L15 - ECG and Heart Blocks Flashcards
Machine to measure ECG
Electrocardiograph
Read out of the ECG machine
Electrocardiogram
What does the ECG rely on
The summation of the cells repolarisation and depolarisation which gives resultant vectors
What about the hearts electrical activity can be sensed by predators
The small ammount which reaches the surface of the skin
Where are the three limb leads placed
What is this known as
Right arm, left arm, left leg
Einthovens triangle
Where is lead I
Right arm to left arm (+)
Where is lead II
Right arm to left leg (+)
Where is lead III
Left arm to left leg (+)
At rest what can be said about current flow
What is this point known as
No net current flow toward an electrode so no deflection on the ECG
What would net current flow toward an electrode cause
Upward deflection of the ECG trace
What deflection is seen when the atria depolarise
Net current movement toward II
Upward deflection in lead II
What deflection is seen when the septum is depolarised
Upward deflection in lead II
As current is moving in the same direction as lead II
What deflection is seen when the ventricles depolarise
Downward deflection in lead II
As current is moving in the opposite direction of lead II
Describe what is occuring at the P wave
Atrial depolarisation
Why is the p wave a relatively small deflection
Atria = small muscle mass = small deflection of the ECG trace
What does the flat line after the p wave represent
That atrial depolarisation is complete
Describe the QRS complex
Ventricular depolarisation begins at the apex
Large deflection as large muscle mass
Why is no wave seen for the repolarisation of the atria
Because it is masked by the QRS complex
What does the flat line after the QRS complex represent
Completion of ventricular depolarisation
What is happening to the blood at the flat line after the QRS complex
Blood being ejected from the ventricles into pulmonary and systemic circulations
What is shown by the t wave
Ventricular repolarisation - beginning at the apex
Occurs in the same direction as depolarisation so has an upward deflection
What is shown by the flat line after the t wave
Ventricular repolarisation is complete
Heart is ready for the next cycle
Normal PR interval
0.12-0.2
Normal QRS interval
0.08-0.1
Normal QT interval
0.4-0.43
Normal st interval
Average of about 0.32
What is shown by the U wave
Possible repolarisation of the purkinje fibres/papilalry muscles
What would a high P wave be indicative of
Atrial hypertrophy
greater muscle mass of the atria so more t depolarise
What would a low T wave be indicative of
Ventricular hypoxia (not enough O2)
What would a prolonged ST interval be indicative of
Acute myocardial infarction
What is electromechanical dissociation
When the heart has been damaged, electrical activity is still conducted however the heart muscle is unable to respond - cardiac output is 0
This can be seen after death
What is meant by arrhythmia
Lack of a rhythm
What situations may cause natural variations in a hearts rhythm
What are these two variations called?
Is this normal?
Exercise, sinus arrhythmia
Bradycardia and tachycardia
Normal
What occurs in sinus arrhythmia
Increase in HR by 15% on inspiration
Decrease in HR by 15% on expiration
What is non-exercise tachycradia
Where HR is in the region of 150-200 and no exercise is being performed
What HRs are achieve when the heart is in a flutter statee
200-300
What HRs are seen when the heart is in fibrilation
300
What is meant by a heart block
Impairment of the hearts conducting pathways
What sort of things could cause a heart block
Heart disease/artery disease/infarction
What is a first degree heart block
Block that occurs between the SA and AV nodes
Slowing SA –> AV node conduction
What does a first degree heart block lead to on ECG
Increased PR interval
What are the two types of second degree heart block
Mobitz I
Mobitz T2
Describe a mobitz type 1 second degree heart block
Some SA impulses fail to evoke QRS complex
PROGRESSIVE PROLONGATION of the PR interval leads to a non conducting p-wave
In a mobitz 1 when is the PR interval longest
Immediately prior to the dropped beat
In a mobitz 1 when is the PR interval shortest
Immediately after the dropped beat
Describe a mobitz type 2 second degree heart block
Intermittent non conducted beats remain constant
PR interval in conducted beats REMAINS CONSTANT
(P waves march through at a constant rate)
PR interval is an exact multiple of the preceeding PR interval (ie. double if one beat is dropped)
What causes a mobitz type 2 heart block
Failure at the level of the purkinje system (below the AV node)
What is a mobitz 2 likely to be caused by
Structural damage to the conducting system
Why is mobitz 2 described as ‘all or nothing’
His-purkinje cells suddenly and unexpectedly stop working
No pattern or fixed relationship
What is a type 3 heart block caused by
Complete absence of AV conduction
None of the supraventricular impulses are connected to the ventricles
Is perfusing rhythm maintained in a type 3 heart block
Yes at rest
BUT not under stress
What are the consequences of 1st degree heart block
Benign
Only usually noticed in athletes
What are the consequences of 2nd degree heart block
Mobitz 1 usually benign (beat dropped and then recovery)
What is the most common form of arrhythmia
Atrial fibrillation
Describe atrial fibrillation
Rapid beating of the atria
Starts with a brief episode that becomes more constant
What are the symtpoms of atrial fibrillation
Usually is asymptomatic but some symptoms associated with a high HR are often seen
Congestive symptoms
Treatments of atrial fibrillation
Flecanidide B-blockers Amidrarone Dronedarone Digoxin
What is one of the risks with atrial fibrillation
What may be prescribed to alleviate such risk
Clots forming on the wall of the atria
Warfarrin
What is meant by circus movement and re entry
Where the electrical signal doesn’t complete the normal circuit but an alternative one
The refractory muscle which normally prevents re-excitation being reexcited to early
Non refractory by: Unidirectional block or transient bi-directional block
