Flashcards in ECG theory and Practice Deck (78)
What happens in sinus rhythm?
SA node conducts generates AP automatically that conduct rapidly through the atria causing atrial contraction
Relatively slowly through the AV node
-Rapidly through the bundle of His and the left and right bundle branches
- Rapidly through the purkinje fibers causing ventricular contraction
What is an ECG?
Recording of potential changes detected by electrodes
positioned in the body surface that allows the electrical activity of the heart to be monitored in a simple manner which is non invasive
How do potentials at the body surface occur?
Arise from currents that flow when the membrane potential of the myocardial tissue is changing (depolarization or repolarization)
What info can an ECG provide?
Electrical axis of heart
Assess for myocardial ischaemia and infarction
What does electrical activity within and between cardiac muscle cells cause?
Current flow within the heart
Current flow within the surrounding tissues
Potential differences between different sites on the body surface
What is a recording between potential differences at different sites of the body called?
Electrocardiogram (ECG) or EKG
Simplified physics of an ECG?
The AP propagating in sequence through the conducting system and muscle of the heart causes separation of charge or differences in potential between cardiac regions
Charges that are separate are called an electrical dipole which is a vector with components of magnitude and direction (eg atria to ventricle)
What does a dipole represent?
Why is an electrical vector important in an ECG?
Clinically important as it allows the electrical axis of the heart to be estimated
Has components of magnitude and direction (orientation of arrow)
What is magnitude of an electrical dipole?
Determined by mass of cardiac muscle that is involved in generation of signal
What is direction of an electrical dipole determined by?
Overall activity of the heart at any instant in time and varies during cardiac cycle
What is an ECG lead?
Imaginary line (not the wire) between 2 or more electrodes
In a lead what do the two electrodes act as?
Recording electrode (red)
Referencing electrode ( green)
What happens on the ECG when depolarization moves toward the recording electrode?
Generates an upward deflection
What happens on ECG when depolarization moves away from recording electrode?
Generates a downwards deflection
What is a flat line on ECG referred to as?
What do you call it when there is no movement on an ECG?
No movement towards or away from recording electrode
What does a 12 lead ECG comprise of?
3 standard limb leads - 1,2 and 3- termed bipolar
3 augmented voltage leads (aV) - aVR right, aVL left and aVF foot - termed unipolar
6 Chest leads V1-V6 (precordial leads)
What do the 12 leads provide?
Comprehensive picture of the heart on different planes and from different directions
What leads show the vertical (frontal) plane?
Leads 1, 2 and 3
aVR, aVL, and aVF
What leads show the horizontal plane?
Where is Lead 1?
RA (-ve)to LA (+ve)
Where is lead 2?
RA (-ve) to LL (+ve)
Where is lead 3?
LA (-ve) to RL (+ve)
What direction does lead 2 see the heart from?
Describe what is seen for lead 2 and why?
Normally upward deflection (P wave)
Atrial depolarization spreads from the SA node inferiorly and to the left (depolarization is moving towards the recording electrode in lead 2
Which type of electrode is +ve?
How long should a P wave last?
0.12 seconds or less
What is the downward deflection preceeding an R wave called?