Transcript Mechanisms of Tachycardia

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)
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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