Hoiter, Loop Recorder, and Event Counter Capabilities of Implanted Devices JOHAN E.P. WAKTARE and MAREK MALIK From the Department of Cardiological Sciences, St. George's Medical School, London. United Kingdom WAKTARE, J.E.P., ET AL.: Hoiter, Loop Recorder, and Event Counter Capabilities of Implanted Devices. The current generation of cardiac pacemakers and implantable cardioverter defihrillators almost all have some capabilites to store data regarding device activity and patient events for future retrieval. This infor- mation may provide valuable information regarding device function and whether this is proving valuable in patient management. Examples include "pace-sense" counters, which can reveal under sensing of pa- tient events, and serial lead impedance measurements, which are able to demonstrate trends not seen on isolated measurements. Hoiter capabilities become vital in more advanced devices for documenting the utility of, and fine tuning the programming of, features such as antitachycardia pacing, rate-responsive- ness, and mode-switching. Finally, the ability to store patient events as marker channels and even intrac- ardiac electrograms adds a diagnostic capability not available through external monitoring. This role has now been advanced by the development of a purely diagnostic implantable loop recorder. (PACE 1997; 20[Pt. 111:2658-2669) cardiac pacemaker, implantable cardioverter defibrillator, implantable loop recorder, event counters, electrogram storage Introduction The abilities of implantable devices to deliver arrhythmia therapy has evolved rapidly since the first pacemakers in the 1960s. Bradycardia pacing has evolved from single chamber fixed rate de- vices to units offering dual chamber sequential stimulation with variable atrioventricular (AV) delay, a plethora of different rate response sen- sors, mode switching in response to atrial tach- yarrhythmias, and diurnal rate variation. Over a mnch shorter time period, implantablo car- dioverter defihrillators (ICDs) have undergone a similar evolution: from shock only therapy de- tected hy simple rate based detection to advanced arrhythmia recognition and tiered therapy for ven- tricular tachycardia with ramp and burst pacing. The benefits of these advanced features, and whether they will improve the patients symptoms during normal daily activity rather than in the Address for reprints: Johan Waktare. M.D., Cardiological Sci- ences, St George's Hospital Medical School, Cramner Terrace, London, SW17 ORE, UK. Fax: +44-181-767-7141: E-mail: jwak- tareCfrsghms.ac.iik Received May 14, 1997: revised June 25, 1997; accepted June 25, 1997. electrophysiology laboratory, can not be accu- rately predicted. For this reason, a capability to store information about cardiac events by im- planted devices has evolved in tandem with other technological advances. Several different types of data may he stored. The most simple are counters of the proportion of time during which the patient was paced or num- ber of times when mode switching occurred. At a slightly higher level of sophistication, ICDs can store information regarding the detected events before and after therapy delivery. More recently, ICDs and advanced pacemakers have allowed the storage of electrograms that approach surface ECG recordings in quality and hence diagnostic value. They have become essentially implantable event recorders in addition to their therapeutic capabil- ities. In addition, similar quality electrograms are obtainable from a newly developed purely diag- nostic device that is currently patient-activated but requires only subcutaneous implantation to determine the cause of undiagnosed syncope. This article explores the current and future capabilities of this technology, but particularly aims to examine the practical value to the patient. Traditionally, recording technology has been uti- 2658 October 1997, Part II PACE, Vol. 20