ECG Flashcards
What is an ECG
Monitors electrical activity of the heart by recording potential changes on the body surface via electrodes that is simple and non-invasive
How do the potentials at the body surface arise
From currents that flow when the membrane potential of only large masses of myocardial tissue changes
What information does an ECG provide and what diseases does it detect (5)
Cardiac rate Cardiac rhythm Chamber size The electrical axis of the heart Main test to assess myocardial ischaemia and infarction
Electrical activity within and between myocytes causes (2)
Current flow within the heart and surrounding tissues
Potential differences between distant sites on the body surface that is recorded by the electrocardiograph
Physics of the ECG (3)
The AP propagating in sequence through the conducting system and heart muscle causes separation of charge
Charges that are separated constitute an electrical dipole which is a vector
The net dipole during contraction moves from negative to positive
Why is the electrical vector vital clinically
It has a magnitude and direction that allows the electrical axis of the heart to be estimated
Magnitude is determined by
The mass of cardiac muscle that generates the signal
Direction is determined by
The overall activity of the heart at any instant in time during the cardiac cycle
An ECG lead is the (2)
Imaginary line - The lead axis between 2 or more electrodes
It is NOT the wire that connects the electrode to the electrocardiograph
Measured potential is the greatest when
The lead axis is parallel to the direction of the dipole and zero when they are perpendicular
In a lead one electrode acts as a _________ while the other is the _________
Reference electrode
Recording (positive) electrode
When depolarization moves towards the recording electrode what deflection is produced
An upward deflection
When depolarization moves away the recording electrode what deflection is produced
A downward deflection
The 12 lead ECG comprises
3 standard limb leads (I,II and III) - These are bipolar and vertical (or coronal)
3 augmented voltage leads (aVR - right), (aVL - left) and (aVF - foot) - These are unipolar and vertical (or coronal)
6 chest leads (V1 - V6) - These are percordial and transverse
Standard Limb Lead placements and terminals (3)
Lead I : Right Arm negative to Left Arm positive
Lead II : Right Arm negative to Left Leg positive
Lead III : Left Arm negative to Left Leg positive
P wave (3)
Duration shows atrial depolarization to be complete
Depolarization moves towards recording electrode in lead II producing an upward direction
Duration is 80 to 100 ms
QRS complex (4)
Represents ventricular depolarization
Has a duration of 100 ms or less
A downward (negative) deflection preceding an R wave is called a Q wave
A deflection upwards (positive) irrespective of the Q wave is called an R wave
A downward (negative) deflection following an R wave is called an S wave
QRS complex vectors (3)
Q wave is down-right
R is down-left
S is up-right
T wave (2)
Represents ventricular repolarization
Its an upwards (positive) deflection due to wave of repolarization spreading away from recording electrode
PR interval (3)
Reflects the time for the SAN impulse to reach the ventricles
Duration is 120 - 200 ms
It is influenced by the delay of conduction through the AVN
ST segment (2)
Point of ventricular systole
Normally isoelectric
QT interval (3)
Reflects the time for ventricular depolarization and repolarization
Duration is 440 ms in males and 460 ms in females
Prolongation predisposes to disturbances of cardiac rhythm
Augmented Limb Lead Placements and terminals (3)
aVR is Right Arm (+) to Left Arm and Left Foot (-)
aVL is Left Arm (+) to Right Arm and Left Foot (-)
aVF is Left Foot (+) to Right Arm and Left Arm (-)
Limb Lead Records (2)
Leads I and aVL are lateral leads – each has the recording electrode on the left arm and views the heart 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
