S70 Use of the Implantable Loop Recorder in Evaluation of Patients with Unexplained Syncope ANDREW D. KRAHN, M.D., GEORGE J. KLEIN, M.D., ALLAN C. SKANES, M.D., and RAYMOND YEE, M.D. From the Division of Cardiology, University of Western Ontario, London, Ontario, Canada Implantable Loop Recorder. Syncope is a complex symptom with multiple potential etiologies that can be difficult to establish. The major obstacles to diagnosis are the periodic and unpredictable nature of events and the high spontaneous remission rate. Short-term ECG monitoring often is unproductive when initial noninvasive testing is negative due to the low probability of recurrence during the brief monitoring period. Implantable loop recorders extend the ability to monitor cardiac patients, enhancing the diagnostic yield to as high as 85% in difficult to diagnose syncope. Several recent studies suggest that prolonged monitoring with an implantable loop recorder has a role in patients with syncope and conduction disturbances, negative tilt testing, and unexplained seizures, and may be superior to conventional testing with tilt and electrophysiologic studies. (J Cardiovasc Electrophysiol, Vol. 14, pp. S70-S73, September 2003, Suppl.) syncope, monitoring, diagnosis, loop recorder Introduction Syncope affects 30% of adults at some point during their life; it is responsible for 1% to 6% of emergency room visits and 0.6% to 1.0% of hospital admissions. 1-3 There is a peak in age of presentation during adolescence and early adult- hood that is predominantly neurocardiogenic syncope, and a second peak in the sixth and seventh decades reflecting more arrhythmias from organic or degenerative heart disease, as well as orthostatic causes. Syncope is a complex symptom with multiple etiologies that often are difficult to discern. The major obstacles to diagnosis are the unpredictable and infrequent nature of events and the high spontaneous remis- sion rate. 4,5 Clinicians rely on clinical assessment and abnor- mal laboratory results to make a diagnosis by inference in many cases. Monitoring of key physiologic parameters dur- ing spontaneous syncope may provide a seldom-attained gold standard for diagnosis of cardiovascular causes of syncope. Advances in long-term cardiac monitoring techniques have added a powerful tool for arrhythmia detection. Implantable Loop Recorders The implantable loop recorder (ILR) is ideally suited to patients with infrequent recurrent syncope because it per- mits prolonged monitoring without external electrodes. The only commercially available ILR at this time is the Reveal © (Medtronic Inc., Minneapolis, MN, USA). The current itera- tion of the ILR has a pair of sensing electrodes 3.7 cm apart on the shell, measures 6.1 × 1.9 × 0.8 cm, weighs 17 g, and has a battery life of 14 months. It is easily inserted subcuta- neously in the left chest using standard sterile technique and local anesthetic. It has been implanted in the right parasternal, Supported by Grant NA3397 from the Heart and Stroke Foundation of On- tario. Dr. Krahn is a Research Scholar of the Heart and Stroke Foundation of Canada. Address for correspondence: Andrew Krahn, M.D., London Health Sci- ences Center, University Campus, 339 Windermere Road, London, Ontario, Canada N6A 5A5. Fax: 519-663-3076; E-mail: akrahn@uwo.ca subcostal, and axillary regions with an adequate albeit lower amplitude signal. The recorded bipolar ECG signal is stored in a circular buffer capable of retaining 21 minutes of uncom- pressed signal or 42 minutes of compressed signal in one or three divided parts. Because the compressed signal quality is negligibly different from the uncompressed signal, it is used most often to maximize the memory capacity of the device. The memory buffer is frozen using a nonmagnetic hand-held activator provided to the patient at the time of device im- plant. The episodes are downloaded after interrogation with a standard Medtronic 9790 pacemaker programmer. The cur- rent version of the device (Reveal Plus © ) has programmable automatic detection of high and low heart rate episodes and pauses. The resultant memory configuration allows for divi- sion of multiple 1-minute automatic rhythm strips in addition to 1–3 manual recordings. This permits automated backup of manual activations to detect prespecified extreme heart rates or pauses (typically <30 beats/min, >160 beats/min, and pauses >3 sec). This also permits detection of asymptomatic heart rate changes that may influence clinical judgment as to the likely cause for syncope in the absence of symptomatic recurrence (Fig. 1). The implant procedure is similar to creation of a smaller and more superficial pacemaker pocket. Cardiologists and cardiac surgeons have performed the implant in an operat- ing room and in the electrophysiology or cardiac catheter- ization laboratory setting. Some physicians have implanted the device in an office-based procedure room. An adequate signal can be obtained anywhere in the left thorax, without the need for cutaneous mapping. 6 Mapping optimizes the sensed signal and is recommended for patients in whom au- tomatic detection is desirable to prevent oversensing of T waves and double counting leading to automatic detection of high rate episodes. Mapping usually leads to device insertion in an oblique orientation in the high left parasternal region (unpublished observation). Right parasternal sites have been used to optimize p wave amplitude. The patient, along with the spouse, family member, or friend, is instructed on use of the activator at the time of implant. Use of prophylactic an- tibiotics usually is recommended to prevent pocket infection, similar to that for implantation of permanent pacemakers.