ECG Flashcards
What’s an ECG?
graphic representation of electrical pd changes against time of myocardium in cardiac cycle
What needs to be considered when looking at QRS complex?
- orientation of recording leads
- concept of heart as an electrical dipole
- change in dipole size + orientation during ventricular excitation
What are the different ECG leads?
6 coronal (attached to limbs) 6 transverse (attached to chest)
What do the 6 frontal plane/ limb leads consist of?
3 bipolar leads (I, II, III)
What are the 3 bipolar leads derived from?
Einthoven bipolar lead
How are the bipolar leads designated?
each records difference in electrical potential between limbs
What are the 3 unipolar leads?
derived leads: I, II, III, to form aVR, aVL, aVF L–left wrist R–right wrist F–left ankle N–right ankle
What are the diff horizontal ‘Chest’ leads?
V1-6
Where’s V1 placed?
4th IC, R sternal edge
Where’s V2 placed?
4th IC, L sternal edge
Where’s V3 placed?
equidistant V2-V4
Where’s V4 placed?
5th IC, midclavicular line
Where’s V5 placed?
L anterior axillary line in horizontal line w V4
Where’s V6 placed?
mid axillary line, horizontal w V4 + V5
Why P wave?
Hump before QRS Atrial depolarisation Electrically one structure Starts at SAN (SR) P wave morphology changes dependant on origin of depolarisation
Why PR interval?
Onset of P wave to the onset of the QRS complex
Time taken from SAN to ventricles
Measures AVN conduction time
120-200ms
Why QRS complex?
Ventricular depolarisation
Occurs after AVN/His/Purkinje system traversed
What’s QRS interval?
Beginning of QRS to its end
1st deflection can be negative (Q) or positive (R)
Duration of ventricular depolarisation
80-120ms
What’s T wave?
Follows the QRS complex
ventricular repolarisation
heart reverts back to normal
What’s QT interval?
Onset of QRS to end of T wave
Ventricular depolarisation + repolarisation
Long QT interval syndrome–> sudden death
350-420ms
What’s ST segment?
Flat segment between the end of QRS and start of T wave
Depression = ischaemia
Elevation = MI
How’s an external electrical field is set up around the
dipole?
During flow of current along the cell surface
Why will a pd occur between a part of a cardiac cell + another dipole?
When cardiac cell is depolarising or repolarising, diff currents flow across membrane at various points
When is there no electrical field?
When cell is depolarised or repolarised + at RMP, there’s no diff in membrane potential at diff points along cell surface
(despite pd between inside + outside cell)
Typical Voltage Scale?
10mm=1Mv
Speed?
25mm/s
1 small square?
0.04s
