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
How long does the P wave last?
0.08 sec
What causes the PR interval to last more than 0.2 seconds?
Type 1 heart block
How long is the PR interval?
0.2 seconds
What marks out PR interval?
End of P wave to start of QRS complex ( PQ interval in reality)
Describe what state the heart is in in the PR interval
Isoelectric period
Why is PR interval largely flat?
Represents time taken for conduction through AVN (delay between atrial and ventricular excitation)
How long does the QRS complex last?
0.1 seconds
What causes abnormally big R wave on the ECG?
Large septum and hypertrophy
What is ST segment?
Plateau phase of ventricular AP (represents heart in isoelectric state)
Why is T wave same orientation as QRS complex?
Repolarisation occurs in the opposite direction to depolarisation
Differences in timings of APs between endocardium and epicardium (endocardium depolarises first as closer to septum), so sharp R wave
Epicardium repolarises faster (more K+ channels).
Why is there no deflection for atrial repolarisation?
Very slow so doesn’t produce a sharp electrical repolarisation
Why is QRS complex not a simple upright deflection?
Ventricle electric dipole changes its angle
Describe the stages that cause QRS complex
10msec: wave front at angle, dipole moving away from left arm (Q wave)
50msec: wavefront enters ventricles points toward left arm, (R wave)
90msec: dipole upwards away from left arm towards head, (S wave)
When is T wave almost as big as R wave?
Too much extracellular K+ (hyperkalaemia) shortens APs of epicardium disproportionately
How do you diagnose 2nd degree heart block?
2 P waves before QRS complex
How can you diagnose a heart attack?
Elevated ST
Hoe can you diagnose atrial fibrillation?
Lack of P wave and irregular heart rate
Where are the heart sounds relative to the ECG?
1st: after QRS
2nd: after T wave
Describe the ECG of 3rd degree heart block..
Third-degree AV block exists when more P waves than QRS complexes exist and no relationship (no conduction) exists between them.
