Implantable Cardioverter-Defibrillators

Question 1

What is the difference between secondary and primary prevention and what are the associated high risk factors?

Answer 1

Primary prevention indication is to avoid a severe cardiac event from happening for the first time. It is a preventative measure in patients with reduced ejection fraction.
Secondary prevention indication is to prevent a life-threatening event from re-occurring, since the probability of a seconds event is very high post aborted SCD, prior VT-VF or unexplained syncope associated with poor LV function

Question 2

Name the main trials supporting primary prevention indications for an ICD

Answer 2

• MADIT 1996: Mortality reduction 54% in patients with LVEF < 35%, prior MI, nsVT, inducible VT at EPS
• MADIT II 2002: mortality reduction 31% in patients with LVEF <30% and prior MI
• MUSTT 1999: mortality reduction 51% in patients with LVEF < 40%, including patients with CAD, asymptomatic nsVT
• SCD HeFT 2005: mortality reduction 23% in patients with LEVF <35% and CHF

Question 3

What is the Class I guidelines (American and European) consensus for ICD indication? (general consensus that ICD is indicated)

Answer 3

1. Cardiac arrest due to VF/VT not due to a transient or refractory cause
2. Spontaneous sustained VT in association with structural heart disease.
3. Syncope of undetermined origin with clinically relevant, hemodynamically significant sustained VT/ VF induced at EPS when drug therapy is ineffective, not tolerates or not preferred.
4. nsVT in patients with CAD, prior MI, LV dysfunction, and inducible VF or sustained VT at EPS that is not suppressed by a class I antiarrhythmic drug (Class I drugs decrease the conduction velocity of the cardiac tissue).
5. Spontaneous sustained VT in patients without structural heart disease that is not amenable to other treatments.

Question 4

What are the Class II guideline consensus (American and European) indications for an ICD? (divergence of opinion or conflicting evidence about usefulness of treatment)

Answer 4

Class IIa (in favor of usefulness): Patients with LVEF <30%, at least 1 month post-MI and 3 months post coronary artery revascularisation
Class IIb (usefulness less well established):
1. Cardiac arrest presumed due to VF when EPS not possible
2. Severely symptomatic sustained VT in patients awaiting cardiac transplant
3. Familial or inherited cardiac conditions with a high-risk of SCD, such as LQTS or HCM.
4. nsVT with CAD, prior-MI, LV dysfunction, and inducible VT/VF on EPS
5. Recurrent syncope of unknown origin in the presence of LV dysfunction and inducible ventricular arrhythmia at EPS (when other causes for syncope excluded)
6. Unexplained syncope with family history of unexplained SCD in association with typical or atypical Brugada syndrome.
7. Syncope in patients with advanced structural heart disease with no obvious cause after investigations

Question 5

What are the Class III indications for an ICD? (evidence or agreement that treatment is not useful or harmful)

Answer 5

1. Syncope of undetermined cause in a patient with no inducible ventricular arrhythmias or structural heart disease
2. Incessant VT or VF
3. VT or VF resulting from arrhythmias amenable to surgery or cardiac ablation
4. Ventricular tachyarrhythmia due to reversible causes
5. Terminal illness with life expectancy < 6 months
6. Psychiatric illness that may be aggravated by ICD implantation
7. Patients with CAD, LV dysfunction prolonged QRS in the absence of spontaneous or inducible sustained VT/VF and are due CABG
8. NYHA IV drug-refractory HF patients who are not candidates for heart transplant

Question 6

What is the pathophysiological principle behind genetic cardiac disorders and the source of arrhythmia?

Answer 6

Gene mutations may cause the loss or gain in function of ion channels influencing the repolarisation process and thus the duration of the myocardial action potential.

Question 7

What is the pathophysiological mechanism behind long QT syndrome?

Answer 7

**Ca channels can be partially reactivated in phase 2 and 3 of the repolarisation and the overall action potential is prolonged. **
Torsades the pointes are often induced by early afterdepolarisation after a compensatory pause following a PVC

Question 8

What happens to the action potential in Brugada syndrome?

Answer 8

Is shortened

Question 9

What happens to the action potential in short QT syndrome?

Answer 9

Action potential is very short and the repolarisation process very fast

Question 10

What is arrhythmogenic right ventricular dysplasia/ cardiomyopathy?

Answer 10

Is a genetic disease characterised by a progressive firbrofatty replacement of the RV myocardium.

Question 11

What is catecholaminergic polymorphic ventricular tachycardia and what is it´s arrhythmogenic mechanism?

Answer 11

Is a genetically determined arrhythmogenic disease. The genetic mutations affect the amount of Ca++ released by the sarcoplasmatic reticulum during adrenergic stimulation. The elevated intracellular Ca++ levels may cause delayed afterdepolarisation.

Question 12

Describe the arrhythmogenic mechanism of hypertrophic cardiomyopathy

Answer 12

HCM is a inherited myocardial disorder with autosomal dominant trait associated with microscopic evidence of myocardial fiber disarray which causes the hypertrophy

Question 13

What is a “voltage delay” and how it affects the charging time of the HV capacitor?

Answer 13

A chemical buildup on the battery cathode takes place if the battery hasn´t been used to charge the capacitor for more than 3 months. This chemical buildup increases the internal resistance of the battery and causes the voltage to suddenly drop. Since the loaded battery voltage with this chemical buildup is much lower than normal voltage, the charging time of the HV capacitor will be a lot longer and delivery of a shock will be delayed. This is why capacitors have to be fully charged periodically (capacitor reforming) to remove the chemical buildup from the vanadium reduction processes.

Question 14

Why ICD batteries don´t use lithium-iodine as pacemakers?

Answer 14

The battery of the pacemaker has to deliver a rather small current to pace.
The battery of an ICD has to deliver very high charge densities/ high current charges.

Question 15

How are ICDs able to deliver high current charges until the end of life?

Answer 15

Due to the properties of the battery. The reduction of silver to its metallic state increases the conductivity of the cathode during the depletion of the battery, hence the decline in voltage is not caused by an increase in resistance and the battery retains its ability to deliver high current charge throughout. ICD´s battery resistance decreases initially at the beginning of life until middle of life, and then increases towards end of life. A resistance in the circuit impairs the transient flow of current/ electricity and prolongs the charging time.

Question 16

What is the construction of the defibrillation coil?

Answer 16

In order to deliver a large current a large surface area of the electrode is required. The defibrillation electrodes/ coils have a folded construction, thus a flattened coil so that both sides of the electrode can deliver current.

Question 17

What is a high voltage capacitor? What´s its use in an ICD?

Answer 17

A capacitor is a reservoir of stored electricity (i.e an electric charge). A capacitor consists of two conducting metallic plates (such as aluminium) and a insulation between them (air or plastic). The capacitor allows to rapidly deliver charges/ shocks, whereas the battery itself cannot deliver a large amount of charge all at once.

Question 18

What are the formulas for Capacitance and Charge?

Answer 18

Charge (Coloumb) = Capacitance x Voltage (Volt)
Capacitance = Charge (Coloumb)/ Voltage (Volt) - is the amount of liquid in the glass (i.e the amount of electricity in the capacitor)

FARAD is the special unit of capacitance.
1F = 1C/1V

A capacitor has a capacitance of 1 Farad (1F) when a voltage of 1 volt (1V) creates charge of 1 coloumb (1C) or 1 ampere x 1 second (1C = 1As).

1microFarad = 1x 10(-6)

Question 19

What is the charging time equation?

Answer 19

Charging time of the capacitor depends upon its capacitance (C) and the total resistance of the circuit (R). t= C x R

For all practical purposes, we may accept that a capacitor is completely charged after a time of 5RC.

e.g. a capacitor of 100 microFarad is charging via a resistor of 20 kiloohms.
The time constant ( = Rx C) is: 100x10(-6) x 20 x 10 (3) = 2 sec
The charging time ( = 5RC): 5x 2= 10 sec

Question 20

What is capacitor maintenance on ICDs? Why is important?

Answer 20

Capacitor maintenance consists of a periodic charging of the capacitor in order to keep the dielectric intact and prevent “voltage delay” of the battery.

Question 21

What is a dielectric in an ICD?

Answer 21

A dielectric is a very thin layer of aluminium oxide. Deforming is a problem of the dielectric. With a low voltage or absence of a voltage across the terminals of the capacitor, the thickness of the aluminium layer is slowly reduced. And so when a higher voltage is applied leakage of the current is greater and the ICD performance decreases, taking longer to charge the capacitor.

Question 22

What are two potential causes for a delayed charging time in an ICD?

Answer 22

1. Due to inadequate maintenance (reforming) of the HV capacitor
2. Due to a lower voltage of the battery (close to ERI)

Question 23

When should you pay attention to the charge time on an ICD?

Answer 23

CHT > 12 sec - needs investigation
CHT >15 sec - needs a generator replacement
When performing a manual capacitor reforming test, if the CHT is too long when the battery is OK there might be malfunctioning of the charger or the capacitor.

Question 24

What are the desired properties of a defibrillation coil?

Answer 24

The electric resistance of the coiled conductors should be as low as possible. Hexafilar coiled conductors have lowest resistance.
The electric current delivered by the HV electrode should be uniform over its entire surface.

Question 25

How does defibrillation work?

Answer 25

A shock defibrillates by resynchronizing the myocardial cells. Recovered cells that are in electrical diastole are recovered, whereas in cells that are already activated the action potential is extended. If the average current of a shock is less than required by the strength-duration curve, some of the myocardial cells are not resynchronized and the fibrillation waves are not terminated.

With the discharging of the HV capacitor the cell membrane voltage increases exponentially until a maximum and then decreases again. At maximum voltage over cell membrane the largest number of cells are captured/ defibrillated.

Question 26

How does a biphasic shock work?

Answer 26

A biphasic shock occurs in two phases. In phase 1 (it is similar to monophasic shock): depolarises some cells and extends the refractory period of already stimulated cells to resynchronize virtually all the ventricular myocites. The function of phase 2 is to remove residual charge on cells that were “not captured” - cells whose refractory period was not extended. This diminishes the amount of borderline stimulated cells which would otherwise remain excitable or “pro-arrhythmic”.
The maximum voltage over cell membrane should ideally be reached at the end of phase 1 and it should return to “0” voltage at the end of phase 2.

Question 27

What influences the defibrillation threshold?

Answer 27

The defibrillation threshold and the optimal pulse durations d1 and d2 to reach maximum possible membrane voltage depend on:
- HV capacitor (type of ICD)
- the shock resistance/ impedance (type and position of the shocking electrodes)
- time constant of the cell membranes of the patient (changing with the patient/ medication)

Answer 28

Energy at a given tilt remains constant regardless of the pulse duration. Studies have shown that tilts between 40-65% are better than 80% tilts. The device delivers a constant energy by varying the pulse duration as a function of the patient´s shock resistance. A smaller resistance between the high voltage electrodes results in a shorter pulse width.
Waveforms >10 ms can re-fibrillate the heart.

Question 29

What is the relationship between shock resistance and delivered shock voltage?

Answer 29

The device delivers an amount of energy that varies as a function of patient´s shock resistance (Rs). A smaller resistance between the high voltage electrodes results in a larger tilt - more energy delivered.

Question 30

Why would a lower device capacitance improve the device´s ability to have a lower DFT?

Answer 30

A lower capacitance device will have lower phase durations, shorter pulses. When the shock waveform is of “constant duration” is the voltage that generates the force to change the status of the cardiac cells.
When the shock waveform is of constant tilt (i.e constant energy/ voltage), the device varies the pulse duration as a function of the patient´s shock resistance Rs. A smaller resistance between the shock electrodes (higher voltage/ energy) results in shorter pulse duration required.

Question 31

How does shock waveform pulse duration in phase 1 and phase 2 affect the defibrillation threshold?

Answer 31

Adjusting the pulse duration both in phase 1 and phase 2 (of a biphasic pulse) allows a fine tuning which results in best defibrillation efficiency, i.e the lowest defibrillation threshold.

Question 32

What is the fine tuning shock waveform programming to maximize device battery?

Answer 32

Programming optimal short pulses with a small tilt and a high average voltage, extends the lifetime of the battery.

Answer 33

The delivered shock energy is always smaller than the stored energy in the capacitor.

Question 34

Describe the SVT discriminators in Boston Scientific

Answer 34

Onset: differentiates between sinus tachycardia and VT by evaluating how sudden V-V interval changes
Stability evaluates R-R interval stability to differentiate between fast conducted AF and monomorphic VT
Sustained Rate Duration: at the end of the programmed SRD (= maximum therapy inhibition) therapy is delivered. Mainly used in patients with exercise induced VT.
Shock if unstable: polymorphic VT discriminator, if a VT rate is unstable the device will skip ATP and go straight to shock (ATPs are not efficient against polymorphic VT)
V rate > A rate: at the end of the programmed duration the A rate is compared to the V rate. V>A takes precedence over all other SVT discriminators
A fib rate threshold

Question 35

Energy in a capacitor

Answer 35

The energy stored in a capacitor is directly proportional to device´s capacitance and to the square of its voltage

E = ½ × C × V²

E (Energy in Joules)
C (Capacitance in Farads)
V (Voltage in Volts)

Question 36

A capacitor of 120 µF is initially charged to 200 V. It delivers a pulse which is truncated at a tilt of 65%.
What is the energy delivered to the patient?