Arrhythmias Flashcards
What two systems are innervated by the vagus nerve?
Heart and GIT
Normal sinus rhythm impulses originate from which node?
SA node
Describe the impulse pathways during AF
Impulses have random, chaotic pathways
Name three conditions which are associated with AF
- Rheumatic disease
- Pericarditis
- Hyperthyroidism
Name three ways a heart can be restored to normal rhythm
- Defibrillation
- Cardioversion
- Transcutaneous pacing
What is an AV block?
Decreased rate of impulse from atria to ventricle through AV bundle
List four physiological causes of heart block
- Ischaemia
- Compression
- Inflammation
- Extreme vagal excitation (parasympathetic)
What occurs in 1st degree AV block?
Atrioventricular conduction is lengthened
Describe the lead II ECG of 1st degree AV block
P wave precedes each QRS but PR interval is >0.2s
What occurs on the ECG of both 2nd degree AV blocks?
Sudden dropped QRS
Describe the lead II ECG of 2nd degree AV block
Intermittently skipped ventricular beat
What occurs in Mobitz 1 AV block?
Progressive lengthening of PR interval until there is a dropped QRS
What occurs in Mobitz II AV block?
Normal PR interval with a randomly dropped QRS
What occurs in 3rd degree AV block?
Impulses originate at AV node and proceed to ventricles. Atrial and ventricular activities are not synchronous
Describe the lead II ECG of 3rd degree AV block
Constant rates of P waves and QRS complexes that occur independently of each other
What can be used to rx bradycardia?
-
Atropine
- Blocks parasympathetic nervous system
-
Isoprenaline
- Activates sympathetic nervous system
- Adrenaline (last resort)
From which node does sinus tachycardia originate?
SA node
Where do the impulses originate in supraventricular tachycardia?
Above the ventricles, usually the atria
Where do the impulses originate in ventricular tachycardia?
Ventricles
Where do narrow complex tachycardias originate?
Above the ventricles
Where do broad complex tachycardias originate?
Usually from the ventricles, though can also be SVT with aberrant conduction
What is done to halt broad complex tachycardia?
Synchronised cardioversion
What can trigger broad complex tachycardia?
- After MI (hypoxia)
- Drugs/toxins
- Inflammation
What is the most common non-psychogenic narrow complex tachycardia?
AV-nodal reentrant tachycardia (AVNRT), accounting for 80-90% of all cases
What causes AVNRT?
Extra pathways in/around the AV node
In AVNRT, does the fast pathway have a short or long refractory period?
Long
In AVNRT, does the slow pathway have a short or long refractory period?
Short
Describe the conduction process of AVNRT
- Signal comes down from the SA node and goes down both pathways, reaching the end of the fast pathway before the slow pathway
- Signal goes down into the ventricles as well as back up the slow pathway to meet the slow signal where they cancel each other out
- Both pathways enter their refractory periods, with the slow pathway recovering sooner
- Another signal comes down and starts down the slow pathway while the fast is still in refractory
- As the signal reaches the end of the slow pathway the fast pathway comes out of refractory
- The signal continues down into the ventricles as well as travelling back up the fast pathway to the atria and reentering the slow pathway
- Cycle continues, causing an atrial and ventricular contraction with every loop
Describe an ECG of AVNRT
- Narrow complex tachycardia without P waves
- P waves may not always be visible (may be buried in QRS, as atria and ventricles are contracting simultaneously)
- May have ‘pseudo-waves’ caused by retrograde P waves
How is AVNRT treated?
- Sometimes terminates itself, ending in a QRS
- Adenosine (transiently blocks AV node)
How does adenosine rx SVT?
- Vascular smooth muscle
- Inhibits Ca++ channels, reducing contraction
- Opens K+ channels, hyperpolarising the cells
- SA/AV node
- Opens K+ channels, hyperpolarising the cells
- Inhibits release of noradrenaline
How does adenosine induce rebound tachycardia?
Decreased cardiac output and vasodilation result in hypotension, triggering the baroreceptor reflex
What is the long term rx for AVNRT?
Catheter ablation - defective tissue is identified then destroyed by high energy (heat, laser, or extreme cold [cryo-ablation]) with a catheter
