The Normal ECG Flashcards
Why is atrial repolarisation not seen on ECG?
Because it happens at the same time as ventricle depolarisation, which is stronger
What does a P wave represent?
atrial depolarisation
What does a T wave represent?
Ventricular depolarisation
What does a U wave represent?
Unsure. Potentially the repolarisation of papillary and accessory muscles
What is a dipole?
A pair of equal and oppositely charged poles separated by a distance.
Reference must be negative and sensor must be positive
How do cells create a dipole?
If one is depolarised and one adjacent cell is resting, there is a dipole created due to the difference in electrical currents
What does a negative reading represent?
When the first cell repolarises, a reversal of the dipole occurs, the current wants to go towards the reference
What does the absence of a dipole represent?
When both cells are repolarised
What does a dipole require to be set up?
A conductive medium e.g. water.
Orientation and position of electrodes matters
How is the dipole measured in the heart when there are many different dipoles in different directions?
It is measured by the net dipole i.e. the sum of all vectors (diretion and magnitude). Generally DOWN when the myoseptum is depolarising
What is a lead?
A pair of electrodes that have a specific orientation, allowing you to measure from a reference point to a sensor
What does depolarisation heading in the same direction as a lead produce?
POSITIVE deflection
What does depolarisation heading in the opposite direction to a lead produce?
NEGATIVE deflection
What does depolarisation heading perpendicular to the direction of a lead produce?
ZERO deflection
What does repolarisatin heading in the same direction as a lead produce?
NEGATIVE deflection
What does repolarisation heading in the opposite direction to a lead produce?
POSTIVIE deflection
What does repolarisation heading perpendicular to the orientation of a lead produce?
ZERO deflection
What happens if a signal is not travelling horizontally or vertically?
The voltage recorded by a lead is simply the component of the depolarisation vector that is running/leaning in that direction
What happens if two leads are measuring at the same time?
The net depolarisation vector can be determined. Each deflection will equate to the strength of the signal in that direction.
What does the PR segment represent?
Delay at the AV node
What does the QRS complex represent?
ventricular depolarisation
what does an isoelectric line represent?
no electrical activity
What are some uses of the ECG?
- measure HR
- determine which part of the heart is acting as the pacemaker
- assess for enlargement
- assess for blockages
- look for signs and location of ischaemic injury
- look for signs of electrolyte abnormalities
What does the Wigger’s diagram show us?
Electrical activity, left ventricular volume and pressure
What is Einthoven’s triangle?
The basic 3 lead ECG
Lead I: RA-LA
Lead II: RA-LL
Lead III: LA - LL
What are augmented limb leads?
Leads that are produced without extra cables. They use two existing points as a negative
What are the three augmented limb leads?
aVF: femoral: LL - (RA+LA)
aVR: Right arm: RA - (LA+LL)
aVL: Left arm: LA- (RA+LL)
What is the axial reference system? What does it measure?
Axial reference system is created by superimposing all 6 limb leads over the heart
Can give an indication of the location of the heart in the thoracic cavity and hence any hypertrophy
What is a normal cardiac axis?
-30 to +90
What is Right axis deviation? What does it indicate?
90 - 180
indicates RV hypertrophy
What is left axis deviation? What does it indicate?
-90 to -30
LV hypertophy
What is extreme axis deviation?
No man’s land
180 to -90
What are chest leads and what do they do?
6 leads that allow us to look at the heart in 3D and show us the exact location of where the ischaemia or issue is. all measure the current coming towards them
