Mechanism of arrythmias

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Mechanism of arrythmias

  1. 1. Mechanism of Arrhythmias Dr Fuad Farooq
  2. 2. Normal Sinus Rhythm Implies normal sequence of conduction, originating in the sinus node and proceeding to the ventricles via the AV node and His-Purkinje system EKG Characteristics: Regular narrow-complex rhythm Rate 60-100 bpm Each QRS complex is proceeded by a P wave P wave is upright in lead II & downgoing in lead aVR
  3. 3. The mechanisms responsible for arrhythmias are generally divided into – – – Disorders of impulse formation Disorders of impulse conduction Combinations of both cardiac
  4. 4. Disorders of Impulse Formation Can be – Disorder in Automaticity – Trigger activity
  5. 5. Automaticity Automaticity is the property of a fiber to initiate an impulse spontaneously, without need for prior stimulation Characterized by – Inappropriate discharge rate of the normal pacemaker, the SA node (e.g., sinus rates too fast or too slow for the physiologic needs of the patient) – Discharge of an ectopic pacemaker that controls atrial or ventricular rhythm Ectopic pacemakers are often called latent or subsidiary pacemakers
  6. 6. Automaticity Ectopic pacemakers are usually suppressed by overdrive suppression by the more rapidly firing SA node or by concealed conduction from neighboring fibers – Manifested when the SA discharge rate slows or block occurs at some level between the SA node and the ectopic pacemaker site – The discharge rate of the latent pacemaker can speed inappropriately and usurp control of cardiac rhythm from the SA node
  7. 7. Recognizing Altered Automaticity on EKG Gradual onset (warming) and termination of the arrhythmia The P wave of the first beat of the arrhythmia is typically the same as the remaining beats of the arrhythmia (if a P wave is present at all)
  8. 8. Decreased Automaticity Sinus Bradycardia
  9. 9. Increased/Abnormal Automaticity Sinus tachycardia Ectopic atrial tachycardia www.uptodate.com Junctional tachycardia
  10. 10. Triggered Activity • Triggered activity is initiated by after-depolarizations, which are depolarizing oscillations in membrane voltage induced by one or more preceding action potentials • Thus, triggered activity is pacemaker activity that results consequent to a preceding impulse or series of impulses, without which electrical quiescence will occurs
  11. 11. Phase 1: Initial rapid repolarization Phase 0 : Rapid depolarization •Closure of the fast Na+ channels •Phase 0 and 1 together correspond to the R and S waves of the ECG •Opening of fast Na channels Phase 4: Resting phase •Associated with diastole portion of heart cycle •Potassium outward current through open, inwardly rectifying K+ channels ( Ikl ) Phase 2 Plateau phase Phase 3 Repolarization •K+ channels remain open •Allow K+ to build up outside the cell, causing the cell to repolarize •Corresponds to T wave on the ECG •Balance between the inward movement of Ca+ and outward movement of K + •Corresponds to ST segment of the ECG
  12. 12. Can be: • Early after-depolarizations (EADs), arise from a reduced level of membrane potential during phases 2 (type 1 – augmented opening of Ca channels) and 3 (type 2 – augmented opening of Na channels) of the cardiac action potential • Late or delayed after-depolarizations (DADs), occur after completion of repolarization (phase 4 – secondary to increase cytosolic Ca++ levels), generally at a more negative membrane potential than that from which EADs arise
  13. 13. • Causes of EAD include: 1. Low potassium blood level 2. Slow heart rate 3. Drug toxicity (i.e. quinidine causing torsades de pointes) • DAD may cause triggered activity due to: 1. Premature beats 2. Increased serum Ca++ level 3. Increased adrenaline levels 4. Digitalis toxicity
  14. 14. Disorder of Impulse Conduction Can be: – Conduction block – Re-entry
  15. 15. Mechanism of Reentry
  16. 16. Mechanism of Reentry
  17. 17. Reentrent Rhythms AV nodal reentrant tachycardia (AVNRT) AV reentrant tachycardia (AVRT) – Orthodromic – Antidromic Atrial flutter Atrial fibrillation Ventricular tachycardia
  18. 18. Recognizing Reentry on EKG Abrupt onset and termination of the arrhythmia The P wave of the first beat of the arrhythmia is different as the remaining beats of the arrhythmia (if a P wave is present at all)
  19. 19. Example of AVNRT
  20. 20. Atrial Flutter Most cases of atrial flutter are caused by a large reentrant circuit in the wall of the right atrium
  21. 21. Atrial Fibrillation Atrial fibrillation is caused by numerous wavelets of depolarization spreading throughout the atria simultaneously, leading to an absence of coordinated atrial contraction
  22. 22. Thank You

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