L14 - Electrocardiogram Flashcards
What produces an ECG?
Summation of cells depolarisation/repolarisation gives resultant vectors
Measurement of vectors produces an ECG
Where can electrical potentials be recorded?
When cardiac impulse passes through the heart, the electrical current spreads to adjacent tissues
- Small amount even reaches the surface of the skin
If electrodes are placed on the skin around the heart, electrical potentials can be recorded
What are the two different ways to record electrical activity?
Surface electrodes - placed on to the skin and connected by cables to the ECG
Standard limb leads – tracing of voltage difference and what is actually produced by the ECG recorder
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How does use of standard limb leads work?
Right arm, left arm, left leg Einthovens triangle - - Lead 1 – RA and LA - Lead 2 – RA and LL - Lead 3 – LA and LL
What causes an upward deflection on an ECG?
Net current flow towards an electrode causes an upward deflection on ECG
What does an ECG show?
The ECG is the spread of electrical excitation through the heart
Electrical excitation causes muscle contraction its progression through the heart
What are the 6 different stages of an ECG?
- P wave - atria depolarisation
o Small muscle mass so deflection is small - Atria depolarisation complete
- QRS wave – ventricular depolarization begins at apex
o Big muscle mass so deflection is big
o Repolarisation of atria happens at the same time but this electrical activity is swamped by ventricular depolarisation - Ventricular depolarisation complete and blood ejected to the lungs/body
- T wave – ventricular repolarisation begins at apex
o Progresses superiorly
o It occurs in the same direction as depolarisation so it also produced an upward deflection - Ventricular repolarisation complete
How long is PR typically?
0.12-0.20 s
How long is QRS typically?
0.8-0.10 s
How long is QT typically?
0.40-0.43 s
How long is ST typically?
0.32 s
What is atrial hypertrophy?
High P wave amplitude
More muscle mass, more to depolarise, bigger deflection
What is ventricular hypoxia?
Low T wave amplitude
Not enough oxygen
What is acute myocardial infarction?
Longer ST interval
What is arrhythmia?
A lack of rhythm
What is it known as when the heart rate naturally varies?
Bradycardia and tachycardia
E.g. exercise from 65-180 bpm
E.g. sinus arrhythmia – 15% increase on inspiration, 15% decrease on expiration
What are some examples of when heart rate varies abnormally?
E.g. non-exercise tachycardia 150-200 bpm
E.g. flutter – 200-300 bpm
E.g. ventricular fibrillation – 300+ bpm
Often caused by interruptions in the hearts conduction pathway
What can cause heart blocks?
Myocardial infarction
Artery disease
What is a first degree heart block?
Interruption between the SA and AV nodes
Slowing SA-AV conduction
This leads to an increased PR interval
What are the two stages of a second degree heart block?
Mobitz 1
Mobitz 2
What occurs during Mobitz 1?
Some SA impulses fail to evoke QRS
Progressive prolongation of PR interval culminating in a non-conducted P wave
- PR interval longest immediately before dropped beat
- PR interval shortest immediately after dropped beat
Progressive fatigue of the AV nodal cells
What occurs during Mobitz 2?
Intermittent non-conducted P wave without progressive prolongation of PR interval
- PR interval in conducted beats remains constant
- PR interval surrounding the dropped beat is an exact multiple of the preceding PR interval
All or nothing - his-purkinje cells suddenly fail to conduct a supraventricular impulse
What are the characteristics seen in Mobitz 2 due to?
Failure of conduction at the level of the his-purkinje system
Structural damage to conducting system
- Infarction, fibrosis
Patient with Mobitz 2 usually have a pre-exsiting?
Patients typically have a pre-existing LBBB or block
- 2nd degree AV block is produced by intermittent failure of remaining fascicle
What occurs during a third degree heart block?
Complete absence of AV conduction – no supraventricular impulses
Perfusing rhythm is maintained by a junctional or ventricular escape rhythm
Patients with a third degree heart block may suffer with?
Ventricular standstill leading to fainting or sudden cardiac death
Severe bradycardia with independent atrial and ventricular rates
What are the consequences of a first degree heart block?
Benign – often seen in athletes
What are the consequences of a second degree heart block?
Benign (type1)
May require pace maker (type 2)
What are the consequences of a third degree heart block?
Chamber contraction out of synchrony
Atrial contraction against closed tricuspid
- Cannon wave in jugular vein
- R atria contract at the same times as R ventricle so pressure wave is sent back up the jugular vein
Reduced perfusion of tissue
- Dizziness and syncope
What is atrial fibrillation?
Most common arrhythmia
Starts with brief abnormal rhythm of atria which becomes longer (sometimes constant)
May be perceived as palpitations or exercise intolerance
Symptoms of atrial fibrillation?
Usually asymptomatic
- Accompanied by symptoms related to rapid heart rate
- May produce angina
- Shortness of breath
- Oedema of ankles
What are the treatments for atrial fibrillation?
Flecainide
Beta blockers
Amiodarone/dronedarone
Warfarin/anticoagulants
What are circus movements?
Electrical signal not completing normal circuit but alternative circuit looping back upon itself rapidly
Refractory muscle normally prevents re-excitation
If wave of excitation meets non-refractory tissue it will carry on
How can circus movements be non-refractory?
Unidirectional block
Transient bidirectional block
