ICD Basics

Your attention is drawn to the following issues.

Components of the ICD

ICDs are superficially similar to conventional pacemakers. The simplest system consists of a single lead to the RV, and a hermetically sealed titanium can containing the computer, battery, and circuitry. The battery is generally larger than that for a conventional unit. The defibrillation shock is generated by a system of capacitors which store charge until it is sufficient to deliver.

The lead or leads are implanted in the same fashion as those for conventional pacing. The RV lead is required to deliver the high-energy shock, but can also be used for pacing and sensing. An ICD lead can be readily recognised on the plain chest x-ray by the appearance of a coil wrapped around the inner core.

Functions of the ICD

Current advanced ICDs can offer treatment in three differing ways:
1. Conventional anti-bradycardia pacing (already discussed)
2. Anti-tachycardia (ATP) pacing (Fig 1, below)
3. Defibrillation shocks

ICDs are extremely sensitive and effective medical devices, but come with a significant cost burden, and issues around implantation. For those reasons, there are some very clear directions to clarify who should be considered for an implant. Further information on this topic can be found within the 2013 ESC guidelines.

Because VT is potentially amenable to termination by pacing, unlike VF, the device will often be programmed to try several repeated bursts of ATP for tachycardia recognised as VT. It is estimated that greater than 95% of VT is successfully pace-terminated in this fashion. It will only deliver shock therapy if the tachycardia persists (i.e. ATP has been unsuccessful), or if VF is recognised.

If defibrillation is unsuccessful, further shocks will be delivered. Many devices now have a limit to the shocks delivered for any one tachycardia episode. Most shocks are delivered at an energy level in the range of 26-36J. Shocks at this level cannot be mistaken by the patient, as it is a violent event. In general terms, if a conscious patient is unsure whether a shock has been delivered, it probably has not.

If a patient has lost consciousness, only formal device interrogation will reveal what therapy has occurred.

Fig 1 ATP pacing. Anti-Tachycardia Pacing (ATP) electrogram. Click the image to see a larger version. Image reproduced with permission from Boston Scientific