ECG Theory and Practice Flashcards
describe ECG
gives electrical activity of the heart, recording of potential changes detected by electrodes positioned on the body surface - allowing electrical activity of the heart to be monitored
provides information abut cardiac rate rhythm, chamber size, electrical axis of the heart and is a main test to assess fr myocardial ischaemia and infarction
ECG is a vector graph of voltage versus time and shows the movement of depolarisation and repolarisation waves across the heart
describe electrical conduction in the normal heart
in sinus rhythm, the SA node generates action potentials automatically that conduct;
rapidly through atria => atria contraction
relatively slowly through AV node
very rapidly through bundle of His and left (2) and right (1) branches
rapidly through purkinje fibres => ventricular contraction
direction of depolarisation - endocardium to epicardium
direction of repolarisation - epicardium to endocardium
describe how ECG recorded
potentials at the body surface arise from currents that flow when the membrane potential of myocardial tissue is changing (depolarisation or repolarisation)
only large masses of cardiac tissue generate sufficient current to be detected at the body surface as potential changes (atrial and ventricular muscle - left ventricular has larger mass and so larger action potentials)
describe how the heart generates potentials upon the skin
electrical activity within and between (gap junctions) cardiac muscle cells cause;
current flow within the heart
current flow within surrounding tissue
potential differences between distant sites on the body surface - detected by electrodes placed on the skin coupled to a sensitive recording device => the electrocardiograph
electrocardiograph is used to record the electrocardiogram
describe physics of ECG
as the action potential propagate through conducting system of the heart it causes separation of charge, or differences in potential between cardiac regions
charges that are separated constitute an electrical dipole which is a vector with components of magnitude and direction (from atria to ventricles)
describe the electrical field generated by the cardiac dipole at one instant time
dipole represents electrical vector
electrical vector has components of magnitude (length) and direction (orientation) - this is clinically important as it allows the electrical axis of the heart to be estimated
magnitude - determined by mass of cardiac muscle that is involved in generation of signal
direction - determined by overall activity of heart at any instant time and varies during the cardiac cycle
describe leads and directions of ECG
lead is an imaginary line, the lead axis, between 2 (or more) electrodes
in a lead;
one electrode - recording (positive) device
when a wave of depolarisation moves towards recording electrode, generates upward deflection on ECG
when depolarisation moves away from recording electrode, it generates downward deflection on ECG
if there is no movement towards or away from recording electrode, there is no deflection on ECG (isopotential)
NOT the wire
describe types of leads in ECG
12 lead compromise a picture of the heart in different directions and planes;
vertical (frontal, coronal) - leads I, II, III, aVR, aVL, aVF
horizontal (transverse) - leads V1-V6
3 standard limb leads (I, II, III) - bipolar
3 augmented voltage (aV) leads - aVR (right) - detects vectors from atria, aVL (left) - detects vectors from left lateral surface of heart, aVF (foot) - detects vectors from inferior surface of heart - unipolar
6 chest leads (V1-V6), precordial leads - unipolar
describe the standard limb leads
bipolar Einthoven's triangle (refer PP) lead I - RA-ve to LA+ve lead II - RA-ve to LL+ve (used as rhythm strip) lead III - LA-ve to LL+ve
describe information given by lead II
sees heart from inferior direction
atrial depolarisation (contraction) spreads from SA node inferiorly and to the left - depolarisation is moving towards the recording electrode in lead II producing upward deflection in this lead => P wave
P wave duration in normal ECG reflects the time for the depolarisation of atrial muscle to be complete
QRS complex represents ventricular activation (depolarisation)
starts in inter-ventricular septum and spreads from left to right causing the small and narrow Q wave
the main free walls of ventricles depolarise causing tall and narrow R wave
ventricles at base of heart depolarise, causing small narrow S wave
T wave represents ventricular repolarisation
it is an upward deflection because the wave of repolarisation is spreading away from the recording electrode
describe intervals of lead II
PR interval - reflects the time for the SA node impulses to reach the ventricle. Strongly influenced by the delay in conduction through AV node (diagnostically important)
ST segment - isoelectric (elevation, depression - diagnostically important) systole
QT interval - reflects the time for ventricular depolarisation and repolarisation. Prolongation predisposes to disturbance of cardiac rhythm - drugs may be causative
TP segment - diastole
describe the augmented limb leads
unipolar
Goldberger’s method - one +ve electrode (recording), 2 others linked as -ve
effectively positions the reference (linked) electrode in the centre of the heart ccto which the recording electrodes look
refer PP
describe views of the heart in the frontal plane provided by the standard and augmented limb leads
see PP
hexaxial reference system - 6 views in total
describe limb lead records
leads I and aVL are lateral leads - each has the recording electrode on the left arm and views of the heart are from the left
leads II, III, and aVF are inferior leads - each has the recording electrode on the left foot and views the heart from an inferior direction
aVR - waves are negative - predominant vector is depolarisation moving away from the recording electrode
II - waves are positive and well resolved - predominant vector is depolarisation moving towards the recording electrode
describe chest (precordial) leads
unipolar
wilson’s leads
electrodes of the standard limb leads are all linked together to effectively provide a reference electrode in the centre of the heart
6 chest leads are between recording electrodes placed on the chest wall at defined positions and the central, reference, electrode
