The ECG Flashcards
What does ECG detect?
The spread of cardiac excitation creates currents in the extra cellular fluid.
The currents generate small potential differences of around 1mv which can be recorded by sensitive voltmeters connected to metal electrodes on the skin surface.
What does the ECG not record?
Contractile events
What produces +ve deflection on ECG?
Flow of -ve charge in extracellular space
What are 3 lead components of ECG?
Lead 1: right arm (-) to left arm (+).
Lead 2 Right arm (-) to left leg
Lead 3 left arm (-) to left leg
What does ECG recording use?
3 electrodes (one on each arm and one on left leg) which make up Einthoven’s triangle
Direction of what is important for ECG measurement?
Dipole, if dipole is perpendicular to measurement axis then won’t see dipole
What does lead one pick up?
Excitation across atria going horizontally
What does lead two pick up?
Aligned with major axis of the heart, so excitation travels down the septum to ventricles
Which lead produces the largest deflection?
2
Describe the ECG deflection produced by SAN, why?
No detectable deflection, SAN too small
What wave does atrial depolarisation produce?
P wave
What wave does ventricular depolarisation produce?
QRS complex
What wave does ventricular repolarisation produce?
T wave
Why is there bigger ECG deflection from ventricles compared to atria?
Bigger wavefront of action potential
When do mechanical events occur in relation to electrical events?
After
