CVS Session 7 (Lecuture 7.2) Flashcards
What is a ‘lead’ ? What can be ascertained by comparing leads?
Electrical view of the heart
By comparing leads can:
- localise abnormalities
- detect changes in electrical axis
What are augmented leads?
Have two negative electrodes connected
First convert two negative to one
Convert into a positive
Combine it with actual positive to give one view
What are the limb leads and what view do they give?
aVR aVL aVF I II III
Around heart ‘top bottom’ in a clockwise fashion
What view do the chest leads give?
View the heart in the horizontal plane around it ‘side to side’
6 chest leads
What does the full ECG consist of ?
12 leads, 10 electrodes
I, II, III
aVR, aVL, aVF
Chest leads V1-V6
What do you look at in the ECG?
Rate Rhythm Axis - direction of R wave e.g. left or right P wave P-R segment - AVN QRS complex - ventricular contraction Q-T interval - systole duration T wave - repol
How do you calculate the HR of an ECG?
Measure R-R interval
Divide 300 by R-R interval in squares
300 squares = 1 minute
Describe how the rhythm of a heart can be judged by an ECG?
Judged from any lead
Choose the one where the relevant components are most visible
Is the rhythm regular or irregular?
What is the relationship between atrial and ventricular depolarisation?
What is absent in atrial fibrillation? Describe other features seen in AF on an ECG
P wave absent (depol of atrial cells)
Rate varies
Scattered pacemakers so ventricles still contract but slower
What does the P-R interval indicate? How long is it usually?
Time taken for impulse to reach ventricles
3-5 small squares (0.12 - 0.2 s)
What is i) first degree heart block, ii) second degree heart block and iii) third degree heart block?
Heart block is the interference of the time taken for an impulse to travel from the SAN to the ventricular muscle.
i) Prolonged P-R interval as a result of a delay along the conduction pathway
ii) Excitation fails to pass AVN or the BoH. (erratic P-R interval)
iii) Atrial contraction is normal but no beats are conducted to the ventricles i.e. no relationship between P wave and QRS complex.
What does the QRS complex tell you about?
Axis of heart and pattern of conduction through the ventricles
How is the axis of the heart determined on the ECG?
Combination of the depolarisaton of the right + left ventricles generates a single vector normally pointing slightly left
How can the axis of the heart change?
Change in relative amount of muscle in right and left heart will rotate the vector
Increased LV muscle = left shift
Increased RV muscle = right shift
How can you determine the axis of the heart from an ECG?
Find lead with smallest and most equiphasic deflection
Net deflection is zero indicating that electrical axis must run at right angles to that view
Usually parallel to lead II
