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Transcript
The Electrical Management of Cardiac Rhythm Disorders Tachycardia Mechanisms of Tachycardia 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 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 Action Potential and Ion Action Action Potential by Cardiac Region 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) 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 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 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) 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 Reentry Triggered Reentry Not Triggered 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 AVNRT on IEGM DOWN THE SLOW PATHWAY UP THE FAST PATHWAY Wolff-Parkinson-White (WPW) Macro-reentry SVT Atrial and ventricular participation Concealed WPW 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) AF with Irregular Ventricular Response 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 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 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) Monomorphic VT Polymorphic VT 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 Ventricular Fibrillation 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 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