Arrhythmia: Overview Flashcards
Define a cardiac arrhythmia
A disturbance to the hearts rate or rhythm
What are the 2 possible causes of cardiac arrhythmia?
- Changes in impulse formation
- Changes in impulse conduction
What are the 2 types of cardiac arrhythmia caused by changes in impulse formation?
Automaticity changes
Triggered activity
What are the 3 types of cardiac arrhythmia caused by changes in impulse conduction?
Re-entry circuits
Conduction block
Accessory tracts
When describing the site of origin of a cardiac arrhythmia what terms are used?
- Supraventricular (atria, AV)
- Ventricular
List the 3 subtypes of cardiac arrhythmia due to changes in automaticity
(i.e. the 3 possible reasons for the loss of overdrive suppression)
- SA node frequency too slow
- Latent pacemakers firing is quick
- Tissue damage (i.e. SA node fails)
Describe the automaticity of components of the cardiac conduction system other than the SA node
A slower spontaneous phase 4 depolarisation
Give the firing rates of the:
- SA node
- AV node
- Purkinje fibres
SA node: (Fastest)
70-80
AV node: (Medium)
50-60
Purkinje fibres: (Slowest)
30-40
Define overdrive suppression
Where the SA node exerts control over heart rate and rhythm by discharging action potentials a frequency greater than other heart structures
Is a change in the automaticity of the heart always pathological?
No, can be due to physiological autonomic control of HR
Describe how a cardiac arrhythmia arises from the loss overdrive suppression
SA node no longer dictates heart rate and rhythm
Its role is taken by a latent pacemaker
Define a latent pacemaker
Pacemaker cells that are normally controlled by overdrive suppression, but can assume autorhythmicity
Describe what happens when a latent pacemaker cell isn’t receiving impulses at its pacemaker frequency
It starts making its own action potentials
Define an escape impulse
An impulse generated by a latent pacemaker cell when it doesn’t receive an impulse from the SA node in time
(occur if SA node is too slow, or is impeded)
Define an escape beat
Define an escape rhythm
- The heart beat that may be produced by an escape impulse
- A series of escape beats
Define an ectopic beat
A beat initiated by a latent pacemaker that is firing at a rate faster than the SA node
Thus overdrive suppression is lost
Define an ectopic rhythm
A rhythm not dictated by the SA node
but instead by latent pacemakers firing at a rate faster than the SA node
Give possible causes of an ectopic rhythm
- Ischaemia
- Hypokalaemia
- Increased sympathetic activity
- Fibre stretch
Can non-pacemaker cells (such as myocytes) assume spontaneous activity?
Yes, under specific conditions
Usually only after tissue damage (e.g. MI)
Define an afterdepolarization
At what points in the action potential can they occur?
An abnormal oscillations in membrane potential
Can occur:
- During repolarization (phase 2 and 3)
- After repolarization (phase 4)
List the 2 types of afterdepolarization
early afterdepolarizations (EADs)
delayed afterdepolarizations (DADs)
Do all afterdepolarizations cause a heart beat?
No, only if they reach the threshold to cause a premature action potential
Describe an early afterdepolarization (EADs)
When does it occur?
What causes it?
Can occur during phase 2 or 3
Phase 2:
- mediated by Ca2+ channels (as Na+ channels are still inactivated)
Phase 3:
- mediated by Na+ channels (as Na+ channels have partially recovered from inactivation)
Is an early afterdepolarization (EAD) more likely to occur during bradycardia or tachycardia?
Why is this?
More likely during bradycardia
They prevent the relaxation of the muscle (doesn't let it reach phase 4) ---> Longer contraction --> Longer QT interval
Thus are associated with a prolonged action potential, and prolonged QT interval
In what type of tissue is an early afterdepolarization (EAD) more likely to occur?
Purkinje fibres
What ECG findings are suggestive of early afterdepolarization (EAD)?
- Prolonged QT segment
- Bradycardia
as this means longer action potentials
Describe an delayed afterdepolarization (DADs)
When does it occur?
What causes it?
Occur in phase 4
Caused by a large increases in intracellular [Ca2+]
Excessive intracellular [Ca2+] causes:
- oscillatory release of Ca2+ from the sarcoplasmic reticulum
- transient inward current (involving Na+) occuring in phase 4
^^(these provide the +ve charges required for a depolarisation)
Is an delayed afterdepolarization (DAD) more likely to occur during bradycardia or tachycardia?
Tachycardia
Describe how cardiac arrhythmia is caused by a re-entrant circuit
A self sustaining re-entrant electrical circuit stimulates an area of myocardium repeatedly
Describe the normal function of an area where a re-entrant circuit could form
Impulse splits at a junction with a non-excitable area
- ->
- Signals travel off in either direction
- Where the impulse paths reconnect the signals, traveling in opposite directions stop each other
- This is because one impulse can’t excite the cells just excited by the other impulse
- Thus the signals cancel out
Describe the re-entrant circuit required for a cardiac arrhythmia
Impulse splits into 2 paths at a junction with a non-excitable area
One path has a unidirectional block, only allowing retrograde conduction
The retrograde conduction is slowed to prevent the impulses canceling each other out
Define anterograde conduction
Forward impulse conduction in a nerve
Define retrograde conduction
Backwards impulse conduction in a nerve
What term describes the movement of current in a re-entrant circuit?
Circus movement
List the types of conduction block
First degree:
Second degree:
- Mobitz type 1
- Mobitz type 2
Third degree:
Describe first degree conduction block
Partial conduction block
Due to slowed conduction
All impulses are conducted, just slower
Describe second degree conduction block
Partial conduction block
Due to an intermittent blocking of conduction
Only some impulses are conducted
2 types:
- Mobitz type 1
- Mobitz type 2
Describe Mobitz type 1 conduction block
And its ECG appearance
PR interval gradually increases from cycle to cycle until AV node fails completely and a ventricular beat is missed
On an ECG:
PR interval gets bigger until it misses a QRS complex, then it restarts with a small PR interval
Describe Mobitz type 2 conduction block
And its ECG appearance
Every set number of beats there is no ventricular depolarisation
PR interval is constant but every set number of beats the QRS is missing
Describe third degree conduction block
Complete conduction block
No impulses are conducted through the affected area
Atria and ventricles beat independently governed by their own pacemakers
(Atria = SA node)
(Ventricles = Purkinje fibres)
Due to lower intrinsic frequency of latent pacemakers this causes bradycardia
Why does third degree conduction block cause bradycardia?
Complete block –> latent pacemakers take over
Bradycardia is due to the lower intrinsic frequency of latent pacemakers
Which latent pacemakers are in control of atrial and ventricular pacemaking in third degree conduction block?
Atria = SA node
Ventricles = Purkinje fibres
This is as its the AV node that is blocked, thus the atria and ventricles beat separately
Describe how accessory tract pathways cause cardiac arrhythmias
Some individuals possess electrical pathways in parallel to the AV node
There is another connection between the atria and ventricles
Ventricles will receive impulses through the normal and accessory pathway
Can also set up a re-entrant circuit
Give a common accessory tract pathway
The bundle of Kent
Can you have multiple early afterdepolarizations as part of the same action potential?
Yes
It’s usually just one, but you can have a few
Describe the relationship between action potential length and delayed afterdepolarization incidence
Shorter AP –> increased DADs
Longer AP –> decreased DADs
What types of drug increase the chance of delayed afterdepolarizations?
Why?
Drugs that increase intracellular Ca2+
e.g. Digoxin and catecholamines
Digoxin:
- Increases the release of stored Ca2+ from the sarcoplasmic reticulum
Catecholamines:
- Increase influx of Ca2+
What type of arrhythmic mechanism causes long QT syndrome and Torsades de Pointes?
Early afterdepolarizations
What conditions can lead to reentrant arrhythmias?
Accessory pathways
Scar tissue from and MI
Congenital heart disease
There are 2 types of reentrant circuit mechanism, describe them
Mechanism 1:
-Impulse splits into 2 paths at a junction with a non-excitable area
- One path has a unidirectional block, only allowing retrograde conduction
- The retrograde conduction is slowed to prevent the impulses canceling each other out
OR
Mechanism 2:
- Impulse splits into 2 paths at a junction with a non-excitable area
- One path has slowed conduction
- This allows the delayed impulse to miss the refractory period of the other impulse, forming a reentrant circuit
