Why induce ?

Generalities

After the implantation of first generation defibrillators, the procedure ended nearly systematically by the induction of VF to verify the integrity of the high voltage system and to confirm the reliable sensing of the endocardial signals during VF and termination of the arrhythmia by the device with an adequate defibrillation safety margin.

Despite some risks involved, this test used to be performed systematically with older devices, which were implanted for secondary prevention indications and in patients at high risk of recurrent sudden cardiac death. Shocks were the only therapies available and defibrillation failures were relatively frequent requiring additional intervention to ensure clinical efficacy.

With contemporary devices (i.e biphasic shocks), defibrillation failures are rare, most patients undergo device implantation for primary prevention indications, and anti-tachycardia pacing terminates a large proportion of rapid ventricular tachycardias. Furthermore, newer ICDs deliver shocks of considerably higher maximum energy, guaranteeing a wider safety margin.

Therefore, if defibrillation testing has been an integral part of defibrillator implantation and has been performed in most of the studies that have validated the positive impact of this therapy on sudden death, there is little evidence that testing improves outcome. The recent results of a large randomized study, SIMPLE, showed the absence of difference in outcome between 2 groups: one with and one without systematic defibrillation testing. This has prompted many implanting centers to reconsider the need for a systematic test at the end of the procedure and, instead, to proceed on a case-by-case basis weighting the risks against the benefits. The question of which patients should have defibrillation testing is still under debate. Factors such as position of the generator (right sided implants have higher defibrillation threshold’s than left sided implants), sensing parameters (R wave <5mV increases probability of undersensing VF), the presence of atrial fibrillation (risk of thromboembolism particularly if not anticoagulated) should be taken into consideration as well as whether it is a new implantation or an ICD generator change.

Interests of a VF induction

Verification of an accurate sensing of ventricular endocardial signals during VF

An indispensable prerequisite for the delivery of appropriate therapy is an accurate detection of the arrhythmia (i.e. all fibrillation waves are detected by the device). One of the main objectives of the test is to confirm the proper detection of induced VF. An accurate detection of VF implies the ability to sense very rapid signals that are often variable in amplitude and rate. The requirements for an appropriate detection of VF are, therefore different from those needed during sinus rhythm or even during organized, monomorphic VT. 

For the test, it is customary to lower the ventricular sensitivity (i.e. increasing the amplitude required to trigger a sensed ventricular event) typically to value of 1 to 1.2 mV. A reliable sensing at such sensitivity value guarantees a comfortable safety margin with respect to the nominal value (0.3 to 0.8 mV) used thereafter.

The correlation between R waves amplitude during sinus rhythm and amplitude of ventricular EGM during VF is weak but the sensing of a >5 mV R wave during sinus rhythm is most often associated with reliable sensing during episodes of VF. Undersensing of VF despite proper sensing during sinus rhythm is rare with state-of-the-art defibrillators. The performance of an induction test to confirm the quality of sensing is indispensible probably only in case of suboptimal sensing (<3-5 mV) during sinus rhythm.

Verification of defibrillation efficacy

The other major objective of an induction test at the time of device implantation is the verification of the defibrillation success. To allow a return to sinus rhythm, the shock must alter the ventricular myocardial transmembrane potential and extinguish the existing fibrillation waves without inducing new ones. Defibrillation threshold is defined as the minimum energy that allows effective defibrillation. The term “defibrillation threshold” is incorrect since, unlike the pacing threshold, it is not an absolute value, above which defibrillation is certainly effective, and below which it is predictably ineffective. Instead, at a given pulse strength, the success of defibrillation is more or less probable. The efficacy of a defibrillation shock is best represented by an exponential curve of probability, the shape of which is relatively uniform among patients.

The objective of testing during implantation is to ensure that the maximum energy shock the defibrillator is able to deliver has a very high probability of successfully defibrillating a VF in a given patient.

Factors that influence the defibrillation efficacy

Several patient-related factors, the implanted instrumentation, the ambient drug therapy or the occurrence of complications may have direct effects on the DFT.

            The following patient characteristics have been associated with elevated DFT:

• Male gender
   
• Large body surface
   
• Low left ventricular ejection fraction
   
• Left ventricular dilatation or hypertrophy
   
• Presence of hypertrophic cardiomyopathy or Brugada syndrome  
   

            The following shock characteristics influence the DFT:

• The shock waveform and duration of the phases of a biphasic shock: the defibrillation threshold associated with biphasic waveforms is lower than with monophasic waveforms, and the risk of immediate arrhythmia re-induction is lower. In the case of a fixed tilt, the shock is interrupted when the residual voltage of the capacitors has reached a predetermined percentage. The duration depends on the impedance while the delivered energy is fixed. In the case of a biphasic shock, the first phase plays the role of a monophasic shock, while the second phase brings the membrane potential as near to zero as possible in order to prevent the re-induction of VF. The optimal duration of the second phase depends on the duration of the first, on the defibrillation impedance and on the membrane time constant. When the defibrillation impedance is high and the tilt fixed, the length of phase 2 may be excessive. Optimization of the duration of both phases is or is not programmable according to the manufacturer.
   
• The shock polarity: the nominal polarity is different according to the manufacturers but it seems that the thresholds are lower with a right ventricular electrode used as an anode for the first phase of a biphasic shock, particularly when the DFT is high (the anodal threshold is the same or lower than a cathodal threshold in 76% of patients whose threshold is high).
   
• The orientation of the shock vector: the shock vector must cover the left ventricle uniformly; this vector depends on the position of the defibrillation coil(s) and of the pulse generator with respect to the heart. The distal right ventricular coil must be entirely contained inside the ventricular cavity, while the proximal coil of a dual-coil electrode must be placed high enough to prevent the dissipation of current at the level of the right atrial cavity. Placing the proximal coil inside the superior vena cava may be challenging. If it is floating inside the right atrium, it might be preferable to use the lead as a single coil.
  The greater defibrillation efficacy contributed by a dual coil lead is currently debated. Since current ICDs deliver maximum shock strengths that are higher than previous generations of devices, this additional gain of energy may have a greater impact on the shock efficacy than the use of a dual coil lead. Moreover, when a dual coil lead needs to be extracted, the presence of a proximal coil promotes the adhesion of the lead to the wall of the superior vena cava, complicating and increasing the procedural risks.
   

            The following medications influence the DFT:

• Amiodarone significantly increases DFT, particularly when it is already high
   
• Class IC antiarrhythmic drugs and verapamil also increase DFT
   
• Sotalol, dofetilide and azimilide lower DFT and can be used preferentially in patients in whom it is high
   

            Procedural complications that influence the DFT:

• The presence of a pericardial or pleural effusion, or of a pneumothorax can increase DFT;
   
• By creating a parallel circuit, the persistence of an old lead’s fragment may theroretically shunt a part of the delivered energy.

Risks and contraindications to the performance of the defibrillation test

Defibrillation testing is not without risk, which may occur as a result of the shock itself or because of the induction of ventricular fibrillation:

• The loss of cardiac output during VF may result in cerebral ischemia, particularly after several ineffective shocks and prolonged periods of ventricular fibrillation
   
• In presence of coronary artery disease, VF can cause myocardial ischemia with electrocardiographic changes and increases in blood troponin concentrations. The shock itself can increase the blood concentration of troponin, though the rise seems greater in patients presenting with coronary disease
   
• The mortality due to VF refractory to internal and external shocks is <0.1%, though it is not zero
   
• Thromboembolism is a risk, particularly in patients who have atrial fibrillation or left ventricular mural thrombus. In patients who have atrial fibrillation and in whom anticoagulation has been stopped or reduced, it is recommended that a trans- esophageal ultrasound be performed prior to implantation to ensure there is no evidence of thrombus in the left atrial appendage. In patients with heart failure, a transthoracic echo pre-procedure often with contrast can be performed to ensure that there is no evidence of left ventricular thrombus.

In whom is it recommended to perform defibrillation testing?

The current generations of defibrillators, which are capable of delivering a high output shock, have a high success rate of successful defibrillation (~95%). The need to change the position of the lead or to change the defibrillator system is less than 5%. Given this acute success rate and the risks associated with induction of VF, many centers have changed their practice to only routinely inducing VF in patients who have a higher probability of failure of defibrillation and those in whom detection may be a problem. This includes the following situations:

• If it is not possible to obtain a sensed R wave greater than 5 mV (as below this value the likelihood of undersensing is increased).
   
• Patients in whom the generator is placed in a position other than the typical left sided position, (for example right sided or abdominal sites) or in whom there is an atypical lead position.
   
• Patients with a high likelihood of an elevated threshold, because of the presents of several risk factors, including young age, obesity, coronary artery disease, heart failure, Brugada syndrome, amiodarone therapy ...

On the opposite, in some situations the risks of VF induction are likely to outweigh the benefits:

• The presence of an intracardiac thrombus, severe aortic stenosis, severe coronary artery disease which has not been revascularised or unstable heart failure.
   
• In patients with atrial fibrillation adequate anticoagulation or trans- oesophageal ultrasound to exclude left atrial thrombus is advised.

In all other patients, the choice to test or not to test the implanted device depends on the practice of individual centers and should take into consideration the indication for implantation (secondary prevention pushing in favor of testing against primary prevention where many centers will not induce routinely), the age and clinical characteristics of the patient. At the time of generator change the decision of whether to perform VF induction should take into account whether there are factors that are likely to make successful induction less likely, i.e. change in medication, heart failure status etc. The success of defibrillation might, therefore, be scrutinized if the test was not performed during the initial implant, in case of heart failure progression or if a therapy was introduced that is known to increase the threshold.