T1 L14: Cardiac Arrhythmias

• "Natural death within one hour of the onset of acute symptoms"?

  sudden death
• "Sudden cardiac death"?

  Sudden death due to cardiac causes
• Disorders leading to risks for sudden cardiac death?

  • CHD / low LVEF (left ventricular ejection fraction)
  • structural heart disease (eg cardiomyopathies)
  • primary arrhythmia
• Broad categories of arrhythmia causes

  Electrical (primary or arrhythmogenic): ion channels and electrical issues at cellular level. Extra conduction pathways at organ level.

  Structural: Unusual shape or size of cardiac tissue that changes signal pathway. Can lead to signal delays that interfere with cardiac conduction cycle.

  Ischaemic: Hypoxia makes local heart tissue electrically unstable. Effectively changes signal pathway, leading to delays that interfere with cardiac conduction cycle.
• Causes of primary arrhythmia

  • Unstable myocardium: often due to damaged or hypoxic tissue, eg atrial fibrillation
  • Ion channel pathologies: also called channelopathies. Eg long QT syndrome
  • Accessory conduction pathways: eg Wolff-Parkinson-White
• Cardiomyopathy


  Pathology when heart size, shape or thickness is abnormal; excluding those due to CAD, hypertension, valve abnormalities, and heart disease present at birth

  Consequences often:

  • risk of pumping dysfunction / low output heart failure
  • conduction abnormalities (because the normal pathways of electrical conduction are altered)
• Two types of cardiomyopathy

  • Dilated (eccentric): inner chambers enlarge, myocardium thins and stretches
  • Hypertrophic (concentric): inner LV chamber shrinks, myocardium thickens
• Tetralogy of Fallot 


  • Developmental defect
  • Four developmental defects:
  • ventricular septal defect (hole between ventricles)
  • pulmonary valve stenosis
  • Right ventricular hypertrophy
  • overriding aorta (misplaced or moved)
  • low blood oxygen saturation & cyanosis - blue baby
  • associated with a higher risk of sudden death, even after surgical correction in childhood

  
• Substrate vs Trigger

  Substrate: ongoing, underlying tissue instability that increases triggers/allows for maintenance and amplification of arrhythmia

  • Electrical defect as substrate: genetic/pharmacological problems with ion channels/electrolytes
  • Structural defect as substrate: fibrosis, inflammation caused by IHD, rheumatic disease etc

  Trigger: brief event required to initiate a period of arrhythmia

  • eg extrasystole (extra beat)
• R on T 


  • ECG phenomenon
  • a type of potential trigger for arrhytmias
  • on ECG, premature QRS complex occurs during previous T wave
  • T wave is a 'vulnerable period': during repolarisation of ventricular AP, refractory period is ending
  • Resulting QRS wave is a 'premature ventricular contraction' (PVC).

  
• Pacemaker


  • implanted electronic device
  • Consistently applies impulses for each heartbeat
  • mostly used for bradyarrhythmias and heart block
• ICD - Implantable Cardioverter Defibrillator 


  • implanted electronic device. Uses large shock waves to reset entire heart
  • applies electrical impulses ONLY when ventricular dysrhythmias detected
  • protects from fast or uncontrolled rhythms

  Indications for implanting an ICD:

  • cardiac arrest due to ventricular fibrillation
  • symptomatic heart failure with low LVEF
  • cardiomyopathies
  • congenital
  • channelopathies

  
• Antiarrhythmic Drugs

  • usually affect ion channel activity or sympathetic drive
  • examples:
  • Amiodarone
  • beta blockers
  • (Digoxin for AF)
  • Mostly for supraventricular arrhythmias (deriving from atria or AV node)
• Reperfusion injury 


  tissue damage caused when blood supply returns to the tissue after a period of ischaemia or hypoxia.

  (tissue damage/hypoxia leads to electrical irregularities & risk)

  • Cause: restoration of circulation results in inflammation and oxidative damage
  • Contexts: myocardium after PCI (percutaneous coronary intervention), brain tissue after ischaemic stroke
  • Preventative treatments: cooling, immunosuppression, oxygen radical scavengers
• Syncope vs Seizures

  both may be asymptomatic so difficult to differentiate

  Syncope:

  • may be registered by Holter monitor
  • 'cumple'

  Seizure:

  • may be registered by EEG/brain imaging
  • stiffness/unusual postures, 'tip over'
• Vasovagal vs Exertional Syncope

  Vasovagal syncope:

  • vagal increase & sympathetic decrease
  • so vasodilatation + low heart rate, resulting in sudden low cardiac output
  • triggered centrally (brain), not heart level
  • most common form; recurrent, common in young adults

  Exertional syncope:

  • benign
  • usually after exercise
• Index case


  the initial patient in the population of an epidemiological investigation; the primary case
• The Proband


  In medical genetics, the index case is the case of the original patient that stimulates investigation of other members of the family, usually referred to as ...?
• Number Needed to Treat (NNT)


  Average number of patients who need to be treated to prevent one additional bad outcome.

  Statistical measurement of the impact of a medicine or therapy.
• Penetrance (Genetic)


  the proportion of individuals carrying a particular allele of a gene and also express the trait. ie have the gene and also affected

  incomplete or reduced penetrance: some individuals will not express the trait even though they carry the allele. Many channelopathies can vary from patient to patient.