12.3 Physiological mechanism of arrhythmias Flashcards
What is the definition of an arrhythmia?
An irregular heart rhythm
What is bradycardia & give examples
- Slow HR <60bpm
- Sinus bradycardias (physiological or Sinus node disease)
- Atrioventricular (AV) block
What is tachycardia & give examples
- Fast HR >100bpm
What things cause arrhythmia?
- Electrolyte disturbances
- Ion channel modification or defects
- Structural/anatomy abnormalities
- Cell damage
These lead to…
- Disturbances in impulse generation
- Disturbances in impulse propagation
- Or a bit of both!
Risk factors of arrhythmia?
-
Cell/structural changes
- Myocardial infarction
- Myocarditis
- Fibrosis
- Toxins (e.g. alcohol)
- Chemotherapy
-
Ion channels
- Long QT syndromes
- Drugs
-
Environmental factors –> cellular dysfunction
- Temperature
- Hypoxia
- Levels of K+, Ca2+, Mg2+
- Acidaemia
What are some disorders of impulse formation?
- Disorder in automaticity
- Triggered activity (EADs & DADs)
What is automaticity & abnormalities of automaticity (show on ECG)?
- The property of a fiber to initiate an impulse spontaneously - without needing prior stimulation
Abnormalities of automaticity
- Inappropriate discharge rate (ie sinus tachycardia, sinus bradycardia)
- “Ectopic” pacemaker takes over and controls atrial or ventricular rhythm
What could decreased automaticity indicate?
Sinus node disease
Explain how cardiac automaticity takes place (steps)
- Pacemaker cells have a pacemaker potential (Phase 4) enabling them to self-generate their own action potentials
- The rate of rise of ion influx at the pacemaker cell sets the intrinsic heart rate
- The SA node rate is then influenced by the autonomic nervous system
- A balance of sympathetic and parasympathetic tone sets the resting heart rate = ~60-100 bpm
Draw a pacemaker action potential & explain the graph & phases
Draw a cardiac myocyte action potential (non-pacemaker) & explain the phases
At what Bpm do different pacemakers function?
SA node = ~100bpm
AV node = ~50bpm
Purkinje = ~35bpm
Explain what is meant by triggered activity
- “triggered activity” = after-depolarizations
- These are depolarising “oscillations” in the resting membrane voltage induced by one or more preceding action potentials
- Ie an unstable depolarisations (activations) that occur when the heart should be repolarising (resting)
- These oscillations can trigger (or be triggered by) extra heart beats
- Can trigger an arrhythmia like Torsades de Pointes or VT
Explain what is meant by EAD (triggered activity) (& how seen AP of cardiac myocyte & ECG and what it can lead to)
- Early after-depolarisations (EAD) arise from an abnormal membrane potential during phase 2 and 3
- Lots of “Aborted” Action potentials usually due to changes in the ion channels
- Ie increased opening of Ca channels
- Increased opening of Na channels
- EAD: Nothing, Prolong QT, Sustained Arrhythmia (Torsades de Pointes)
Explain what is meant by DADs (triggered activity) (& how seen AP of cardiac myocyte & what it can lead to)
- Late or Delayed After-Depolarisations (DADs) occur after phase 4
- Due to increased/altered movement of calcium
- Activate a Na/Ca exchanger triggering an AP
- DADs occurs at a more negative membrane potential
- DAD: Nothing, Ectopic beat, Sustained Arrhythmia (Ventricular Tachycardia)
What are some causes of EAD?
- Low serum K
- Slow HR (Bradycardia)
- Drug toxicity ie quinidine
What are some causes of DAD?
- Increased serum Calcium
- Increased Adrenaline
- Drug Toxicity ie Digoxin
- Myocardial Infarction
- Can be an SVT
What are some disorders of impulse conduction?
- Re-entry
- Conduction block
Explain how a re-entry takes place (disorder of impulse conduction)
- Two possible routes for electrical impulse to flow down
- ‘Fast-pathway’ and ‘slow-pathway’
- Impulse flows down one pathway, back up the other, and gets caught in a loop.
Give some examples of re-entrant rhythm conditions
- AV nodal Re-Entrant Tachycardia (AVNRT)
- AV Re-Entrant Tachycardia (AVRT)
- Atrial Flutter
- Atrial Fibrillation
- Ventricular Tachycardia
What is required in order for re-entry to occur (disorder of impulse conduction)?
Central area of block
- e.g. scar tissue, refractory cells
Area/path of variable blocking
- e.g. dead myocytes, myocytes with different refractory period/ conduction velocity, etc.
Explain supraventricular tachycardia & how it will be seen on an ECG
- Supraventricular Tacchycardia (AVNRT or AVRT)
- Regular tacchycardia between 130-250bpm
- There is 1 p-wave for each QRS, but they may not be visible
- QRS duration is usually narrow (normal), as after the AV node the impulse is conducted normally through the ventricles
Explain how the accessory pathways take place in a patient with SVT
What condition is this?
Left sided accessory pathway (SVT)
- As short PR
- +ve v1 wave means pathway is on the left
What condition is this?
Wolf Parkinson White - narrow complex regular tachycardia
What condition is this?
SVT (doesnt matter what lead you look at)
What condition is this?
Atrial Fibrillation - Irregularly irregular
What condition is this?
What are the disorders of impulse conduction (conduction block examples)?
- 1st Degree AV Block
- 2nd Degree AV block
- Type 1 (wenkebach)
- Type 2
- 3rd Degree (Complete) AV Block
What condition is this?
AV block - 1st degree
What condition is this?
AV block - 2nd dgree
Type 1 - Wenchebach
What condition is this?
AV block - 2nd degree
Type 2
What condition is this?
AV block - 3rd degree
How might ischaemic heart disease cause AV block?
- Right Coronary Artery (RCA) most commonly supplies blood supply to SA and AV node
- Blockage to RCA causes SA and AV node ischaemia
- Sinus bradycardia
- AV Block
What are the consequences of arrhythmia?
- Patients may be asymptomatic
Cardiac output = stroke volume x HR
- Low HR will cause direct drop in CO
- But, stroke volume is reliant on LV filling in diastole
- Tachycardia will limit how much the ventricle can fill, reducing SV, reducing CO
- So both tachy and brady cardias can cause haemodynamic compromise
What is the haemodynamic compromise of arrhythmia?
- Prolonged Arrhythmia can lead to
- Heart Failure
- Cardiac ischaemia due to reduced coronary filling
- Hypotension
- Poor renal perfusion -> renal failure
- Poor liver perfusion-> liver failure
- Poor brain perfusion -> syncope, confusion
