CVS 7 ECGs Flashcards
Describe signal recorded by am extracellular electrode placed near a myocardial cell during systole
Extracellular electrodes only record changes in membrane potential so would see two signals with each systole, one upwards peak for depolarisation and one downward peak for repolarisation
Describe the spread of excitation over the normal heart
Action potential generated by pacemaker cells in SAN and spreads over atria and to AVN where after 120ms delay it spreads down septum via bundle branches then out over ventricular myocardium from inside to outside.
After 280ms cells begin to repolarise which spreads in opposite direction to depolarisation
What will give an electrode an upward signal?
Depolarisation moving towards electrode
Repolarisation moving away from electrode
What causes an electrode to have a downward signal?
Depolarisation away from electrode
Repolarisation towards electrode
What causes a bigger amplitude in electrode signal?
The more muscle depolarising and the more directly towards the electrode it’s moving
What causes the P wave?
Atrial depolarisation
What causes the small downward deflection at Q?
The depolarisation away from the electrode as excitation moves out across axis of heart about half way down septum. Small as not moving directly away
What causes the large peak at R?
Depolarisation spreading through ventricular muscle moving towards electrode. It is large as lots of muscle and moving directly towards electrode
Why is there a downward deflection at S?
Depolarisation spreading upwards from apex of ventricle to base away from electrode
What occurs in the ST interval?
Ventricular systole ~280ms
What causes T wave?
Ventricular repolarisation. Upwards as moving away, medium sized as timing over different cells dispersed
What is a lead?
An electrical view of the heart
What can we do by comparing leads?
Localise abnormalities and detect changes in electrical axis
Where does lead one view from?
Left side of heart
Where does lead II view from?
Apex of heart
Where does lead III view from?
From the bottom of heart
What are augmented leads?
These have two negatives connected.
To work out view convert two negatives to one then convert to positive and combine with actual positive to give one view. It favours negative side as two electrodes.
What are the names of the augmented leads?
AVR, AVL and AVF
How many electrodes does a 12 lead ECG use?
10
4 limb
6 chest
Only 9 actually record
Which electrode is neutral?
Right lower limb
What type of view do the limb leads provide?
Vertical
What are the colours of the limb leads?
Ride- red right upper
Your- yellow left upper
Green- yellow left lower
Bike-blue right lower
What type of view do the chest leads provide?
Horizontal
What are the colours and positions of the chest leads?
V1-RED-4th intercostal space to right of sternum
V2-YELLOW-4th intercostal space to left of sternum
V3-GREEN-directly between V2 and V4
V4-BROWN-5th intercostal space at midclavicular line
V5-BLACK-level with V4 at anterior axillary line
V6-PURPLE-level with V5 at midaxillary line (directly under armpit)
What electrodes make up the lead II view?
Left lower positive
right upper negative
What electrodes make up lead 1 view?
Right upper negative
Left upper positive
What electrodes make up the lead III view?
Positive left lower
Negative left upper
What factors should you look at when reading an ECG?
Rate, rhythm, axis, p wave, p-r segment, QRS complex, Q-T interval, T wave
Which lead is often used for the rhythm strip?
Lead II
How do you calculate heart rate from an ECG?
300/number of squares in R-R interval
0.2s per large square
What is ventricular ectopic beat?
Ventricular cells gain pacemaker activity causing ventricular contraction before the underlying rhythm would normally depolarise ventricles. Resulting ECG often wider and taller than with underlying rhythm. Can occur every other, every third, every fourth beat or in groups like couplets or triplets
What would you see with atrial fibrillation?
No p wave, irregular fibrillation waves.
No regular stimulus reaching AV node so other pacemakers generate rhythm
What is heart block?
Communication problem between atria and ventricles. Lots of different types
What is first degree heart block?
P-R interval elongated from usual 200ms as conduction delay through AV node.
What is type 1 2nd degree heart block?
P-R interval gets gradually longer until a QRS complex is completely dropped and then system reset.
What is type 2 2nd degree heart block?
Where QRS complexes occasionally dropped as electrical excitation fails to pass through AV node or bundle of his.
What is complete/3rd degree heart block?
Atrial contractions normal but no electrical conduction conveyed to ventricles so ventricular pacemakers generate own signal which doesn’t relate to atrial contraction and is usually slow.
What is bundle branch block?
Where damage to conducting pathways alters route of spread and changes shape of QRS complex often lengthening it.
How do you find the electrical axis of the heart?
Find lead with smallest deflections. Here net deflection is 0 so electrical axis must run at right angle to this view. Often parallel to lead II
Which way will the electrical axis move if there has been a left shift or a right shift?
Left- towards lead I
Right- towards lead III
If parts of the myocardium are stressed, dying or dead, how does this affect the ECG?
Produces extra signals in the ST segment
