Electrocardiogram Principles (ECG) Flashcards
Role of an ECG
Monitoring movement of waves of excitation
What does an ECG provide information on?
Timing and direction of cardiac events
(Atrial and Ventricular Depolarisation, Ventricular Repolarisation)
Rate/Rhythm disturbances
(Tahcy/brachy cardia; sinus rhythm; arrythmia)
Conduction abnormalities
(A-V conduction time or abnormalities in spreads across branches)
Mass of active myocardium
(Ischaemic Events)
TRCM
Time - Rhythm Conducted Melodically
Why doesn’t an ECG show Atrial Repolarisation
Atrial repolarisation occurs simultaneously with ventricular depolarisation, which is a much more voltage significant event
Does an ECG indicate force of contraction?
It does not >:(
bitch
How does an ECG work (basic principles)
Imagine a strip of cells placed in a bath filled with conducting solution; positive electrode focally positioned and corners of bath connected to negative electrodes
If a positive impulse (wave of depolarisation) travels across those cells from left to right, an upward deflection is recorded on the trace
What would a unipolar ecg show if depolarisation was moving towards a positive electrode
Upwards deflection
What would a unipolar ecg show if depolarisation was moving away from a positive electrode
Downward deflection
What would a unipolar ecg show if depolarisation was moving perpendicular to a positive electrode
No deflection
What would an ecg show if depolarisation was moving towards a negative electrode
Downwards deflection
What would an ecg show if depolarisation was moving away from a negative electrode
Upwards deflection
Compare the ecg readings of a focally positioned negative electrode recording the same impulse as a diametrically opposite positive electrode
They would yield the same reading
What would a bipolar ecg show
It would simply present the composite deflection of what is seen at each electrode (which is the same at both); so is simply a deflection with twice the magnitude
What might a repolarisation event look like as they move to a positive electrode
This would cause a downward deflection on the ecg trace; this logic follows for all the other directions and polarities
What does an ECG measure in the actual heart versus in a strip of neatly lined up cells
In the heart, electricity flows in various directions, so the ECG measures the mean vector of impulse
