0629 - The ECG - RM Flashcards
What do intervals and segments represent in an ECG?
Intervals are durations:
PR - Start of P to start of Q/R
QRS - Start of Q/R to end of S
S-T - End of S to end of T
Segments are isopotentials. Always run from end of one to start of another.
What conventions are used to record a standard ECG?
Voltage - Vertical:
Major Square = 5mm = 500 microVolt (uV)
Minor square = 1mm = 100 microVolt (uV)
Speed - Horizontal:
Major square = 5mm = 200ms (0.2s)
Minor square = 1mm = 40ms (0.04s)
Calibration pulse of 1mV for 200ms on each channel. Typically ticks every 25mm/1s at bottom of chart.
Counting Squares - Remember, 100uV/40ms per minor square! Must get it to +/- 10ms in exam!
How would you measure an ECG?
- Name, sex, age
- Quality of recording:
Calibration Pulses
Quality Traces
Speed (25mm/s)
Rhythm - Regular? Sinus?
Normal Electrical axis? (is QRS positive in leads I and II?)
Comment on ST intervals and T waves - are they isoelectric and positive respectively?
What causes a cardiac dipole?
As an area depolarises, current will flow into the neighbouring hyperpolarised area. If this wave of depolarisation is flowing from anode to cathode, it is positive on the trace.
Last areas to depolarise are first to repolarise, meaning that the T wave is positive (it’s going the other way, but the positions are reversed).
What are the leads in standard 12-lead ECG?
Frontal plane -
3 bipolar (+/- electrodes on limbs)
3 unipolar (difference against ground) - aVL, aVR, aVF
Precordial Area
6 unipolar leads (V1-V6)
What generates P, QRS, and T waves
P-wave - Atrial Excitation
Q-wave - Early ventricular excitation
R-wave - ventricular excitation
S-wave - Late ventricular excitation
T-wave - Ventricular repolarisation
(U-wave - late ventricular repolarisation)
How does an R wave change in time in the frontal plane?
Depolarisation follows a septal (L to R on Septum) ->mural (down the septum and into the ventricles) ->basal (top corner of L ventricle) pattern (S wave), which generates the QRS complex.
How do you determine the electrical axis in the frontal plane?
- Normal (-30-90 degrees - both positive)
- I positive, II Negative - Leftward deflection (-30 to -90 degrees)
- I negative, aVF positive - Rightward deflection (90 to 180 degrees)
- Negative in I and II - extreme deflection (180 to -90 degrees).
What ECG changes are caused by hypoxia, and by infarcted tissue?
Hypoxia creates an additional dipole during ST period, as the cells cannot effectively hyperpolarise, leaving them positive relative to their neighbours. This results in an ST segment depression.
In a transmural infarct, the ECG evolves:
Initially, there is ST elevation due to impaired action potentials due to an additional dipole against the infarcted tissue.
Later, as the tissue is silent, the lack of electrical activity in that area (normal dipole) leads to a pathological Q wave, and inversion of the T-wave.
How would you draw, label, and roughly scale an ECG trace?
