the electrical management of cardiac rhythm disorders tachycardia mechanisms of tachycardia

The Electrical Management of Cardiac Rhythm DisordersTachycardia

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