Developments in Implantable Cardioverter Defibrillator Therapy


if the arrhythmia is to be terminated, although it may be delayed until consciousness is impaired by the arrhythmia or until sedation can be given.


The development of anti-tachycardia pacing (ATP) allowed the better treatment of harmful arrhythmias not usually associated with cardiac arrest and sometimes without immediate compromise, since ATP is not so disturbing to the patient. In addition, ATP may be an initial treatment strategy for compromising ventricular arrhythmias, with shocks used if ATP fails to terminate the arrhythmia. Unnecessary shocks are those that are delivered for an appropriately detected ventricular arrhythmia, but where the arrhythmia could have terminated spontaneously or by ATP. Improvements in programming may reduce the chance of unnecessary shocks, for example, by the use of ATP for rapid VT and of longer detection times.


Inappropriate shocks arise from inaccurate detection of a ventricular arrhythmia, most commonly because of an atrial arrhythmia conducted at a high ventricular rate. Other causes include sensing of ‘noise’ caused by lead fracture. Improved detection algorithms and discriminators have reduced the incidence of the former and may help to provide early warning of the latter before an inappropriate shock. Inappropriate shocks are common; for example, in SCD-HeFT, of the 32.7 % of trial participants who received shocks, half received inappropriate shocks.4


Shock therapy is associated with higher mortality; while the exact mechanisms are unclear, shocks with energy greater than 9 J have been shown to reduce cardiac index, whereas shocks with lower energy have not had this effect.7


In the SCD-HeFT, patients who had


appropriate shocks had a mortality over double the rate of those who had no shocks; an even higher mortality was noted for the group of patients who had multiple shocks, and for patients with both appropriate and inappropriate shocks.8


strong association between shock ‘dose’ and mortality.9


A recent study also found a A further


meta-analysis of data on inappropriate shocks also confirmed increased mortality with increasing numbers of shocks.10 ATP has not been shown to increase mortality.11


However, Thus, strategies to


reduce shocks may have mortality benefits as well as the clear benefits in terms of reducing patient distress.12


A number of strategies may be used to reduce the number of shocks. For example, the PainFree Rx II study demonstrated that ATP is successful at terminating fast VT (188–250 beats per minute) in 72 % of cases, avoiding unnecessary shocks.13


Some ventricular arrhythmias may be self-terminating and therefore deferring therapy may result in no treatment being needed. Clearly, a balance has to be struck between waiting long enough for any non-sustained arrhythmia to terminate, but not so long that the patient becomes dangerously compromised. In the RELEVANT study,14 ventricular fibrillation detection was 30/40 intervals compared with 12/16 in the control group. The prolonged detection time resulted in reduced therapies (both appropriate and inappropriate), as well as in fewer heart failure hospitalisations, with no increase in episodes of syncope or death. Therefore, delaying detection modestly does not increase adverse events, but does reduce patient shocks.


An interesting new development is the ‘virtual ICD’ – a computer simulation of ICD function based on historical data that allows the prediction of the effects of new detection and treatment algorithms.15


EUROPEAN CARDIOLOGY


Other Advances Remote Monitoring


Traditionally, interrogation and follow-up of pacemakers and ICDs require the patient to attend a clinic, usually held in a hospital outpatient unit. More recently, most of the device manufacturers have developed systems that allow trans-telephonic device interrogation in the patient’s home, with data sent to the ICD centre and/or physician via a variety of means. This allows some follow-up appointments to be remote, which makes routine and unscheduled follow-up more efficient and can be used to alert the medical team urgently of potential problems, such as indicators of lead fracture and the possibility of inappropriate shock therapy. Currently, however, programming changes cannot be made remotely and require clinic attendance.


Many current devices are capable of providing much more diagnostic information to the physician than simple cardiac rhythm monitoring. For example, some devices are able to predict impending deterioration in heart failure symptoms using data derived from thoracic impedance and patient activity to provide an alert of possible clinical deterioration. The ability of the device to send this data automatically when a possible problem is identified has the potential to prevent hospitalisation by facilitating timely assessment.


Lead Connections


Up until very recently, ICD leads bi- or trifurcated at the header connection end, with connections for the pace/sense component (IS-1 connector) and each of the one or two shock coils (DF-1 connections). More recently, a new international standard (DF4) has been devised, with a single connector for the ICD lead incorporating all the components in one plug. This


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This has many potential advantages, allowing the testing of individual algorithms without the need to recruit large numbers of patients into clinical trials, with results becoming available rapidly.


Vascular Access Issues


Currently, almost all ICDs are implanted using transvenous leads. Vascular access is rarely an issue in new implants except in cases of congenital cardiac or vascular anomalies, or where previous vascular interventions have caused complications, but the longevity of current-generation ICD generators and leads is such that younger patients may undergo several lead implants over the course of their lives. With each procedure the risk of vascular damage and infection increases and chronic lead extraction may be required.


To overcome the problems associated with vascular access, a completely extravascular ICD system has been developed.16


While


and have been shown to be effective at terminating detected arrhythmias, both at defibrillation threshold testing16 in clinical practise.18


and A potential concern with subcutaneous ICDs is


that the infection rate related to this large device appears to be higher than that with conventional ICDs, at around 5 %,18


although it


is likely that the extravascular nature of the device makes device infection less likely to lead to severe sepsis or endocarditis.


this is currently unable to pace the heart (other than for transient, post-shock back-up pacing), it may represent a good option for patients who do not require pacing for bradycardia and whose ventricular arrhythmias are unlikely to respond to ATP. The systems may be as reliable as transvenous ICDs in detecting ventricular arrhythmias,17


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