ILR basics

Since its introduction fifteen years ago, the implantable recorder (ILR) has become one of the reference diagnostic tools in the context of unexplained syncope. Syncope represents a major public health problem since 40 to 50% of the general population has already experienced an episode of transient loss of consciousness, and this symptom represents 1 to 6% of hospital admissions. While medical history, anamneses and clinical examination yield the correct etiology in the vast majority of the patients with neurally-mediated syncope, the performance of this conventional strategy is disappointing in the setting of cardiac syncope. The correlation between a syncopal event and any alteration of the ECG is the “gold standard” for the clinician; however syncope rarely occurs during short-term monitoring. The implantable loop recorder is a small device inserted subcutaneously in the left pectoral region that allows for long-term ambulatory recording of a bipolar ECG. The device has a loop memory, which records and memorizes the electrocardiogram upon its activation by the patient or by a bystander following a symptomatic episode, as well as automatically on the basis of pre-programmed detection criteria.

The implantation of an implantable loop recorder is usually performed under local anesthesia in the same operative theater as that used for pacemakers. After a large disinfection of the left thoracic region, the device is positioned between the first intercostal space and the fourth rib. The quality of the future recorded tracings, the ability to avoid artifacts and to discriminate between different types of arrhythmias critically depends on the optimization of the implant procedure. Two elements should be considered: 

1) the optimization of R-wave collection: the Reveal XT package contains a mapping tool (the Reveal Vector Check –RVC- attached to the blister package), which helps selecting the optimal implant site without desterilizing the device itself. The Vector Check has 2 electrodes that are approximately 4 cm apart, and which match the electrode spacing of the device. These electrodes make contact with the device electrodes at one end and with the patient’s skin at the other. The quality of the sensed signal can be checked with the programmer. Small variations in the orientation of the device may lead to significant changes in the amplitude and morphology of the R-wave. Optimal device positioning is associated with R-waves of large and stable amplitude (≥0.3 mV when viewed on the programmer strip chart), a stable R-wave morphology, a good visualization of both the R-wave and the P-wave with a R/T and R/P ratio sufficiently high to avoid any P and T-wave oversensing.
2) reducing the risk of myopotential oversensing: to warrant good reliability of the automatic detection, it is important to reduce device movement within its subcutaneous pocket as much as possible. Accordingly, the pocket should not be too large. It also should not be too small to avoid the externalization of the device. The head of the device is equipped with 2 suture holes for securing the device to the subcutaneous tissue and minimize the risks of rotation and migration of the device after implant.

Detection of atrial fibrillation

The diagnosis of AF is based on an algorithm that tracks the stability of the ventricular electrograms. By measuring the AF burden, the device helps deciding whether a treatment is needed, as well as verifying its efficacy. In 25% of instances, ischemic strokes remain unexplained and are considered cryptogenic after detailed investigations, including telemetry monitoring and recording of a 48-h ambulatory ECG. The likelihood of recording a symptomatic or asymptomatic atrial arrhythmia in a context of cryptogenic stroke increases in parallel with an increase in the duration of ECG monitoring. The absence of correlation between symptoms and arrhythmic episodes may justify an ILR, which automatically records asymptomatic episodes. When episodes diagnosed by the device last >24 h, introduction of an antithrombotic regimen as well a conversation regarding the need to introduce antiarrhythmic medication may be indicated.