Implant procedure & Defibrillation threshold testing

Device: ICD Field: Implantation

1. Basic concepts

Implantation Team

Different laboratories have different requirements and protocols; in general we recommend the team includes the following members:

  • A doctor trained in implantation of Implantable Cardioverter-Defibrillators (ICD)
  • A cardiac physiologist or technician who is experienced in programming and testing of ICD’s
  • An anesthetist if all or part of the implantation is to be performed under general anesthesia
  • A nurse familiar with the implant suite and laboratory

The team must be competent in the management of medical emergencies including cardio-pulmonary resuscitation.

Facilities in the operating room and patient preparation

The operating room should have the following:

  • X-ray equipment of good quality
  • An external defibrillator and life support equipment
  • Equipment to allow continuous monitoring of heart rate, respiratory rate, oxygen saturation and blood pressure throughout the procedure
  • Facilities for delivering general anesthesia

We prepare the patients in the following way:

  • They are consented prior to the procedure, having previously been counseled and given access to the relevant information regarding the procedure as well as the long-term implications of having an ICD.
  • We routinely give antibiotics immediately prior to the procedure.
  • They have continuous monitoring of heart rate, oxygen saturation and blood pressure.
  • They are connected to an external defibrillator with adhesive pads placed on the thorax.
  • Standard surgical aseptic techniques are followed throughout the implantation procedure.
  • The procedure can be performed under general or local anesthesia. Many centers routinely perform implantation under local anesthesia, often in conjunction with short acting medication such as fentanyl, propofol, or midazolam.
  • Implantation on the left side is typically preferred, because the generator casing is usually used as an active and integral part of the vector of defibrillation. The left sided position provides a vector which maximizes the flow of current through myocardial tissue, thereby maximizing the chances of successful defibrillation.

implantation procedure

Many of the techniques for implanting conventional pacemakers also apply to ICD implantation. When
implanting ICD’s, we pay particular attention to the following:

  • We aim to use the cephalic or axillary vein, in order to minimize the risk of subclavian crush, the ICD lead being larger and more fragile than standard pacing leads.
  • We aim to ensure that the entire distal coil of the defibrillation lead is positioned within the right ventricular cavity, in order to maximize the chances of successful defibrillation.
  • We aim to ensure that the final RV lead position results in good sensing and pacing parameters (R wave> 5 mV, pacing threshold <1V/0.5 ms) and aim to minimize the risk of oversensing by ensuring that large T and P waves are not present. Note that signal processing differs between the programmer and the ICD in that the ICD applies more filters and therefore the R wave is often smaller when measured through the ICD.
  • To ensure that the lead is in a stable position and securely fixed, we routinely use active leads.
  • The ICD generator is larger than a pacemaker and therefore special consideration should be given to the positioning of the pocket for the generator. A subcutaneous position is technically simpler to perform and does not require deep dissection. However, it is essential to position the generator adjacent to the pectoral muscle in order to minimize the risk of erosion. The size of the pocket should be appropriate to the device- not too large so that the device migrates and not too small so that the device does not sit uncomfortably. Patient preference should also be taken into consideration; sub-pectoral or even sub-mammary implants usually give a better cosmetic result and may be preferred in patients who are thin, young or women. The pocket should be created anteriorly to avoid interfering with movement of the arm.
  • As with standard pacing meticulous care should be taken when connecting the leads to the header.

defibrillation threshold testing

Traditionally, implantation of an ICD has involved induction of ventricular fibrillation to ensure that it can successfully detect and treat ventricular fibrillation (VF). Historically devices were first implanted for secondary prevention, i.e. a population of patients in whom further episodes of VF are relatively common. The older generation of devices had a higher failure rate for defibrillation and therefore defibrillation threshold was routine in most patients. However, technological advances mean that failure of defibrillation is now much rarer, particularly now that most devices are able to deliver higher energy shocks, which allows better safety margins to be achieved. Therefore many centers do not routinely perform defibrillation threshold in all patients, as the risks need to be weighed against the benefits. The choice of whether defibrillation threshold testing is performed should therefore be made on a case by case basis. 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.

Induction of ventricular fibrillation
VF can be induced by:

  • Rapid pacing: Burst stimulus 1 to 5 seconds continuously at 50 Hz
  • Shock during the vulnerable period of the T wave : right ventricular pacing at a fixed frequency (150 bpm) of 8 beats, then a 1 Joule shock timed to coincide with the peak of the T wave (coupling of about 300 ms)

Evaluation of detection
The first objective of the test is to ensure that sensing is appropriate (i.e. all fibrillation waves are detected by the device) and that as a result the device appropriately detects VF.

  • During the test, the device is typically programmed to low sensitivity (i.e. increasing the amplitude required to trigger a sensed ventricular event typically a value of 1 to 1.2 mV is programmed). If VF is appropriately detected at this value, this implies that there is a good safety margin when the device is programmed to the nominal sensitivity value of 0.3 mV.
  • Undersensing is less likely to occur when the sensed R wave during sinus rhythm is > 5 mV. However, this correlation is not perfect and undersensing of VF can occur even if there is good R wave amplitude during sinus rhythm.


Verification of the effectiveness of defibrillation
The next objective is to test the efficacy of defibrillation. To allow a return to sinus rhythm, the shock must alter the ventricular myocardial transmembrane potential and remove the existing fibrillation waves without inducing new ones.

  • Defibrillation threshold corresponds to the minimum energy that allows effective defibrillation
  • Unlike a stimulation threshold, the defibrillation threshold is not an absolute value above which defibrillation will always be effective and below which it will always be ineffective, rather it provides a probability of success for a given energy.
  • The relationship between probability of success and energy delivered is observed as a sigmoidal dose-response curve. The appearance of this curve is relatively uniform between patients.
  • The objective of testing during implementation 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.

Defibrillation testing in clinical practice
In clinical practice a number of different protocols can be used. Many of these do not aim to determine the actual defibrillation threshold; rather they set out to determine that a device is likely to provide an adequate safety margin. These include the following protocols:

  • Step-down to failure testing, the first shock energy is programmed at 10J less than the maximum output of the device. Progressively lower energies are then used at successive inductions (for example steps of 5J lower energies for each successive test) until there is failure of defibrillation. The defibrillation threshold is taken as the lowest successful energy.
  • A second protocol which is commonly used is to simply test the same energy twice. The first shock is programmed to deliver an amplitude which is less than 10 Joules of the maximum capacity of the device. To verify the effectiveness of the shock, the same amplitude is then tested a second time.
  • At least 3 -5 minutes are allowed between tests to allow hemodynamic recovery and to minimize the cumulative effect of shocks. If the shock delivered by the implantable defibrillator is ineffective, a rescue shock can be delivered either by an external defibrillator or through the implanted defibrillator, which is typically programmed to maximum energy. The internal shock does not carry the risk of thermal injury to the skin, but may be associated with slight delay in delivery due to the time taken for re-detection of VF and for the device to charge.

Upper limit of vulnerability testing
An alternative way of assessing safety margin, is to determine the upper limit of vulnerability. This is the lowest energy above which a shock delivered during the ventricular vulnerable period, does not lead to induction of VF.

  • The defibrillation threshold and the upper limit of vulnerability have been shown to be correlated
  • During sinus rhythm shocks of decreasing value are delivered at the peak of the T-wave
  • The energy which is the last not to induce VF is taken as the upper limit of vulnerability
  • The upper limit of vulnerability has been shown to correspond to an energy level which has 90% success of delivering successful defibrillation. However, the upper limit of vulnerability is an indirect measure and does not provide information with regard to the ability of the device to detect VF.

The defibrillation threshold can be influenced by many factors:
Patient characteristics associated with high defibrillation threshold:

  • Male gender
  • Increased body surface area
  • Impaired ejection fraction
  • Left ventricular dilatation or hypertrophy
  • Hypertrophic cardiomyopathy or Brugada syndrome

Drugs affecting defibrillation threshold:

  • Amiodarone may significantly increases the defibrillation threshold especially if the threshold is already high
  • Anti-arrhythmic drugs in class IC and verapamil may also increase thresholds
  • Sotalol, dofetilide or azimilide reduce thresholds and can be used preferentially in patients with high threshold

Technical factors affecting the defibrillation threshold:

  • Defibrillation thresholds are typically lowest when the right ventricular anode is used for the first phase of a biphasic shock
  • Successful defibrillation requires that sufficient myocardial mass (predominantly left ventricle) is depolarized; this is determined by the shock vector. This vector depends on the position of the defibrillation coils and the housing relative to the heart.
  • The distal right ventricular coil should be located entirely within the ventricular cavity
  • The proximal coil of a dual-coil probe should be positioned high enough to avoid a loss of power due to right atrial blood flow. Sometimes it is difficult to position the coil in the superior vena cava. If the coil is floating in the right atrium, it is generally better to program the device as a single coil device
  • The gain in terms of threshold provided by a dual coil probe is discussed and appears to be lower since the possibility of optimizing the tilt and the respective durations of each phase of the shock from a single-coil probe

Intraoperative complications affecting defibrillation thresholds:

  • The presence of a pericardial effusion or pneumothorax can result in a high defibrillation threshold
  • The presence of an old lead or lead fragment can result in a higher defibrillation threshold by diverting some of the energy delivered by creating a parallel circuit

Management of patients with high defibrillation thresholds:
In the rare patient who is found to have a high defibrillation threshold, we take the following approach:

  • Check the lead position to ensure that it has not moved
  • Check the lead connections to the device
  • Look for reversible causes of a high threshold, such as reversible ischemia, metabolic abnormalities, pericardial effusion or pneumothorax and treat them
  • Change the polarity or vector of the shock to see if this results in a lower defibrillation threshold
  • Try an alternative lead position, by moving the lead to an apical position if it was originally implanted on the septum or vice versa. Care should be taken to ensure that the entire distal coil is positioned within the right ventricle
  • If not already selected, consider switching to a high-energy defibrillator
  • If the threshold remains high despite the above measures then a subcutaneous or coronary sinus electrode can be implanted

Contra-indications and risks of defibrillation testing:

  • 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 patients with ischemic heart disease, VF can result in myocardial ischemia with electrocardiographic changes and increases in troponin. The shock itself can be responsible for a troponin rise but this increase is greater in patients with ischemic heart disease.
  • Mortality due to induction of VF which is refractory to internal and external shocks is less than 0.1%.
  • 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 echocardiogram is performed prior to implantation to ensure there is no evidence of thrombus in the left atrial appendage. In patients with heart failure, we routinely perform a transthoracic echo pre-procedure often with contrast to ensure that there is no evidence of left ventricular thrombus.

Is it necessary 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.

Situations where the risks of VF induction are likely to outweigh the benefits include:

  • The presence of an intracardiac thrombus, severe aortic stenosis, severe coronary artery disease which has not been revascularized or unstable heart failure. In patients with atrial fibrillation, adequate anticoagulation or trans- esophageal echocardiography 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. Another role for induction is to ensure that the leads have been correctly connected to the generator.

post implantation

Post implant management should include:

  • Chest X-ray
  •  12-lead ECG
  • Continuation or re-initiation of anticoagulation.
  • Device interrogation to check lead parameters, and to optimize programming.


3. Take home messages

  1. At the end of the implantation procedure, VF is induced by:
  • 50Hz rapid pacing
  • Shock in the vulnerable period of the T wave
  1. The three objectives of VF induction at the end of the implantation procedure are:
  • To check the electrical integrity of the hardware and connections
  • To check the quality of ventricular detection
  • To verify the effectiveness of defibrillation
  1. During VF induction, the standard programming includes:
  • One VF zone
  • Ventricular sensitivity approximately 1 mV
  • Programming discrimination algorithms 'off'
  • First defibrillation shock with a safety margin of at least 10 Joules below the maximal deliverable energy
  • Second defibrillation shock of maximal amplitude
  1. The defibrillation threshold:
  • Is not an absolute value above which defibrillation will always be effective and below which defibrillation will always be ineffective
  • The relationship between probability of success and energy delivered can be modeled by an exponential curve. There is a strong correlation between defibrillation threshold and upper limit of vulnerability.
  • The decision to perform a test during implantation depends on preferences of the center, indication for implantation, patient characteristics and quality of ventricular detection in sinus rhythm.
  1. The induction of VF during implantation is contraindicated in patients with:
  • Intracardiac thrombi
  • Suboptimally anticoagulated or non- anticoagulated patients with AF
  • Severe aortic stenosis
  • Unstable coronary artery disease or decompensated heart failure