Defibrillators Flashcards
What are defibrillators?
- device that shocks a dysfunctional heart back to life
VF
A medical emergency as blood cannot be pumped out
Cardiac arrhythmias
- conditions in which failure in the timing and/or coordination of contraction occurs
- mainly by abnormal formation or propagation of excitation wave
Most dangerous type of arrhythmia
Re-entrant arrhythmia
Ectopic pacemaker
- abnormal pacemaker cells sitting outside SAN
- can cause additional beats (premature beats)
- normally suppressed
- can take over normal pacemaker activity
- can result in tachy or bradycardia
What changes can an ectopic pacemaker result in?
- change in conduction
- reduction in speed of conduction
- longer spread through myocardium
- QRS complex changes shape (increases duration by >10s)
What is re-entry?
- reoccurrence of same action potential through same pathway
- requires 3 conditions to be met
3 conditions to be met to allow re-entry
- critical timing (tissue needs to be excitable as action potential leaves path)
- in local region with a block in its pathway
- length of refractory period of the tissue
Re-entrant arrhythmia
- signal does not complete normal propagation circuit, but loops
- circuit movement reentry (anatomic or functional) (one of the most dangerous types of arrythmia)
Anatomical re-entry
- occur because of anatomical block
- re-entrant wave excites same tissue over and over again
- Wolff Parkinson White syndrome -> SVT
- narrow QRS and tachy
- can also be caused by spiral wave front rotating around blockage (vessel/scar)
- if reentry occurs faster at sinus rate, it will take over
What is the refractory period?
- period immediately after depolarisation when another action potential cannot be initiated
Functional re-entry
- no anatomically defined route
- altered region of tissue
- circulates around central core
- through to be main cause of ventricular tachycardia
- harder to reset, pace or manage than anatomical as no clear gap
How to treat tachycardia?
Cardioversion
Methods of cardioversion
Antiarrhythmic drugs
Ablative techniques
Electrical therapy = defibrillator
Antiarrhythmic drugs for cardioversion
- IV (acute) or oral (LT)
- suppress abnormal firing of pacemaker tissue = increase length of effective refractory period (change in ion movement across membrane)
Problems with Antiarrhythmic drugs for cardioversion
- drugs must be taken daily for life
- SE = proarrhythmia (get worse or new arrhythmias begin)
What are ablative techniques
- physically destroy cardiac tissue causing tachycardia (functional reentry)
Examples of ablative technqiues
- surgery = local heating & cooling of tissue
- transcatheter approach = targeted electrocautery in the heart
Pros of ablative techniques
- can cure tachycardia so antiarrhythmic medication not needed
- transcatheter ablation rapidly becoming treatment of choice for many SVTs
Types of defibrillator
- external or implantable
- implantable = automated (implanted cardioverter defibrillator)
The refractory period
- central idea for defibrillation
Theories of defibrillation
- Critical mass theory
- upper limit of vulnerability theory
Critical mass theory
- not all tissue needs to be activated to terminate fibrillation
- a critical amount of myocardium has to be depolarised to reach refractory period to extinguish arrhythmia
- small areas could still support wavefronts
- studies mapping electrical activation after failed shocks lead to disagreement with theory
Upper limit of vulnerability theory
- failed shocks reinitiate defibrillation rather than completely stop it
- shock propagates out from application points (voltage gradient drops towards edges, fibrillation reinitiated in regions of low shock intensity - away from shock location)
- upper limit defines when shock energy is sufficiently strong to depolarise all myocytes = no risk of reinitiating fibrillation
Lower and upper limit of volunerability
Lower limit = minimum electrical stimulus strength that induces VF
Upper limit = maximum electrical stimulus strength that induces VF
Electric shock dose
The energy delivered during the shock (joules)
Energy = current x voltage x time
What is the form of delivered shock?
DC discharge
Direct surgical defibrillation energy
5-30J
Cardioversion energy
- Atrial flutter & SVT (50-100J)
- AF (100-200J)
- VT (100-200J)
Emergency defibrillation energy
- initially 200J, then again 200J or 360J
Defibrillation threshold
- in reality no such thing
- success of defibrillation influenced by multiple factors and best modelled as random variable
What is more important than electric shock dose?
Voltage gradient!
Key aspect in determining whether shock will success or fail
Also different waveforms
= So pattern of current flow + total energy production are both important
Waveforms
- monophasic
- biphasic
External defibrillator types
Manual or automated
Manual external defibrillators
- confined to use in ambulances or hospitals
Automated external defibrillators
- fully automated
- frequency seen in workplace environments
- generally limited to treating VF or VT
- automatically analyse ECG and make a decision whether to deliver shock
- detect VF with specificity and sensitivity
- public follow instructions
- need to be maintained in environment so in safety panel to hide higher functions for trained users and in alarmed storage containers
Key factors of automated external defibrillators (AEDs)
- cannot interfere with response at all
- automatically detects heart rate
- automatically shocks if needs to
- override functions are available but need to know how to activate them
- takes 10-20 seconds to detect rhythm and decide what to do, a trained clinician can be much faster
Components of External Defibrillators
- control box (holds all electronics & user controls), (must be able to generate 7kV to charge the capacitor)
- power source (lithium, NiCd batteries)
- electrodes (high voltage paddle electrodes or adhesive pads)
- gel needed to increase conduction and prevent from skin burns
- cables
- connectors
Unsychronised defibrillation
Unsynchronised (asynchronous)
- shock at any time
- initially 200J, then again 200 or 360J
- treat VF
Problems with unsychronised defibrillation
- treating AF (vulnerable point) (still have QRST -> still have T period -> if shock here = VF)
Synchronised defibrillation
- DC cardioversion
- cardioversion = synchronised defibrillation
- lower energy shock
- shock 30ms after R wave
- cells contract simultaneously, terminate abnormal rhythm
- AF = 100-200J
- atrial flutter and SVT = 50-100J
When to use an implantable cardioverter defibrillator
When individuals who are risk of needing one
Implantable Cardioverter Defibrillators (ICDs)
- small, full implanted LT defibrillation device for high risk patients
- monitors electrical rhythm to detect and treat dangerous fast heart rhythms (VT & VF)
- placed in upper chest, just below left collar bone
How do ICDs work?
1) Sense cardiac rhythm
2) Analyse electrocardiogram for VT, VF etc.
3) Shock if required (some devices may attempt overdrive pacing prior to synchronised cardioversion)
ALL OF THIS REQUIRES = POWER (battery!) (high power requirement for charging capacitor and advent of devices limited by battery technology)
Main components of ICD
GENERATOR - Battery - Capacitors - DC-DC convertor - computer circuitry - programmer SWITCHES LEADS
Developments has meant reduction x10 in ICD size
Batteries for ICDs
- lithium silver vanadium oxide
- high energy storage
- relatively stable voltage output over lifetime
- battery status determined by voltage and charge time on capacitor
- battery cannot provide enough instantaneous energy to defibrillate heart
Capacitors
- ICD require 2x 390V, 200uF capacitors to deliver 30J
- similar to photoflash capacitors (manage high discharge currents without over heating)
- dielectric constant is the key factor in reducing size
- hybrid designs in latest units
- highly competitive market
ICD Leads
First ICD devices: - 2 HR sensing electrodes (pace/sense) - 2 large patch defibrillating electrodes (shock) - open heart surgery required NEWER DEVICES - 1 transvenous lead - rate sensing electrodes at the end - defibrillating electrodes along length
Implanting ICD Leads
Modern design of ICD leads made surgery simple:
- 2 inches incision in upper chest
- lead taken through vein into heart
- local anaesthetic (patients are aware)
- connect lead wires to ICD and program device
- inserted ICD in pocket beneath skin
- most people stay in hospital overnight and go home next day
Problems with ICD
- May fire constantly or inappropriately
- bleeding incision/catheter site
- blood vessel damage
- infection
- tear/bleeding in heart muscle
- pneumothorax
- lead dislodgement
- DVT (rare)
Why is it bad if ICD fires constantly or inappropriately
- medical emergency as dangerous but also depletes battery life
- significant discomfort and anxiety to patient and may trigger VF
- emergency services equipped with ring magnet to place over device, effectively disables shock function
