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Clinical Measurements > Defibrillators > Flashcards

Defibrillators Flashcards

1
Q

What are defibrillators?

A
  • device that shocks a dysfunctional heart back to life
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2
Q

VF

A

A medical emergency as blood cannot be pumped out

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3
Q

Cardiac arrhythmias

A
  • conditions in which failure in the timing and/or coordination of contraction occurs
  • mainly by abnormal formation or propagation of excitation wave
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4
Q

Most dangerous type of arrhythmia

A

Re-entrant arrhythmia

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5
Q

Ectopic pacemaker

A
  • 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
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6
Q

What changes can an ectopic pacemaker result in?

A
  • change in conduction
  • reduction in speed of conduction
  • longer spread through myocardium
  • QRS complex changes shape (increases duration by >10s)
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7
Q

What is re-entry?

A
  • reoccurrence of same action potential through same pathway

- requires 3 conditions to be met

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8
Q

3 conditions to be met to allow re-entry

A
  • 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
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9
Q

Re-entrant arrhythmia

A
  • signal does not complete normal propagation circuit, but loops
  • circuit movement reentry (anatomic or functional) (one of the most dangerous types of arrythmia)
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10
Q

Anatomical re-entry

A
  • 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
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11
Q

What is the refractory period?

A
  • period immediately after depolarisation when another action potential cannot be initiated
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12
Q

Functional re-entry

A
  • 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
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13
Q

How to treat tachycardia?

A

Cardioversion

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14
Q

Methods of cardioversion

A

Antiarrhythmic drugs
Ablative techniques
Electrical therapy = defibrillator

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15
Q

Antiarrhythmic drugs for cardioversion

A
  • IV (acute) or oral (LT)
  • suppress abnormal firing of pacemaker tissue = increase length of effective refractory period (change in ion movement across membrane)
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16
Q

Problems with Antiarrhythmic drugs for cardioversion

A
  • drugs must be taken daily for life

- SE = proarrhythmia (get worse or new arrhythmias begin)

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17
Q

What are ablative techniques

A
  • physically destroy cardiac tissue causing tachycardia (functional reentry)
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18
Q

Examples of ablative technqiues

A
  • surgery = local heating & cooling of tissue

- transcatheter approach = targeted electrocautery in the heart

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19
Q

Pros of ablative techniques

A
  • can cure tachycardia so antiarrhythmic medication not needed
  • transcatheter ablation rapidly becoming treatment of choice for many SVTs
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20
Q

Types of defibrillator

A
  • external or implantable

- implantable = automated (implanted cardioverter defibrillator)

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21
Q

The refractory period

A
  • central idea for defibrillation
22
Q

Theories of defibrillation

A
  • Critical mass theory

- upper limit of vulnerability theory

23
Q

Critical mass theory

A
  • 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
24
Q

Upper limit of vulnerability theory

A
  • 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
25
Q

Lower and upper limit of volunerability

A

Lower limit = minimum electrical stimulus strength that induces VF
Upper limit = maximum electrical stimulus strength that induces VF

26
Q

Electric shock dose

A

The energy delivered during the shock (joules)

Energy = current x voltage x time

27
Q

What is the form of delivered shock?

A

DC discharge

28
Q

Direct surgical defibrillation energy

A

5-30J

29
Q

Cardioversion energy

A
  • Atrial flutter & SVT (50-100J)
  • AF (100-200J)
  • VT (100-200J)
30
Q

Emergency defibrillation energy

A
  • initially 200J, then again 200J or 360J
31
Q

Defibrillation threshold

A
  • in reality no such thing

- success of defibrillation influenced by multiple factors and best modelled as random variable

32
Q

What is more important than electric shock dose?

A

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

33
Q

Waveforms

A
  • monophasic

- biphasic

34
Q

External defibrillator types

A

Manual or automated

35
Q

Manual external defibrillators

A
  • confined to use in ambulances or hospitals
36
Q

Automated external defibrillators

A
  • 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
37
Q

Key factors of automated external defibrillators (AEDs)

A
  • 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
38
Q

Components of External Defibrillators

A
  • 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
39
Q

Unsychronised defibrillation

A

Unsynchronised (asynchronous)

  • shock at any time
  • initially 200J, then again 200 or 360J
  • treat VF
40
Q

Problems with unsychronised defibrillation

A
  • treating AF (vulnerable point) (still have QRST -> still have T period -> if shock here = VF)
41
Q

Synchronised defibrillation

A
  • 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
42
Q

When to use an implantable cardioverter defibrillator

A

When individuals who are risk of needing one

43
Q

Implantable Cardioverter Defibrillators (ICDs)

A
  • 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
44
Q

How do ICDs work?

A

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)

45
Q

Main components of ICD

A 
GENERATOR
- Battery
- Capacitors
- DC-DC convertor
- computer circuitry
- programmer
SWITCHES
LEADS

Developments has meant reduction x10 in ICD size

46
Q

Batteries for ICDs

A
  • 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
47
Q

Capacitors

A
  • 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
48
Q

ICD Leads

A 
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
49
Q

Implanting ICD Leads

A

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
50
Q

Problems with ICD

A
  • May fire constantly or inappropriately
  • bleeding incision/catheter site
  • blood vessel damage
  • infection
  • tear/bleeding in heart muscle
  • pneumothorax
  • lead dislodgement
  • DVT (rare)
51
Q

Why is it bad if ICD fires constantly or inappropriately

A
  • 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

Clinical Measurements (11 decks)

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