Pathophysiology of Arrhythmias Flashcards
What are the main types of arrhythmias?
– Bradycardias
– Tachycardias
How are disturbances of the cardiac rhythm diagnosed ?
With an ECG
Define decremental conduction. Where is it found ?
In AV node, the more frequently the node is stimulated the slower it conducts. Prevents rapid conduction to the ventricle in cases of rapid atrial rhythms, such as atrial fibrillation or atrial flutter.
Define fast conduction. Where is it found ?
In His Purkinje system, conduct faster and faster until it stops conducting, when refractory.
Describe the phases of a typical cardiomyocyte AP. Relate this to the AP of a pacemaker cell.
Phase 0 = rapid depolarisation Phase 1 = partial repolarisation Phase 2 = plateau Phase 3 = repolarisation Phase 4 = pacemaker potential
Pacemaker cells do not have phases 1 or 2.
What are the main kinds of bradycardias, where classified by location.
Sinus bradycardias and AV blocks (latter occurs in or below AV node)
What are the causes of sinus bradycardia ?
SINUS BRADYCARDIA ALWAYS A SECONDARY EVENT, TO: – Drugs (e.g. Beta Blockers, Diltazem) – Vagal activity – Hypothyroidism – Sinus Node disease – Electrolyte abnormalities
What are the causes of AV block bradycardia ?
– Vagal activity
– Myocardial infarction
– Electrolyte abnormalities
– Degenerative diseases
What are the main kinds of AV blocks ? What is this classification based on?
1st, 2nd and 3rd degree AV blocks.
By ECG abnormality
Describe the main feature of 1st degree AV block.
– Lengthening of the PR interval
Describe the main feature of 2nd degree AV block.
– Mobitz Type 1 (Wenckebach block).
• Progressive lengthening of PR interval until P wave blocked and then PR short again
– Mobitz Type 2
• Block after 2 or 3 conducted beats in regular pattern
Describe the main feature of 3rd degree AV block.
– Complete AV dissociation (atrial activation (usually from the sinus node) is independent from ventricular activation (originating from the AV junction, His-Purkinje system, or ventricles))
What are the main treatments for bradycardia ?
• Pacemakers
– Temporary
– Permanent
• Only if needed, treatment for:
– Symptoms of syncope dizziness
– Prophylactic at time of operations
– Post AMI
What are the main types of tachycardia ? What is this classification based ?
Based on ECG
- Narrow Complex / Supraventricular Tachycardias (whatever is causing it is above AV node and sending it below AV node)
- Broad Complex Tachycardias (origin is below AV node and therefore arising from ventricle)
What is the main danger with broad complex tachycardias ?
Broad complex tachycardias are intrinsically unstable, can move into VF and to death.
Are all broad complex tachycardias VTs ?
No, not all broad complex tachycardias are VT
What are the main types of narrow complex / supraventricular tachycardias ?
– Atrial Tachycardias (Focus in the atrium)
– Junctional Tachycardias (Arise at junction of the heart)
– AVNRT + AVRT (Involve the AV node intrinsically or AV node and accessory pathway)
– Atrial Flutter (narrow and regular)
– Atrial Fibrillation (narrow but irregular. Could still be AF is broad and irregular)
What are the main types of broad complex tachycardia ?
– Ventricular Tachycardia
• Monomorphic (every complex is the same) and polymorphic (complexes are different) VT
– SVT (supraventricular tachycardia) with aberration (= acquired, rate- dependent bundle branch block) (e.g. AVNRT + AVRT ???)
– SVT with a pre-existing BBB morphology on ECG (e.g. SVT of antedromic tachycardia in WPW)
Explain how an SVT with aberration occurs.
QRS complex looks narrow but when driven at higher rate, gets rate dependent BBB, so complex widens.
An SVT that comes on as ventricular tachycardia and induces a rate dependent BBB will be broad, this is SVT with aberration.
What are the main basic mechanisms of tachycardia ?
- Ectopic Focus – i.e. tissue with rapid pacemaker function
- Re-entry / circus movement
- Fibrillation – independent wavelets of activity
Explain the mechanism of re-entry/circus movement.
Following myocardial infarction, dead zones of myocardium cannot conduct electrical impulses. However, because of the syncytial nature of the myocardium, electrical impulses simply flow around the dead zone and activate the remaining healthy myocardium. However, during conditions of ischemia and ischemic injury, ischemic portions of the myocardium will allow action potentials to proceed through them in one direction but not in the opposite direction. This is called unidirectional blockade. If action potentials are able to leak through this damaged area and emerge in the healthy myocardium after that area is past its refractory period, this “reentry” action potential will reactivate the healthy myocardium, sending another action potential generation through the injured tissue path. This process will then repeat again and again in an endless circle. This condition is called a circus rhythm and, as impulses generated from this circus cycle radiate outward through the syncytium, they activate the heart as a whole, at the rate set by the circus rhythm. Such circuits thus become secondary pacemakers. They may involve a few myocardial cells or large portions of the myocardium and often end up pacing the heart at an abnormally high rate
Describe the ECG appearance of AVNRT or AVRT.
Regular, narrow complex tachycardia without P waves activity
What are the tachycardias which involve the AV node ?
AVNRT and AVRT
What is usually the cardiac history of patients with AVNRT and AVRT ?
No history of cardiac disease
Define AVNRT.
AVNRT = AV nodal re-entrant tachycardia
– Tachycardia where re-entry circuit is through juxtanodal material
Define AVRT.
• AVRT = AV re-entrant tachycardia
– Tachycardia where re-entry is through an accessory pathway
• Revealed accessory pathway means Wolff Parkinson White (if conduct over them in sinus rhythm)
• Concealed accessory pathway means normal non- tachycardia (i.e. the accessory pathway only conducts in a retrograde manner)
What is meant by accessory pathway through which re-entry occurs in AVRT ?
His Purkinje material that has breached the AV ring somewhere other than the AV node
Distinguish between revealed and concealed accessory pathway.
The accessory pathway through which re-entry occurs in AVRT may either by revealed or concealed.
• Revealed accessory pathway means WPW (if conduct over them in sinus rhythm)
• Concealed accessory pathway means normal non- tachycardia (i.e. the accessory pathway only conducts in a retrograde manner)
How are AVRT and AVNRT terminated ?
Using IV adenosine
Define Wolf Parkinson White Syndrome.
- Pre-excitation (of the ventricles)
* Anatomical atrio-ventricular bypass tract with non-decremental conducting properties