ECG Flashcards
How do the ventricle depolarise?
From endocardium to epicardium
What are the purkinje fibres?
Fine branches of the bundle of His
Which allow rapid spread of depolarisation throughout ventricular myocardium
Where can you fin the Bundle of His?
In the intra-atria septum of the heart
Briefly describe how the heart depolarises
- SAN depolarises
- Impulses spreads through atria
- Held up at AVN (want atria to finish contraction)
- Spreads to ventricle via Bundle of His (endocardium to epicardium)
- Spreads rapidly down R and L bundles and purkinje system (septum is depolarised first)
- Myocytes of intra-ventricular septum depolarised first
- Apex and RV and LV free walls are depolarised next
- Base of the ventricles are last to be depolarised
- Repolarisation of the ventricles happens in the reverse order
How do we see negative and positive complexes on an ECG?
If the signal is going from negative to positive then it will give a positive complex (ie a Hill) and then the reverse for a negative complex (a trough)
How are negative and positive complexes formed on an ECG during repolarisation?
If the signal goes form positive to negative you get a negative complex and vice versa
*it is the opposite of depolarisation
When viewing electively activity form the apex of the heart, what causes the difference in the size of the positive and negative complexes and why, at points, would you see both together?
If the depolarisation/repolarisation is heading in the same direction as the electrodes then the peaks/troughs will increase in size
Movement at 90 degrees gives both positive and negative complexes
(Ie if the depolarisation wave is going directly away from the electrode- +ive to -ive then you see a large trough)
Where is SAN located?
In the top right hand corner of the RA
Near junction of SVC and RA
Why does the SAN depolarisation not register on an ECG?
Because the signal is insufficient
What is the P wave of an ECG?
Atrial depolarisation (Spreads along atrial muscle fibres and internodal pathways
This is a small positive complex because the movement is towards the positive electrode
What is the point of the delay of signal at the AV node?
Allows time for atrial contraction to fill ventricle
What separate the atria and ventricles in electrical terms? How is this bridged?
A fibrous ring
Bundle of His bridges the gap between them
What makes up the flat line after the p wave on an ECG?
The AVN impulse delay and the depolarisation of the Bundle of His
How does the intra-ventricular septum depolarise? (In which direction)
How is this seen on an ECG?
From left to right
Produces a small downward deflection (called the Q wave) because the signal is moving away from the positive electrode but not directly towards the negative electrode so the negative complex is only small
What causes the R wave seen on an ECG?
The depolarisation of the apex and free ventricular wall
The signal is moving directly towards the positive electrode so there is a large positive complex (large becuase large muscle mass-more electrical activity)
In what situation will the R wave be even taller than normal?
If LV was hypertrophied
What causes the s wave of the QRS complex of an ECG?
The depolarisation of the base of the ventricles
Signal going away from positive electrode so negative complex is formed
What surface does repolarisation occur from?
Begins on epicardial surface
Spreads in opposite direction to depolarisation
What causes the T wave of the ECG?
Ventricular repolarisation
Because signal is towards the negative electrode then we see a positive complex (medium wave)
What is the QRS wave representing?
Ventricular depolarisation (NOT CONTRACTION)
When recording an ECG how many electrodes are there and where do they go? How many views/leads does this give of the heart?
10 electrodes: 4 on limbs, 6 on chest
Gives 12 views/leads
What are the 6 limb leads?
aVR aVF aVL I II III
Which limb leads are best for looking at the inferior surface of the heart?
- aVF
- II
- III
Which limb leads are best for looking at the lateral LV?
- I
- aVL
In what plane do the chest leads view the heart?
Transverse plane
What are the septal leads?
V1 and V2
What are the anterior leads?
V3 and V4
What are the lateral leads?
V5 and V6
What does lead V1 face?
RV and septum
What does lead V2 face?
RV and septum
What does lead V3 face?
Apex and anterior wall of ventricles
What does lead V4 face?
Apex and anterior wall of ventricles
What does lead V5 face?
LV
What does Lead V6 face?
LV
Why does lead V1 show a negative complex during ventricular depolarisation even though there is electrical signal coming towards it?
The ventricular walls depolarise simultaneously and the electrical signals recorded look at the sum of the opposing effects in the 2 ventricles. LV is much thicker so determines curve and the LV depolarisation is moving away from V1 so it creates a negative complex
What is the normal ECG paper speed?
25 mm/sec
25 small squares=1 second
5 large squares = 1 second
If 5 large boxes= 1 second, how long is 1 large square? And how long is 1 small square?
1 large square = 1/5 of a second= 0.2 seconds (200ms)
1 small square =1/5th of 0.2 seconds= 0.04 seconds (40 ms)
How many squares on an ECG chart represent 6 seconds and then 1 minute?
6 sec= 30 large squares
1 min= 300large squares
How do you calculate heart rate from an ECG?
Count the number of large boxes between complexes (easier to count R-R intervals)
How many complexes could be fitted into 300 large boxes (ie in 1 minutes?)
Eg 300/(number of large boxes you count) = HR
Eg 300/4=75 bpm
How do you calculate the heart rate if rhythm is irregular?
Count the number of QRS complexes in 6 seconds (30 large boxes), then x by 10
*this method can be used for regular heart rhythms too
How long should the PR interval be?
0.12-0.20 seconds
Or
3-5 small boxes
Prolonged if >1 large box
**PR interval starts just before the P wave and just ends just before the Q wave
How long should the QRS complex be?
<0.12 seconds
Or
<3 small boxes
Prolonged=might show ischaemic bundle of His or purkinje fibres
How long should the QT interval be?
Varies with heart rate
Show us how long the heart is taking to repolarise
Upper limits are:
0.45/0.47 (m/f) seconds
Or
11-12 small boxes
What is normal sinus rhythm?
Depolarisation is initiated by SA node
The bottom strip of an ECG gives the rhythm- reading from apex
How do you determine if a heart beat is in sinus rhythm?
- is the rhythm regular?
- heart rate? (60-100)
- are there p waves (shows that sinus node has depolarised)
- are P waves upright in leads I and II?
-is PR interval normal? (3-5 small boxes)
- every P wave followed by QRS?
- every QRS preceded by a P wave?
-normal QRS width (<3 small boxes)
It is possible to be in sinus rhythm with a heart rate outside of 60-100bpm?
Yes, it is call sinus bradycardia or sinus tachycardia
What causes the normal delay between the P wave and the QRS complex?
AVN delay
Which might cause a prolonged PR interval? (6)
Prolonged PR-AV block
AV nodal disease Enhanced vagal tone Myocarditis Acute myocardial infarction Electrolyte disturbances Medication
What does the QT interval show?
Ventricular depolarisation and repolarisation
What forms the inferior border of the heart and which coronary artery usually supplies this region?
RV
Right marginal artery
NB:
QT interval gets shorter with increasing heart rate and to assess this accurately it needs to be corrected for heart rate
Why are leads II and aVR ECG traces all but mirror images of one another?
Because they’re travelling in opposite directions but aren’t quite going in the same plane
Why, when looking at different leads on an ECG, will the deflections be smaller/bigger?
Because the leads are looking from different angles so the depolarisation that they experience may be bigger/not as big
What region of the heart face the anterior of the chest wall?
RV, septum and little bit of the LV
What coronary artery supplies the anterior aspect of the heart?
LAD (left anterior descending)
