CVS Session 7 Flashcards
Why do electrodes outside the cell ‘see’ two signals with each systole?
Can only ‘see’ changing membrane potential therefore one signal for depolarisation and one for repolarisation
In which direction does the large changing electrical field generated by the myocardium travel?
Radiating away from the myocytes through the body to the skin
What does an ECG show?
Effects of depolarisation
Effects of repolarisation
Spread of electrical field alteration over the heart
How does conduction spread over the atria?
Starts at SAN
Spreads over atria to AVN
AVN delays conduction for 120 ms
How does spread of excitation cause ventricular depolarisation?
Activity spread down septum then out over ventricular myocardium
Endocardial –> epicardial
What causes depolarisation to spread in the epicardial direction?
A uniform thin layer of depolarisation in the endocardial surface
Where does the last signal seen originate from?
Activity of cells at base of valves
How does repolarisation occur?
After 280 ms it spreads in the opposite direction to depolarisation
Why does repolarisation occur in the opposite direction to depolarisation?
To unravel the fibres arranged in a figure of eight pattern that have twisted during depolarisation
What is a view in relation to an ECG?
Imaginary direction in which you are looking at the heart depending on the position of the electrode relative to the spread of activity
What creates an upward signal on an ECG?
Depolarisation moving towards the electrode
Repolarisation moving away from the electrode
What causes a downward signal on an ECG?
Depolarisation moving away from the electrode
Repolarisation moving towards the electrode
What does the amplitude of the signal depend on?
How much muscle is depolarising
Vector of movement of excitation
Considering the apex view, why does atrial depolarisation cause a small upwards deflection?
Small amount of muscle
Moving towards electrode but not directly
How does excitation spread from the septum?
Spreads ~1/2 down septum then out across the axis of the heart
What restricts the amplitude of the signal caused by spread from the septum given that a large amount of muscle is depolarising?
Its relative direction
How does excitation spread through the ventricular myocardium?
Through ventricular muscle along an axis slightly to the left of the septum
Why does the epicardium repolarise first?
Cells in epicardium happen to spontaneously repolarise first
Why is the area under the R wave equal to that under the T wave?
Same cells are involved
What does the QRS complex show?
Spread of excitation to endocardium and subsequent spread across ventricles
What does the P wave show?
Atrial systole
What does the Q wave show?
Septal depolarisation spreading to ventricle
What does the R wave show?
Main ventricular depolarisation
What does the S wave show?
End of ventricular depolarisation
What does the T wave show?
Ventricular depolarisation
Why is atrial repolarisation not seen on an ECG?
It is lost in the QRS complex as the signal is small and swamped by ventricular changes
Starting from the apex view, as you move the view clockwise around the heart, what happens to the R wave?
Amplitude decreases –> becomes -ve –> becomes more -ve –> neutral –> small +ve –> large +ve
How does the amplifier in an ECG utilise a +ve and -ve electrode?
Invert -ve electrode
Add signal to +ve input making signal detectable
In which direction does lead I look from?
Left side
In which direction does lead II view the heart?
Towards apex
In which direction does lead III view the heart?
From the bottom
In which plane do the augmented leads measure electrical activity?
Vertical
Using clock positions, what views do the augmented leads have of the heart?
aVL = 2 o' clock aVF = 6 o' clock aVR = 10 o' clock
Which plane do the chest leads give views in?
Horizontal
Where are the electrodes for V1-3 placed?
V1 = 4th intercostal space, right eternal edge V2 = 4th intercostal space, left eternal edge V3 = 5th rib b/w V2 and V4
Where are the electrodes for V4-V6 placed?
V4 = 5th intercostal space, mid-clavicular line V5 = 5th intercostal space, b/w V4 and V6 V6 = 5th intercostal space, mid axillary line
What standard rate do all ECG machines run at?
300 squares per minute
How can heart rate be calculated?
300/ squares of RR interval
~12 beats present normally
How do you asses the rhythm of a heart trace?
Choose lead where relevant components are most visible to see if R waves are regular/irregular
How is the axis determined?
Estimate the direction of arrow that generates the R wave by looking at the most positive leads
When are P waves absent?
Atrial fibrillation
How long should a normal PR interval be?
~120 ms
What 3 different PR interval abnormalities can be seen and what do they indicate?
Prolonged = first degree heart block
Erratic = second degree heart block
No relationship b/w P and R = third degree heart block
What is heart block?
Something that prevents transmission b/w atria and ventricles
What type of damage can the QRS complex be used to identify?
Ventricular
What does the QT interval indicate?
Length of systole
What is the T wave examined for?
To see if repolarisation is in the right direction
Why are intervals between R waves irregular in atrial fibrillation?
Pacemaker other than SAN is controlling contraction at a slower than usual rate
Why is atrial fibrillation dangerous if the heart continues to function?
Pooling occurs in corners of atria which increases the risk of a clot being pumped into the arteries
If the R waves are further apart than the P waves, what does this indicate?
Atria are contracting more frequently than ventricles
How is the P wave seen in third degree heart block?
Normal shape but not related to the QRS complex
What causes natural axis deviation?
Size of person
Why is the normal axis of the heart a single vector pointing slightly left?
Combination of R and L ventricular depolarisation with thicker ventricle wall on left pulling net arrow left
How do the limb leads appear in right axis deviation?
I = -ve II = small III = +ve
How do the limb leads appear in left axis deviation?
I = +ve II = small III = -ve
If the net deflection is 0, what does this tell you about the axis?
At right angles to that view
What causes bundle branch block?
Damage to conducting pathways altering route of spread of depolarisation
What changes are detected on an ECG of a patient with bundle branch block?
QRS complex changes shape - almost always increased width
‘Bunny ears’ may be present
What causes ‘bunny ears’ in bundle branch block?
Spread of depolarisation turns around and moves back along path it has just taken due to block –> two R waves generated
How is damage to the myocardium identified on an ECG?
Changes to the ST segment
When does damage to the myocardium occur?
If there are problems w/perfusion to the myocardium - if it is stressed, dying or dead
What does damage to the myocardium affect?
Spread of electrical activity during systole
What do extra ST signals indicate?
Extra electrical current generated in systole due to stressed/dying/dead myocardium
What does ST depression indicate?
Transient hypoxia
Why will the heart rate often be high on an ECG displaying ST depression?
It is investigated by exercise test
What does ST elevation indicate?
Dying tissues generate injury currents due to lack of oxygen
What three ECG changes may be identified during myocardial infarction?
ST elevation (sometimes)
Pathological Q waves
Inverted T waves
What causes pathological Q waves in MI?
Dead myocardium replaced by fibrous tissue that electrical activity has to flow around
How are pathological Q waves identitified on an ECG?
> 0.04s wide (1 small square)
How would the ECG of someone who has not presented with an Mi show that they had in fact suffered a previous MI?
Pathological Q waves - they persist after other changes are resolved
In myocardial infarction, what does the view with the most prominent abnormality help identify?
Which coronary artery is blocked
Whether full/partial ventricular wall thickness affected
What type of MI are pathological Q waves seen in?
Full thickness
Which leads most commonly show the most prominent abnormality in an MI?
Chest leads
