Session 7 - The ECG Flashcards
What do will you see on an ECG if there is no change in membrane potential
Nothing as the ECG only records changes
Describe the conduction over the heart
Starts in the SA node then spreads over the atria to the AV node where there is a delay of c.120ms. Then spreads down the septum from L to R then over the ventricular myocardium from the inside to the outside
Describe repolarisation
Happens after about 280ms
Spreads in the opposite direction to ventricle depolarisation - outside to inside
What would you see if depolarisation was spreading towards an electrode
An upward signal
What would you see if depolarisation was spreading away from an electrode
A downward signal
What would you see if repolarisation was spreading towards an electrode
A downward signal
What does the amplitude of a signal show
How much muscle is depolarising and how directly towards the electrode the excitation is moving
What does the P wave show
Atrial depolarisation
What does the QRS complex show
Ventricular depolarisation
Why is atrial repolarisation not seen
As is lost in the QRS complex
What does the T wave show
Ventricular repolarisation
What is the cardiac axis
The average direction of spread of depolarisation as seen from the front and estimated from leads 1,2 and 3
What would you see if there was left axis deviation and why might this occur
Biggest signal displacement would be seen in lead 1 not in lead 2
Left ventricular hypertrophy
What would you see in right axis deviation and why might this occur
The greatest deflection is seen in lead 3
Right ventricular hypertrophy
What do you see in atrial fibrillation
No P waves and irregular contraction of the ventricles
What does the P-R interval measure and what are its normal values
The delay between atrial depolarisation and ventricular depolarisation
Normally 120-220ms (3-5 small squares)
What do you see in first degree heart block
Elongated PR interval indicating a conduction problem
What do you see in second degree heart block
Some P waves that are not followed by a QRS complex. Ratio is not 1:1
What do you see in third degree heart block
No relationship between P waves and QRS complex
In third degree heart block why are the QRS complexes a different shape
They are longer as there is an abnormal focus of depolarisation within the ventricle
What might you see in an MI
Pathological Q waves
ST elevation
T wave inversion
What is the normal paper speed
25mm/second
