ECG Flashcards
How are the electrical currents of the SAN detected?
The currents radiate through body fluids and so can be detected by electrodes on the skin.
What is a lead?
A view of the heart
What does an ECG measure?
The current flow of an electrical event through waveforms, doesn’t measure the AP. It monitors the phases of the cardiac cycle to be monitored.
How many leads can an ECG have?
12 leads
- 6 chest/precordial leads
- 6 limb leads
Where are the limb electrodes placed?
R (red) - Right arm L (yellow) - Left arm F (green) - Left leg N (black) - Right leg Placed on wrists and ankles away from bone
What type of leads produced by the limb electrodes?
3 augumented unipolar leads - Measure current from the centre of the chest radiating out to one of the electrodes.
3 standard bipolar leads - Measure the current between two electrode.
What is the purpose of N electrode?
To reduce and filter out any electrical interference
Name the limb lead views and which electrodes are used to produce them.
Bipolar: I = R to L II = R to F III = L to F (number of L's from the limbs = number of the lead)
Unipolar:
avR = centre to R
avL = centre to L
avF = centre to F
What is the difference between the recordings produced by chest leads and limb leads?
Chest leads have larger peaks as they are closer to the heart and is able to determine beats from the L or R side of the heart.
Where are the chest electrodes placed?
V1 - R 4th ICS (adjacent to sternum)
V2 - L 4th ICS (adjacent to sternum - tricuspid area)
V3 - Between V2 and V4
V4 - L 5th ICS midclavicular line (apex - mitral area)
V5 - L 4th ICS anterior axillary line
V6 - L 4th ICS midaxillary line
What do the electrodes measure?
The dipole seperation between depolarising and resting cells that is generated as the heart contracts.
What determines the shape of the recording produced?
The direction of current determines the shape of the deflection:
Towards the electrode = Upward
Away from electrode = Downward
What determines the size of the deflection?
Size is proportional to tissue mass.
What reading is produced if the flow travels at 90 degrees
No reading
What happens to the deflections during repolarisation?
The deflections are opposite:
Towards the heart = Downwards
Away from the heart = Upwards
What recording results from no net flow?
Isoelectric / flat line
Why is atrial repolarisation not shown on an ECG?
Overlaps with ventricular depolarisation
Which lead is normally used to read the ECG?
Lead II
What does the P wave represent?
Atrial depolarisation - SAN depolarises and spreads through atrium from R to F (upward deflection)
What does the Q wave represent?
Travels down bundle of his and spreads to R ventricle, moving away from F (Downward deflection)
Is it normal to have an absent Q?
Can be due to anatomical variation
What does the R wave represent?
Spread through ventricles from R to F (upwards)
What determines the size of R?
The muscle mass of the ventricles
What does S wave represent?
Spread up through ventricles away from F (downwards)
What does the QRS complex show?
Ventricular depolarisation
What does the T wave represent?
Repolarisation - It repolarises from outside to inside form F to R (upward as travelling away from F but is opposite in repolarisation)
How can HR be calculated from an ECG?
R to R peak = 0.6-1 seconds
How can the AVN delay be calculated?
P to R peak = 0.12-0.2 seconds
What is the average speed for ventricular depolarisation?
0.06-0.1 seconds
