CRM12: ICD Flashcards
What arrhythmias do defibrillators treat?
-VT
-VF
What conditions cause high risk of SCD?
-Brugada
-ARVC
-Long QT
-HCM
-Valve disease
-Non-ischaemic CM
-HCM
Indications for ICD
-Survived a cardiac arrest caused by either VT or VF
-Sustained VT causing syncope or significant haemodynamic compromise
-Sustained VT with LVEF of 35% or less
-Familial cardiac condition with high risk of sudden cardiac death (long QT, HCM, Brugada, ARVC)
-Undergone surgical repair of congenital heart disease
Primary prevention vs Secondary Prevention
Primary
-At risk of SCD but no documented arrhytmia
-Preventative
Secondary
-Survived threatening arrhythmia
-Probability of another event is high
What class indication is given to ICDs in primary prevention?
-Class I indication
-LVEF<35% due to prior MI
-Who are at least 40 days post MI
-NYHA Class II/III
Contraindications for ICDs (Class 3 indications)
-Syncope of undetermined cause
-Incessant VT or VF
-Terminal illness with projected life expectancy of <12 months
-Drug refractory HF, NYHA class IV who are not candidates for transplantation
Class I, II and III for ICD
Features of ICD battery
-Made from Lithium Silver Vanadium Oxide
-Large charge needs to be stored in a small volume
-Long shelf life is essential
-Battery current produced by process of oxidation and reduction
-Distinctive discharge curve (2 regions of nearly constant voltage before ERI)
Features of ICD capacitor
-Capacitor acts as reservoir. It can store electricity
-Ability of capacitor to store an electrical charge is called capacitance
-Capacitor consists of 2 conducting surfaces, separated by insulating material
-Capacitor charged by removing charge from one plate to another
-Automatic capacitor maintenance
-Increased charging time due to lower battery voltage
Features of ICD electrodes
-Multilumen lead construction
-Active lead fixation
-Coiled conductors have low electrical resistance to deliver charge effectively
-Single or dual coil leads
Different types of ICD electrodes
DF-1:
2 x high voltage
1 x pacing
-Can remove HV component
DF-4:
-4 pole defibrillator and pacemaker connector system
How do you identify pacemaker or defibrillator?
-Lead with coil (thicker) is defibrillator lead
When would you use dual coil leads instead of single?
-Very dilated heart which requires more energy (heart failure)
-If ICD is on right side, you can capture more of the myocardium with 2 coils
Types of sensing vectors in ICD and downsides
Dedicated/True bipolar - traditional sensing
-Sensing/pacing between tip and ring
-Can get T wave oversensing
Integrated bipolar - larger sensing vector, can often get larger R waves
-Sensing/pacing between tip and coil
-Depending on position of lead can lead to Atrial oversensing
Mechanisms that cause VT and VF
-Abnormal automaticity
-Re-entry
-Triggered activity - early afterdepolarisations and delayed afterdepolarisations
How do shocking vectors work?
-Shocking occurs between a combination of the device and coil
-If the device is part of the shocking system it is called active can
-Device can be removed from system in dual coil lead system
How are shocking vectors reversed in the event of an ineffective shock?
What is the waveform of shock therapy?
-Biphasic waveforms shown to improve defibrillation
1st phase: designed to depolarise/extend refractory period of all ventricular cells
2nd phase: designed to remove residual charge from cells that were not captured
What is ATP (AntiTachycardia Pacing)?
-Pain free way of terminating VT
-A precisely timed extrastimulus delivered during the excitable gap can terminate the tachycardia
-Most effective in VT caused by re-entry mechanism (e.g. monomorphic slower VT)
What are the 2 main types of ATP?
-Burst
-Ramp
Burst vs Ramp ATP
Burst
-Programmable number of stimuli are delivered at fixed cycle length
-Faster than the rate of VT
Ramp
-Programmble number of stimuli are delivered at increasingly shorter intervals within the ramp
Complication of ATP
-ATP can be pro-arrhythmic
-ATP can cause acceleration of VT
-Ramp has been shown to cause acceleration more than Burst
What 3 measurements are done at implant?
-Impedance
-Sensing
-Capture
Equipment in ICD implant
-Device
-ICD lead - single coil vs dual coil
-Pacing lead for atrium of dual chamber device
-Peelaway sheaths - size appropriate
-Testing cables
-Programmer
-External defibrillator
-ECG
What is defibrillation threshold testing?
-Induction of VF under controlled conditions and assessment of defibrillation safety margin
-VF induced through either delivery of high voltage pacing or pacing on the T wave
-Not used as standard testing anymore due to risk
Where are nearfield and farfield signals recorded between?
Nearfield: Between tip electrode and ring electrode
Farfield: Between coil on the lead and the device
What is dynamic sensing?
ICD adjusts the sensitivity to:
-Avoid T wave oversensing
-Allows very high sensitivities later in the cardiac cycle to enable small VF potentials
-When ventricular event is sensed, the peak value of the signal is measured and threshold is determined
-Sensing is decremented after short periods to avoid double counting
What detection criteria is used in ICD before delivering therapy?
-First looks at rate
-To avoid delivering therapies for non-sustained events it uses a counter
What are the 2 types of counter?
Probabilistic:
-x out of y beats
-Appropriate for polymorphic VT and VF
Consecutive
-Certain number of consecutive beats
-Appropriate for monomorphic VT
What is morphology discriminator?
-Morphology discrimination is based on comparison of VEGM of suspected tachyarrhythmia to stored VEGM of normally conducted sinus beat
-Morphology has to match above a programmable percentage in order to avoid therapy
-Device uses farfield EGM for morphology template
What is onset discriminator?
-Takes an average of the RR intervals
-Looks to see how long the new average intervals are in comparison
-VT sudden increase in cycle length vs Sinus Tachycardia gradual increase in cycle length
What is stability discriminator?
-Looks for fluctuation over an average of intervals
-Differentiates between VT and AF
-VT is stable, AF is not
What is A and V discriminator?
V>A: VT
A>V: AF, AT
A=V: Sinus, AVRT, AVNRT
Follow up and remote monitoring for ICD
-6/12 month follow up for ICD
-Assessment of battery, charge time, lead measurements, diagnostics and arrhythmias
-Appropriate or inappropriate therapy
-Mediation review
-Symptom review
-Remote monitoring
Features of subcutaneous ICD
-No venous access required
-Reduced risks associated with transvenous systems - infections, venous occlusion
-Does not have pacing function
-Cannot deliver ATP, only shocks
-Primary prevention for VF
-Delivers 80J shocks
Alternative to SICD
Extravascular ICD
-Can deliver ATP
-Delivers 40J shocks
-Smaller than SICD
-Lacks long term data
Does this patient need a device?
Dual chamber ICD
-Intermittent CHB
What is happening?
-Oversensing of T wave
-Delivered shock because rate and percentage has been met
-Change configuration or back in the lab
What has happened here?
-RV lead has moved into RA
