The Electrical Management of Cardiac Rhythm Disorders Tachycardia Mechanisms of Tachycardia

1. The Electrical Management of Cardiac Rhythm DisordersTachycardiaMechanisms of Tachycardia

2. Unique Properties of Cardiac Tissue • Selective permeability • Only certain molecules or ions can pass through the cell membrane at certain times • Excitability • Sequential depolarization and repolarization • Communication with nearby cells • Propagation of electrical signals • Conductivity • Transmitting an electrical impulse from one cell to the next • Automaticity • Ability of myocardium to depolarize spontaneously

3. Ions • Ions are charged particles (positive or negative) • They travel in and out of cardiac cells in response to stimuli • Sudden movement of ions across the cell membrane will cause a change in electrical potential that can actually be measured

4. Action Potential and Ion Action

5. Action Potential by Cardiac Region

6. Mechanisms of Arrhythmias • Enhanced automaticity • Abnormal acceleration of phase 4 • Cellular or metabolic causes • Ischemia • Acid-base imbalances • Drug toxicity • Defibrillation is often ineffective in such patients (cardiac tissue is refractory for longer periods of time, so defibrillation cannot work)

7. Mechanisms of Arrhythmias • Triggered automaticity • Affects phase 4 • Triggered by • Pause-dependent arrhythmias • Catecholamine-dependent arrhythmias • Premature beats • Has many features in common with reentry tachycardias • Torsades-de-pointes

8. Mechanisms of Arrhythmias • Reentry • Most common form of ventricular tachyarrhythmias • Requires certain pre-existing conditions • A conduction pathway with two limbs with different conduction times • Unidirectional block • A triggering event • Patients must have the above conditions but do not necessarily have to have acute illness or chronic heart disease • Can be atrial, supraventricular, or ventricular • Defibrillation was designed to treat these specific arrhythmias

9. Supraventricular Tachycardias (SVTs) • SVTs originate above the ventricles • But may involve rapid ventricular response • Types of SVTs • Atrial fibrillation (AF) • Atrial flutter • Intra-atrial reentry tachycardia • Sinoatrial node reentry tachycardia • AV nodal reentry tachycardia (AVNRT) • AV reciprocating tachycardia (Wolff-Parkinson-White syndrome)

10. Macro-Reentry versus Micro-Reentry • Macro-reentry involves a large reentry circuit (can encompass both atria and ventricles) • Micro-reentry involves a small reentry circuit (within one chamber) • Atrial flutter is a macro-reentry atrial tachycardia

11. Reentry Triggered

12. Reentry Not Triggered

13. AVNRT • AV Nodal Reentry Tachycardia (AVNRT) is a common form of SVT • Micro-reentry • Reentry circuit is entirely within the AV node • Atria and ventricles are activated as “bystanders” • AVNRT will show up on an ECG as rapid atrial and rapid ventricular activity

14. AVNRT on IEGM DOWN THE SLOW PATHWAY UP THE FAST PATHWAY

15. Wolff-Parkinson-White (WPW) Macro-reentry SVT Atrial and ventricular participation

16. Concealed WPW

17. Atrial Fibrillation (AF) • AF is a common form of SVT • Can be extremely challenging to treat • Appears chaotic • Is associated with increased risk of stroke • Three main types • Paroxysmal • Resolves without treatment, often asymptomatic • Persistent • Requires treatment to convert, typically causes symptoms • Permanent • Medically refractory, symptomatic (can be severe)

18. AF with Irregular Ventricular Response

19. How to Treat AF • Many approaches to AF, but not all are right for every patient • Pharmacological therapy • Cardioversion • Chemical • Electric • Radiofrequency (RF) ablation • Surgical approaches • For device patients, the AF Suppression™ algorithm

20. Ventricular Tachycardia • Automatic VT • Acute illness • Metabolic cause • Abnormal phase 4 acceleration • Reversible if underlying cause is corrected • Triggered automatic VT • Rarest form of VT • Caused by an underlying chemical disturbance which leads to an electrical disturbance • ICDs have not been proven effective for these types of VTs

21. Ventricular Tachycardia • Reentry VT • Most common form of VT • ICDs were designed to treat this type of VT • Often involves an area of fibrosis on the heart (possibly from prior heart attack or ischemia) • Disrupted electrical pathways • Areas of slow conduction • Scar tissue can be ablated but ablation may just leave new scar tissue! • Monomorphic VT (from one source or focus) • Polymorphic VT (from multiple foci)

22. Monomorphic VT

23. Polymorphic VT

24. Ventricular Fibrillation (VF) • VF is a disorganized and potentially life-threatening arrhythmia • VF rates are so high that cardiac output drops to zero • QRS complexes cannot be clearly identified on ECG • Hemodynamic collapse • ICDs were designed to treat VF • VF can be lethal in minutes

25. Ventricular Fibrillation

26. Treating Ventricular Tachyarrhythmias • Pharmacological therapy (works on action potential) • Ablation • Can be curative in some cases • Most effective in early stages of disease • Device-based options • Combination therapy • Drugs to control tachycardia and reduce ambient arrhythmias • Defibrillation for any potentially dangerous ventricular tachyarrhythmias that might break through

27. Conclusion • There are three main mechanisms of tachyarrhythmias • Automaticity • Triggered automaticity • Reentry • Reentry is the most common form and it is the type of tachyarrhythmia that ICDs are designed to treat • Ventricular tachycardia • Ventricular fibrillation • Arrhythmias are named for the place in the heart where they originate • Supraventricular tachyarrhythmias • Ventricular tachyarrhythmias