Physiological Mechanisms of Arrhythmia

• What is a cardiac arrhythmia?

  An abnormality of the cardiac rhythm
• What may arrhythmias cause?

  Sudden death

  Syncope

  Heart failure

  Chest pain

  Dizziness

  Palpitations

  Can have no symptoms at all
• What are the 2 types of arrythmia?

  Bradycardia

  Tachycardia
• What are tachycardias subdivided into?

  Supraventricular trachycardias (SVT) - arise from atrium or AV junction

  Ventricular tachycardias - arise from ventricles
• When myocardial function is poor, arrhythmias are more symptomatic & are potentially life-threatening
• P wave = sinus

  A) SAN

  B) AVN
• Sinus arrhythmia is usually predictable - due to irregularities of pulse.
• What is sinus bradycardia?

  Sinus rate of less than 60 bpm

  Usually asymptomatic unless rate is very slow

  Sinus brachycardia is normal in athletes -> due to increased vagal tone
• What are the 2 main types of arrhythmia production?

  Distrubances in impulse generation

  Distrubance in impulse propagation

  Can cause bradycardia or tachycardia
• What are the causes of EADs?

  Slow HR

  Prolonged APs

  Ceratin antiarrhythmic drugs (e.g. quinidine) - prolongs AP
• What causes DADS?

  Increased serum Ca+2

  Increased adrenaline

  Drug toxicity (e.g. digoxin)

  MI
• What could you see in pts with EADs?

  Nothing

  Prolonged QT

  Sustained arrhythmia (Torsade's de pointes)

  
• What could you see in pts with DADs?

  Nothing

  Ectopic beat

  Sustained arrhythmia (ventricular tachycardia)

  
• What is heart block?

  Block in either the AVN or bundle of His -> AV block

  Block lower in the conduction system -> Bundle branch block
• What are the 3 forms of heart block?

  1st degree

  2nd degree

  3rd degree
• What is 1st degree AV block?

  PR interval is uniformly prolonged (more than 0.2 s)

  Every atrial depolarisation is followed by conduction to the ventricles, but with delay
• What is are the 2 types of 2nd degree AV block?

  Mobitz I

  Mobitz II
• What is Morbitz I AV block?

  Sinus rhythm

  PR interval lengthens progressively with successive beats, until 1 P wave is not conducted at all

  PR interval before blocked P wave is much longer than the PR interval after blocked P wave

  
• What is Morbitz II AV block?

  Sinus rhythm present

  PR interval is consistent

  1 P wave is not conducted at all

  Dropped QRS complex is not preceded by progressive PR interval prolongation ('no warning')

  Usually, wide QRS complex (> 0.12 sec)

  
• What is 3rd degree (complete) AV block?

  Atrial rate is faster than ventricular rate

  PR intervals are completely variable

  Complete loss of signalling between atria & ventricles (each are contracting, but not in sync)

  P waves fall on T waves or can be 'lost' in QRS complexes

  Normal width QRS complexes

  
• How can MI cause an AV block?

  Right coronary artery (RCA) supplies SAN & AVN

  So, blockage of RCA -> SAN & AVN ischaemia -> AV block

  Bundle of His 'takes over' -> narrow QRS complex, rate 50-60 bpm

  If Bundle of His is damaged, Purkinje fibres 'take over' -> broad QRS complex, rate < 40 bpm
• What does Right Bundle Branch Block (RBBB) show on ECG?

  Secondary R wave in V1

  Slurred S wave in V5 & V6

  Due to right bundle will be delayed -> left ventricle will depolarise first

  
• When does RBBB occur?

  Normal healthy individuals

  PE

  Right ventricular hypertrophy

  Ischaemic Heart Disease

  ASD (WHAT?)

  VSD (WHAT?)

  Tetrology of Fallot
• What is WPW Syndome?

  Wolff-Parkinson-White syndrome
• Define flutter.

  Regular, rapid atrial contractions

  250 - 305 bpm
• Define fibrillation.

  Sawtooth (F waves)

  Typically 2:1, 3:1 or 4:1 conduction ration

  More than 300 bpm
• What are the different mechanisms for producing disturbances in impulse generation?

  Disorder in automaticity

  Trigerred activity (EADs or DADs)
• What are the different mechanisms for producing disturbances in impulse propagation?

  Re-entry

  Conduction block

  Both (atrial flutter)
• How does accelerated automaticity lead to arrhythmia?

  Mechanism of slow depolarisation to reach threshold potential is altered (threshold increased/decreased)

  Reduced threshold -> increased rapid AP firing

  Increased threshold -> decreased AP firing

  Can lead to sinus tachycardia, escape rhythms & accelerated AV nodal rhythms
• How does triggered activity lead to arrhythmia?

  Myocardial damage -> oscillations ('after-depolarisations') of transmembrane potential at end of AP -> may reach threshold potential & produce arrhythmia

  If occurs before the transmembrane potential reaches its threshold -> early after-threshold AP (EADs)

  If occurs after transmembrane potential is completed -> delayed after-threshold AP (DADs)
• What can abnormal oscillations be exaggerated by?

  Pacing

  Catecholamines

  Electrolyte disturbances

  Hypoxia

  Acidosis

  Some medications
• Describe re-entry.

  Fibrous tissue/scar does not conduct electricity (electricity has to go around it)

  Problem - one of the routes is slower than normal (image - alpha is fast & beta is slow)

  Alpha pathway has a longer refractory period than beta

  When AP travels along both pathways, by the time the b pathway has sent its activity around the fibrous tissue, the a pathway has recovered and the b pathway ends up activating the a pathway (and going back up)

  When AP has reached the top (C), the b pathway will have recovered, AP goes back round -> ends up producing a run of tachycardia

  
• What is sinus bradycardia due to?

  Either extrinsic factors that influence a relatively normal sinus node, or to intrinsic sinus node disease

  Can be acute & reversible OR chronic & degenerative
• What are the common extrinsic causes of sinus bradycardia?

  Hypothermia

  Hypothyroidism

  Jaunduce

  Raised intracranial pressure

  Drugs (beta blockers)

  Neurally mediated syndromes (carotid sinus syndrome & vasovagal attacks)
• What are the common intrinsic causes of sinus bradycardia?

  Acute ischaemia & infarction of sinus node

  Chronic degenerative changes (fibrosis of atrium & sinus node)
• What is sick sinus syndrome?

  Condition where the sinoatrial node (SAN) dysfunction causes bradyarrhythmias or tachyarrhythmias

  Pts develop episodes of sinus bradycardia or sinus arrest & commonly have paroxysmal atrial tachyarrhythmias
• What is sick sinus syndrome caused by?

  Idiopathic fibrosis of SAN

  Ischaemic heart disease

  Cardiomyopathy

  Myocarditis
• What is 2:1 or 3:1 (advanced) block?

  Occurs when every second or third P wave conducts to ventricles

  PR interval prior to dropped P wave is always the same

  Form of 2nd degree block, neither Mobitz I or II

  
• What are the different causes of complete AV block?

  Congenital

  Idiopathic fibrosis (Lev's (older pts), Lenegre's (younger))

  Ischaemic heart disease

  Non-ischaemic heart disease

  Cardiac surgery

  Iatrogenic (pacemaker implantation)

  Drug induced

  Infection (endocarditis, lyme disease)

  Autoimmune (SLE, RA)

  Neuromuscular disease (DMD)
• What is the pathophysiology of complete AV block?

  Complete dissociation between atrial & ventricular activity

  P waves & QRS complexes occur independently of one another

  Ventricular contractions maintained by spontaneous escape rhythm originating below site of block from…

  • His bundle -> narrow QRS, rate 50-60 bpm OR
  • His-Purkinje system -> broad QRS, rate < 40 bpm (associated with dizziness & blackouts, Lev's & Lenegre's)