Understanding an ECG Flashcards
p waves represent
atrial depolarisation
in healthy individuals there should be a p wave
preceding each QRS complex
the QRS complex looks. like
what is the PR interval
begins at the start of P wave and ends at the beginning of the Q wave
what does the PR interval reppresent
time taken for electrical activity to move between the atria and the ventricles
what does the QRS complex represent
represents the depolarisation of the ventricles
what is the ST segment
starts at the end of the S wave and begins at the beginning of the T wave
what does the ST segment represent
the ST segment is an isoelectric line representing the time between depolarisation and repolarisatiion of the ventricles (ie. ventricular contractions
what is the T wave
the T wave represents ventricular repolarisation
it appears as a small wave after the QRS complex
what is the RR interval
the RR interval begins at the peal of one R wave and ends at the peak of the next R wave
what does the RR interval represent
the time between two QRS complexes
what is the QT interval
begins at the start of the QRS complex and finishes at the end of the T wave
what does the QT interval represent
the time taken fo the ventricles to depolarise and then repolarise
where is the PR interval
begins at the start of P wave and ends at the beginning of the Q wave
where is the PR interval
begins at the start of P wave and ends at the beginning of the Q wave
each small square represents
0.04 seconds
each large square represents
0.2 seconds
5 large squares represents
1 second
300 large squares represent
1 minute
what is an ECG electrode
a conductive pad that is attached to the skin to record electrical activity
what is an ECG lead
a graphical representation of the heart’s electricaal activity which is calculated by analysing data from several ECG electrodes
how does a 12-lead ECG works
records 12 leads, producing 12 separate graphs on a piece of paper
only 10 physical electrodes are attached to the patient to generate the 12 leads
there are 6 chest electrodes and 4 limb electrodes
what are the chest leads
V1: septal view of the heart
V2: septal view of the heart
V3: anterior view of the heart
V4: anterior view of the heart
V5: lateral view of the heart
V6: lateral view of the heart
what are the other leads
lead I: lateral view (calculated by analysing activity between the RA and LA electrodes)
lead II: inferior view (calculated by analysing activity between the RA and LL electrodes)
lead III: inferior view (calculated by analysing activity between the LA and LL electrodes)
aVR: lateral view (calculated by analysing activity between LA+LL -> RA)
aVL: lateral view (calculated by analysing activity between RA+LL -> LA)
aVF: inferior view (calculated by analysing activity between RA+LA -> LL)
if the R wave is greater than the S wave
it suggests depolarisation is moving towards that lead
If the s wave is greater than the R wave
depolarisation is moving away from the lead
if the R and S waves are of equal size
depolarisation is travelling at exactly 90° to that lead
what is a deflection
when the electrical activity within the heaart travels towards a kead you get positive deflection
when electrical activity travels away from the lead you get negative deflection
the height of the deflection (wave) on the ECG represents the amount of electrical activity flowing in that direction (toward the lead)
how do you localise pathology using ECG
leads represent anatomical territory
which leads show the inferior view of the heart
II, III, aVF
which leads show the lateral view of the heart
I, aVL, aVR, V5, V6
which leads show the anterior view of the heart
V3, V4
which leads show the septal view of the heart
V1, V2
in healthy individuals, how does electrical activity flow
begins at the sinoatrial node then spread to the atrioventricular (AV) node
it then spreads down the bundle of His and then Purkinje fibres to cause ventricular contraction
how is a positive deflection produced
when electrical activity moves towards a lead
how is negative deflection produced
when electrical activity moves away from a lead
the cardiac axis
in healthy individuals, what is the normal cardiac axis
between -30° and +90°
where should you see a positive deflection
the overall direction of electrical activity is towards leads I, II, III, therefore you should see a positive deflection in all of these leads, with lead 2 showing the most positive direction
where should you see a negative deflection
in the aVR lead
due to the aVR providing a viewpoint of the heart from the opposite direction
what is right axis deviation
Right axis deviation (RAD) involves the direction of depolarisation being distorted to the right (between +90º and +180º)
what causes right axis deviation
The most common cause of RAD is right ventricular hypertrophy. Extra right ventricular tissue results in a stronger electrical signal being generated by the right side of the heart. This causes the deflection in lead I to become negative and the deflection in lead aVF/III to be more positive.
what is right axis deviation associated with
RAD is commonly associated with conditions such as pulmonary hypertension, as they cause right ventricular hypertrophy. RAD can, however, be a normal finding in very tall individuals.
what does right axis deviation look like
the deflection in lead 1 is more negative and the deflection in lead aVF/III is more positive
what does left axis deviation look like
Left axis deviation (LAD) involves the direction of depolarisation being distorted to the left (between -30° and -90°). This results in the deflection of lead III becoming negative (this is only considered significant if the deflection of lead II also becomes negative).
what causes left axis deviation
LAD is usually caused by conduction abnormalities.
