ECG Flashcards
(60 cards)
What is an ECG concerned with?
collection of cells
rapid conduction between cells via intercalated discs
In a unipolar electrode, what deflection would depolarisation moving towards a positive electrode give?
Upward deflection
In a unipolar electrode, what deflection would depolarisation moving away from a positive electrode give?
Downward deflection
When is amplitude maximal?
When positive electrode is on the vector
When is amplitude minimal/biphasic?
When positive electrode is perpendicular
No charge
In a unipolar electrode, what deflection would depolarisation moving towards a negative electrode give?
Downward deflection
In a unipolar electrode, what deflection would depolarisation moving away from a negative electrode give?
Upward deflection
Ie. Focally positioned negative electrode records same thing as diametrically opposite positive electrode
Describe a model for unipolar electrode
Strip of cells in bath with conducting fluid
Focally positioned positive electrode
Corners of bath connected to negative electrode
How is the strip affected in a unipolar electrode when it depolarises?
Inside positive, outside negative
Cells closest to electrode most influenced
Indifferent electrode sees average potential of strip
What does the potential difference reflect?
Movement of current in relation to positive electrode
Describe bipolar electrode
2 recording electrodes
Strip of cells in bath with conducting fluid
Focally positioned positive and negative electrode
How is the strip affected in a bipolar electrode when it depolarises?
Inside positive, outside negative
Wave of depolarisation moving towards positive - upward
Also moving away from negative - upward
Combined means bipolar deflection is larger than unipolar potential difference
How is the strip affected in a unipolar electrode when it repolarises?
Potential change is opposite to depolarisation - inside negative, outside positive
Towards positive - downward deflection
Away from positive - upward deflection
How does electrical current flow in the heart?
What does this allow?
Flows as if between 2 termInals in a volume conductor
Intra-thoracic contents also act as volume conductors
This allows surface recordings of potential differences provide 3D picture of electrical events in the heart
What is the recorded potential difference at any instant?
Vectorial resultant (mean vector) of several differently directed wavefronts
What would be recorded on ECG I if the positive electrode is at apex at atrial depolarisation?
Upward deflection - toward positive electrode
What would be recorded on ECG I if the positive electrode is at apex afteratrial depolarisation?
No deflection, due to delay as current passes through AV node
What would be recorded on ECG I if the positive electrode is at apex at ventricular depolarisation?
- Septum (LV to RV) - small negative, away from electrode (small number of cells)
- Main ventricular depolarisation - large upward, towards electrode (left bias, more cells, endo to epi)
Repolarisation - Base of ventricles - small negative, away from electrode
What would be recorded on ECG I if the positive electrode is at apex at ventricular repolarisation?
Away from electrode, therefore positive deflection
Which ventricular cells are first to repolarise?
Those which are last to depolarise (epicardial cells) - shorter action potentials
Limb leads of unipolar
One focally positioned positive electrode, others are indifferent (RL earth)
aVR
Right shoulder
Positive RA
aVL
Left shoulder
Positive LA
aVF
Feet (groin direction)
Positive left leg
