Lecture 13- Arrhythmias Flashcards
abnormal rhythms (arrhythmias) may arise from
the atria
the ventricels
what are arythmias called if they occur in the atria
supraventricular arrythmia
where can arrhythmias arise from int he atria
SAN
atrium itself
AV node
what are arrhythmias that arise in the vetricels called
ventricular arrythmias
name the three main types of arrhythmias
- Atrial fibrillation
- Ventricular ectopic beats
- Ventricular fibrillation
All arrhythmias originate
somewhere outside of the normal conduction system of the heart- atria or ventricles.
you can determine where an arrhthmia originates from by looking
at the width of the QRS complex
if the QRS complex is narrow
rhythm is originating in the atria and traveling down the ventricles via the normal conduction path
if the QRS complex is broad
rhythm is originating in the ventricles and not travelling via the normal conduction path (taking longer for ventricles to depolarise)
Two ways abnormal rhythm can start to cause arrhythmia
- Ectopic focus
- Re-entry loop
1) Ectopic focus
Many tachycardic rhythms are caused by ectopic beats. These are impulses that are generated by an area (focus) in the myocardium, not the SAN. Ectopic impulses can be generated by a small area of highly excitable myocardium which can spontaneously depolarise and cause a wave of depolarisation.
Features seen on ECG depends on where the ectopic impulse originates: Atrial ectopic
an abnormally shaped P wave that appears early and is usually followed by a QRS complex due to the impulses being conducted to the ventricles e.g. atrial fibrillation is caused by an ectopic foci
Features seen on ECG depends on where the ectopic impulse originates: Atrioventricular junctional ectopics
these ectopics can activate the ventricle by travelling via the His-Purkinje system- normal QRS.
Impulses can also retrogradely activate the atria (impulse travels backwards from the AV node to spread across the atria) to give an inverted P wave- however this can be masked by the QRS complex as two events happen at the same time
Features seen on ECG depends on where the ectopic impulse originates: Ventricular ectopics
impulses generated by ectopics in the ventricles do not travel via the His-Purkinje system, and instead spread comparatively slowly over the myocardium. Therefore these ectopics give a broad QRS complex as the time taken for the impulse to travel is longer. There can be a compensatory pause as the ventricles have to repolarise before contracting again when the SA node next fires. E.g. ventricular tachycardia
Supraventricular vs ventricular arrythmias
2) re-entry loops
In the conduction system of the myocardium, there are areas in the pathway where electrical impulses can split and travel down two paths. This usually isn’t a problem because when the two impulses meet each other again, they cancel each other out. This allows the myocardium to contract in an even and efficient way.
However, problems arise when there is damage to areas of myocardium that disrupt the normal pathway of electrical impulses, or there are structural abnormalities. In the case of re-entry loops, there is an area of myocardium that is damaged and causes a unidirectional conduction block (electrical impulses can only travel one way through the damaged tissue, and one direction is blocked). Therefore electrical impulses will be able to retrogradely travel through the damaged tissue.
The impulse in a re-entry loop will travel back on itself and take alternative routes thorough the myocardium, causing abnormal contraction of the heart. This is best shown in the diagram below.
If there are re-entry loops, patient can have:
AV nodal re-entry
Atrioventricular re-entry
Atrial flutter
AV nodal re-entry
if there are ‘fast’ and ‘slow’ pathways in the AV node, there can be a re-entry loop which can cause a supra-ventricular tachycardia.
Atrioventricular re-entry
an accessory pathway between atria and ventricles leads to a re-entry loop. E.g. Wolff-Parkinson-White syndrome.
Atrial flutter can be caused by
can be caused by a re-entry loop in the atria.
atrial arrythmias 93)
Atrial tachycardia
Atrial flutter
Atrial fibrillation
atrial tachycardia is defined as a heart rate above
100bpm
how may atrial tachycardia appear on an ECG
As the heart is beating so fast, the T waves and P waves can be occurring at the same time and merge on the ECG
Causes of atrial tachycardia
- re-entry loop in the AV node
- digoxin toxicity
- ischaemic heart disease
- rheumatic heart disease
Atrial flutter is caused by
re-entry loop in the atria
atrial flutter and the AV node
- AV node cannot keep up with the rapid atrial depolarisation because it cannot conduct impulses more than 200bpm
- Re-entry loop sends off impulses at around 300 bpm-making the most common heart rate 150 bpm
causes of atrial flutter
Hypertension
Ischaemic heart disease
Hyperthyroidism
Alcoholism
Cardiomyopathy
How may atrial flutter appear on an ECG?
- Some P waves are not conducting resulting in AV block
- Most commonly 2:1 AV block i.e. 2 atrial beats (2 P waves) for every 1 ventricular beat (one QRS complex)
atrial fibrillation occurs due to
Arises due to multiple ectopic foci in the atrial myocardium
causes of atrial fibrillation
- dilated left atrium
- hypertension
- ischaemic heart disease
- hyperthyroidism
- alcohol
- cardiomyopathy
how does atrial fibrillation display on ECG?
- Rapid, chaotic impulses
- No P wave- just wavy baseline
- Irregular R-R intervals
why are there irregular R-R intervals in atrial fibrillation
Impulses reach AV node at rapid irregular rate- not all conducted
When conducted ventricles depolarise normally-so normal QRS
atrial fibrilation variations
can be :
- Slow- ventricular response <60bmp
- Fast- ventricular response >100 bpm
- Normal rate- 61-99 bpm
- Can be coarse (amplitude >0.5mm)
- Can be fine (amplitude <0.5mm)
Haemodynamic effects of atrial fibrillation
- Atrial contraction lost- just quiver
- Ventricles contract normally
- Heart rate and pulse are irregularly irregular
- Loss of atrial contract leads to increased blood stasis in left atrium- flow velocity reduced along with impaired contractility of left atrial appendage–> small clots form
- Risk of ischaemic stroke
arrhythmias can be classificed into either
Bradycardia
- heart block or
- simply bradycardia
Tachycardia
- narrow complex
- AF/Flutter
- Sinus tachycardia
- supraventricular tachycardia (atria)
- broad complex
- ventricular tachycardia
- ventricular fibrillation
Premature ventricular ectopic contractions (PVCS)
- Ectopic focus in ventricle muscle
- Impulse does not spread via fast His-Purkinje system
- Therefore much slower depolarisation of ventricular muscle (wider QRS)
- Premature because it occurs earlier than would be expected for the next sinus impulse
- May be ASx or cause palpitations
ventricular tachycardia (VTACH)
Run of >3 consecutive premature ventricular ectopic contractions (PVCs)
how is VTACH seen on an ECG
These are seen as very broad and bizarre QRS complexes on the ECG.
VT is a dangerous rhythm as it can
degenerate into ventricular fibrillation.
causes of bventricular tachycardia (VTACH)
- myocardial infarction
- ischaemic heart disease
- hypertrophic/dilated cardiomyopathy.
Ventricular Fibrillation (VF)
- Abnormal, chaotic, fast ventricular depolarisation
- Impulses from numerous ectopic sites in ventricle
- No coordinated contraction
- Ventricles quiver
- No cardiac output
- If sustained cardiac arrest
causes of ventricular fibrillation
VF is commonly associated with myocardial infarction, but can also be as a result of Torsades de Pointes (discussed next).
VF is the most important
shockable rhythm
how does VF present on ECG
no discernable pattern
Torsades de pointes
Polymorphic ventricular tachycardia- there is VF, however QRS complexes all look very different.
causes of Torsafes de pointes
- Anti-arrhythmic drug treatment
- Electrolyte abnormalities- long QT interval
