4
where is the chest electrode 5 and what lead does it form?
5: 5th intercostal space anterior axillary line
lead = V5
where is the chest electrode 6 and what lead does it form?
6: 5th intercostal space mid-axillary line
lead = V6
what gives the negative reference electrode for the chest leads?
all 3 limb leads combines
roughly the ecentre of the heart
what happens to the limb leads if chest leads arent working?
nothing
what happens to the chest leads if the limb leads aren’t working?
chest leads don’t work either
what plane do the limb leads show?
frontal plane
what plane do the chest leads show?
horizontal plane
how long should the P wave be?
0.08- 0.10 seconds
between 2/3 small boxes
how long should the PR interval be?
0.12 - 0.2s
between 3/5 small boxes
what should the normal callibration of ECG be?
paper speed = 25mm/sec
10mm amplitude = 1mV
what is one small square on ECG paper?
1mm, 0.04s
what is one big square on ECG paper?
5mm, 0.2s
how many big squares and little squares are there per second?
5 big squares per second
25 small squares per second
how do you figure out the heart rate?
300/
no.of big squares between QRS complexes
what is the ECG rhythm strip?
a prolonged recording of lead 2 which allows you to detect rhythm distrubances
what is the step by step approach to analysing an ECG rhythm strip?
- clarify patient
- date/time of ECG
- check calibration of the machine
- is electrical activity present?
- is the rhythm irregular or irregular?
- what is the heart rate?
- are there P waves present?
- is the PR interval normal?
- is each p wave followed by a QRS complex
- is QRS duration normal
- look for individual leads for changes
how would you tell if the cardiac rhythm is regular or irregular from a rhythm strip?
if rhythm is regular, QRS complexes occur at regular intervals
how can you calculate the heart rate from a rhythm strip if the rhythm is irregular?
count number of QRS complexes within 30 big squares and times that by 10
what can it mean if the PR interval is too long?
problem with AV conduction
what does it mean if each P wave is followed by a QRS complex?
sinus rhythm
how long should a QRS complex be?
less than 0.10 seconds
less than 3 small boxes
