1- ECG - theory & practice Flashcards
(34 cards)
what is the simplified physics of the ECG?
action potential through conducting system & muscles of the heart causes separation of charge/differences in potential between cardiac regions
- charges that are separated constitute an electrical dipole which is a vector with components pf magnitude & direction e.g. from atria to ventricles (these potential differences are what electrodes detect)
what is being recorded in an ECG?
recording of potential differences (potential differences in the order of a millivolt (mV] between distant sites on the body surface - detected by electrodes placed on the skin coupled to a sensitive recording device) against time
how many electrodes are placed on skin to record 12 lead ECG?
10 electrodes
How do potentials arise?
at the body surface, arise from currents that flow when membrane potential of myocardial tissue is changing (depolarisation or repolarisation)
- only large masses of cardiac tissue (i.e. atrial & ventricular muscle) generate sufficient current to be detected at body surface as potential changes
what is an ECG lead?
an imaginary line, the lead axis, between 2 or more electrodes (it is NOT the wire that connects electrode to recording device)
- in an ECG lead 1 electrode acts as a recording (positive) electrode (think like a seeing electrode)
- and when depolarisation moves towards recording electrode an upward deflection created and when depolarisation moves away from recording electrode, it generates a downward deflection on ECG
what is isopotential?
when no movement towards or away from the recording electrode and therefore no deflection on the ECG
what is each recording electrode detecting?
lead I = RA negative to left atrium positive
lead II = RA negative to LL positive
lead III = LA negative to LL positive
what is interpretation of lead I detection?
Atrial depolarisation spreads from SA node inferiorly and to the left - depolarisation is moving towards the recording electrode in lead II producing a (normally) upward deflection in this lead - the P wave represents atrial depolarisation
(from RA negative to LL positive)
what does RA, LA, LL stand for?
right arm, left arm, left leg
what is the interpretation of lead II?
all about QRS complex = ventricular depolarisation
- downward deflection preceding R wave is Q wave
- deflection upwards (+ve) is R wave (irrespective is preceded by Q wave)
- downward negative deflection following R wave is S wave
how long does QRS complex last?
0.1 seconds or less
what causes QRS complex?
- ventricular depolarisation starts in the interventricular septum and spreads from left to right causing the small & narrow Q wave
- subsequently the main free walls (walls not connected to middle) of the ventricles depolarise causing a tall & narrow R wave
- finally, the ventricles at the base of the heart depolarise, producing a small & narrows S wave
what is interpretation of lead III?
about T wave = represents ventricular repolarisation
- it is an upward (positive) reflection because the wave of repolarisation is spreading away from the recording electrode (think of it as negative charge moving away from recording electrode which is equivalent of positive charge moving towards it)
for each wave - is depolarisation moving towards or away from recording electrode?
P wave - towards (atrial depolarisation)
Q wave = away (left to right depolarisation of interventricular septum)
R wave = towards (depolarisation of main ventricular mass)
S wave = away (depolarisation at base of heart)
T wave = repolarisation away from recording electrode but like opposite of depolarisation so like positive moving towards it so positive inflection
what is the PR interval?
from the start of the P wave to start of the QRS complex
= reflects the time for SA node impulse to reach the ventricles (normally 0.12 - 0.2 seconds)
- very strongly influenced by the delay in conduction through the AV node
what is the ST segment?
from the end of the QRS complex to the start of the T wave - normally isoelectric
= ventricles contract (systole)
what is the QT interval?
from start of QRS complex to end of T wave
- primarily reflects the time for ventricular depolarisation & repolarisation
- normally 0.36 - 0.44s at heart rate of 60bpm (but needs to be corrected for heart rate)
what does 12 lead ECG comprise of?
3 standard limb leads (I, II, III) - termed bipolar
3 augmented voltage (aV) leads (aVR, aVL, aVF) - termed unipolar
6 chest leads (V1-6) - aka precordial leads
what do the 12 leads do?
provide a comprehensive picture of the heart in different directions & planes
vertical (frontal/coronal) = leads I, II, III, aVR, aVL, aVF
horizontal (transverse) = leads V1-6
what does looking at leads from different directions allow you to do?
- determine the axis of the heart in thorax
- look for any ST segment or T wave changes in relation to specific regions of the heart. This is crucial e.g. in diagnosing ischaemic heart disease
- look for any voltage criteria changes. This is crucial e.g. in diagnosing chamber hypertrophy
what is the hexaxial reference system?
6 views - of the heart in frontal plane provided by standard & augmented limb leads
what are lateral leads?
leads I and aVL - each has recording electrode on left arm and views from the left
what are the inferior leads?
leads I, III and aVF - each have recording electrode on left foot and views the heart from inferior direction
what are the V1-6 leads?
the 6 chest leads - recording electrodes placed on chest walls at defined positions
- the electrodes of the standard limb leads are all linked together to effectively provide a reference electrode in the centre of the heart
