7 The Electrocardiogram Flashcards
Explain how excitation spreads over the heart
Starts at the SA node. Excitation spreads over the atria and to the AV node. It is held for 120 milliseconds. It then spreads down the septum and over the ventricular myocardium. After 280ms the cells repolarise.
What is an ‘electrode view’?
What an electrode detects depending on its position relative to the spread of activity.
What direction will the signal be when the spread of DEPOLARISATION is moving TOWARDS an electrode?
Upward
What direction will the signal be when the spread of DEPOLARISATION is moving AWAY FROM an electrode?
Downward
What direction will the signal be when the spread of REPOLARISATION is moving TOWARDS an electrode?
Downward
What direction will the signal be when the spread of REPOLARISATION is moving AWAY FROM an electrode?
Upward
What determines the signal amplitude on an ECG?
How much muscle is depolarising
How directly towards the electrode the excitation is moving
What produces the P wave?
Atrial depolarisation
What causes the P-Q interval?
The 120ms delay at the AV node
What direction is the Q wave facing at lead II?
What causes this?
Downward deflection
Depolarisation spreads outwards from the septum across the heart muscle
What causes the large R peak?
Large amount of muscle depolarising down the septum in the direction of the apex.
What causes the drop (S) after the large R peak?
Depolarisation spreads to the base of the ventricles
What causes the S-T interval?
The end of systole- 280ms gap
What causes the T wave?
Repolarisation of the cardiac muscle away from the apex. The cells do not all repolarise at the same time so the T peak is smaller and longer than the R peak.
Why is atrial repolarisation not seen on an ECG?
It is lost in the QRS complex
How is 1 view obtained from 2 leads?
A positive and negative electrode are used. The negative signal is inverted and added to the positive signal. The total is then amplified.
What is a ‘lead’ with regard to ECGs?
An electrical view of the heart
What is an ‘augmented lead’?
Two negative electrodes are inverted and then combined. The combination is then added to the actual positive. (3 electrodes used)
How many leads does a full ECG consist of?
Name them
12 leads
I, II, III
aVF, aVL, aVR
Six chest leads V1-V6
How long does a large square on an ECG represent?
0.2 seconds
How is the heart rate calculated from an ECG?
Divide 300 by the number of squares in the R-R interval.
When is the P wave not present on an ECG?
Atrial fibrillation
How long is a normal P-R interval?
3-5 small squares
How does i.) First degree heart block ii.) Second degree heart block iii.) Third degree heart block present on an ECG?
i. ) Prolonged P-R interval
ii. ) Erratic P-R interval
iii. ) No relationship between P wave and QRS complex
What does the QRS complex tell you about?
The axis of the heart
The pattern of conduction through the ventricles
How is the heart apex position estimated from an ECG?
The combination of the signals of the right and left ventricles depolarising will produce a vector pointing slightly to the left.
What can shifts in the position of the heart apex vector show?
Changes in the relative amount of muscle in the left and right side of the heart
How many signals do skin electrode record per beat?
2
One on depolarisation and one on repolarisation
What causes lengthening of the QRS complex?
Bundle branch block
What does ST depression show?
Temporarily short of O2, angina
What does ST elevation show?
Dying tissues, injury currents
How does an MI present on an ECG?
ST elevation
Pathological Q waves- more than 0.04s wide (full thickness MI)
Inverted T waves
