ECG Flashcards
describe how repolarisation spreads through the heart tissue
epicardium to endocardium
describe how depolarisation spreads through the heart tissue
endocardium to epicardium
what is an ECG
recording of potential changes detected by electrodes positioned on the body surface, allows monitoring of heart activity
where do the potentials detected arise from
currents that flow when the membrane potential of myocardial tissue is changing (de/repolarisation)
why does the left ventricle have a bigger influence on the ECG
as bigger mass
can you detect the electrical activity in the nodes
no too small
is it the intra and extracellular current that is detected by an ECG
extra
what is an electric dipole
electrical vector-separated charges, move from atria to ventricles, positive charge first
what are the components of the vector electrical dipole
magnitude and direction
what allows the electrical axis of the heart to be estimated
lines of potential created by the cardiac dipole and their direction
what determines the mass of the electrical vector
the mass of the cardiac muscle
what determines the direction of the electrical vector
overall activity of the heart
describe the direction of depolarisation during the P wave on an ECG
atrial depolarisation moving towards the recording electrode
describe the direction of depolarisation during the Q wave on an ECG
left to right depolarization of the interventricular septum moving slightly away from the recording electrode
describe the direction of depolarisation during the R wave on an ECG
depolarization of the main ventricular mass moving towards the recording electrode
describe the direction of depolarisation during the S wave on an ECG
depolarization of ventricles at the base of the heart moving away from the recording electrode
describe the direction of repolarisation during the T wave on an ECG
ventricular repolarization moving in a direction opposite to that of depolarization accounts for the usually observed upward deflection
what is an ECG lead
the electrical picture obtained of the heart, not the wire
what type of deflect does depolarisation cause when moving towards the electrode
upward deflect
what type of deflect does depolarisation cause when moving away from the electrode
downward deflection
describe an isopotential deflection
no movement of current, no deflection
what are the bipolar leads
standard limb leads (I,II and III)
decsribe the reletionship between the augmented voltage leads
one recording two linked as reference (three all together)
what are the three augmented voltage leads
aVright, aVleft, aVfoot
are the aV leads unipolar or bipolar
unipolar
are the chest leads unipolar or bipolar
unipolar
what leads provide a picture of the heart from a vertical plane
I, II, III, aVR, aVL, and aVF
what leads provide a picture of the heart from a horizontal plane
V1 to V6 (chest leads)
what is the recording electrode for lead 1
RA-ve to LA+ve
what is the recording electrode for lead 2
RA-ve to LL+ve
what is the recording electrode for lead 3
LA-ve to LL+ve
what is bipolarity
A lead composed of two electrodes of opposite polarity is called bipolar lead
what is unipolarity
A lead composed of a single positive electrode and a reference point is a unipolar lead
what is the right legs role in an ECG
is earthed
from which direction does lead 2 see the heart
from an inferior direction
why is the p wave an positive deflection when shown in lead 2
as depolarisation spreads from SA node inferiorly and to the left
what is a normal duration for a P wave in an ECG and what does it represent
time for atrial muscle depolarisation. normally less than 0.120s
what is the downward deflection preceding and R wave called
Q wave
what is an R wave
an upwards deflection irrespective of whether it is proceeded by a Q wave
what is an s wave
a downward deflection following an R wave
what does the QRS complex represent
ventricular depolarisation
how long does a normal QRS complex last
0.1s or less
what causes the Q wave as seen via lead 2
as ventricular depolarisation starts in the inter ventricular septum and spreads from left to right
describe the R wave and what causes it
following the Q wave the main free walls of the ventricles depolarise causing a tall and narrow R wave
describe the S wave and what causes it
finally the ventricles at the base of the heart depolarize, producing a small and narrow S wave
what does the T wave represent
ventricular repolarisation
describe the deflection of the T wave seen from lead 2
an upward (positive) deflection because the wave of repolarization is spreading away from the recording electrode
where does the PR interval start and end
from start of P wave to the start of the QRS complex
what does the PR complex reflect
time for the SA node impulse to reach the ventricles
what is the PR interval normally
0.12 – 0.2 s
what is the PR interval strongly influenced by
delay in conduction through the AV node
what is the position of the ST segment
is from the end of the QRS complex to the start of the T wave
describe the ST segment
normally isoelectric – elevation, or depression, is diagnostically important
what is the QT interval
from the start of the QRS complex to the end of the T wave
what does the QT interval reflect
primarily reflects the time for ventricular depolarization and repolarization
what is the normal duration of the QT interval in males and females
0.44s in males, 0.46 in females
what does prolongation of the QT interval predispose an individual to
disturbances of cardiac rhythm
describe goldberger’s method
one +ve electrode (recording), two others linked as –ve. This effectively positions the reference (linked) electrode in the center of the heart to which the recording electrodes ‘look’
does the machine use the positive or negative electrode to see the lead
positive, negative used as a reference. Line of site= neg to pos electrode or in unipolar average of neg electrodes to pos electrodes (e.g augmented)
what is the negative reference for the chest leads
average of all limb leads, same for all chest electrodes
what is the hexaxial reference system
6 views of the heart in the frontal plane via standard (1-3) and augmented leads
describe aVR waves
negative as predominant vector is depolarisation moving away from the recording electrode
describe lead 2 waves
are positive and well resolved – predominant vector is depolarization moving towards the recording electrode
what are lateral leads and which leads are they
I and aVL- each has the recording electrode on the left arm and views the heart from the left
what are inferior leads and which leads are they
II, III and aVF- each has the recording electrode on the left foot and views the heart from an inferior direction
describe the view of the heart provided by the chest leads
different positions in the horizontal plane
what do leads V1 and V2 look and from what direction
the interventricular septum from the right
what do leads V3 and V4 look and from what direction
anterior of the heart
what do leads V5 and V6 look and from what direction
later aspect (left ventricle) of the heart
in V1 what is the first positive defection in the QRS complex and the negative deflection that immediately follows
R wave then S wave
what happens to the R wave and S wave that follow as you go from V1 to V6
R wave progressively increases and S wave progressively decreases
where is V1 placed
fourth intercostal space immediately right of sternum
where is V2 placed
fourth intercostal space immediately left of sternum
where is V3 placed
mid way between V2 and V4
what is V4 placed
fifth intercostal space in the midclavicular line
where is V5 placed
same horizontal level as V4 in the anterior axillary line
where is V6 placed
same horizontal level as V4 in the mid axillary line
where are the ECG waves bets seen
lead 2
how long does a P wave usually last
0.8 to 10 sec
how long does a QRS complex last
less than 0.10 sec
at what part of the ECG do the ventricle contract
ST segment (systole)
at what part of the ECG do the ventricles relax
TP segment (diastole)
what is the P wave and how long does it usually last
atrial depolarization (0.08 - 0.10 sec)
what does the QRS complex represent
ventricular depolarisation
what does the T wave represent
ventricular repolarisation
what does the QT interval represent
depolarisation and repolarisation
how do you calculate heart rate from an ECG
300/number of large squares between beat (R-R intervals)
what time does a big box on an ECG represent
0.2 seconds
what time does a small box on an ECG represent
0.04 seconds
what in the ECG rhythm strip
prolonged recording of one lead (usually lead 2) which allows you to detect rhythm disturbance
what reasons (3) show the need for 12 leads
- to determine the axis of the heart
- look for any ST segment or T wave that changes in relation to any specific region of the heart (crucial for ischaemic heart disease)
- look for any voltage criteria changes (crucial for chamber hypertrophy)
what are the 6 key steps in analysing an ECG
1 Verify patient details: name and date of birth
2 Check date and time ECG was taken
3 Check the calibration of the ECG paper
4 Determine the axis, if possible
5 Work out the rhythm
6 Look at individual leads for voltage criteria changes OR any ST or T-wave changes
what 7 questions do you ask yourself to workout rhythm
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?
how many small squares should the PR interval be
3-5 small/ 1 big
what are three heart diseases which have a normal resting ECG
myocardial infarction (may or may not have ECG changes). intermittent rhythm disturbance, stable angina (do exercise ECG)
how long is each small square
0.04 seconds
how long is each big square
0.2 seconds
how do you calculate the heart rate when its irregular
count number or QRS complexes in 30 big squares and times by 10
what leads are inferior and what coronary artery is this
II, III, aVF
right coronary artery
what leads are anterior and what coronary artery is this
V1-4
left anterior descending
what leads are lateral and what coronary artery is this
I, V5-6
circumflex
how is right axis deviation shown on an ECG
lead I down, lead aVF up
how is left axis deviation shown on an ECG
lead I up, lead aVF down
how long is the PR interval usually
bigger than 3 small, smaller than 1 big
what are the 6 steps in reading an ECG
1-verify patient details
2-check date and time when ECG was taken
3-check calibration, 25mm per second and 1cm/1mV
4-determin the axis- look at lead I and aVF- if both upright axis normal
5-rhythm strip; electrical activity? Regular/ irregular? Rate? P-waves present? What is the PR interval? Each P wave followed by a QRS complex? QRS duration normal?
6-individual leads for voltage criteria changes OR ST or T wave changes
when is ST elevation significant
when at least 2mm in 2 adjacent chest leads
or
1mm in limb leads
what does no P waves and irregularity mean on an ECG
A fib
what does tall QRS’s in V4, 5 and 6 mean
LVH
what leads show anteroseptal
V1-4
what leads show anterolateral
I, aVL, V1-6
which coronary artery is usually affected in an inferior MI
right coronary
what coronary artery is usually affected in an anterior MI
left coronary
