Periodic tracing

The BioMonitor was developed to allow an automatic recording of episodes diagnosed as atrial fibrillation, bradycardia, asystole, sudden drop in rate or high ventricular rate. As observed on the previous tracing, the recording may be triggered by the patient following a symptomatic episode. The device can also periodically transmit an electrocardiographic tracing, the frequency of which is programmable. By definition, these random periodic tracings are not intended to be of diagnostic value and their analysis is often overlooked. However, they allow verifying the quality of the tracings even in the absence of recorded episodes. The reliability and diagnostic capacity of the system is largely contingent on the quality of the tracings and the possibility of correctly identifying the various components of the electrocardiogram (P waves, QRS complexes, T waves) without interference from an external source (myopotentials, external noise, etc.). The implantation of an implantable loop recorder is usually performed under local anesthesia in the same operative theater as that used for pacemakers. After extensive disinfection of the left thoracic region, the device is positioned between the first intercostal space and fourth rib. The quality of future recorded tracings, the ability to avoid oversensing of artifacts saturating the memory and the ability to correctly discriminate between different types of arrhythmia directly depend on the optimization of the implantation procedure. Two elements require particular consideration:

1. the optimization of R wave collection; small variations in the orientation of the device may lead to significant variations in amplitude and morphology of the sensed signals. There are two possible types of recording: patient-triggered recordings following occurrence of symptoms and automatic recordings in conjunction with the detection of bradycardia or tachycardia. During a patient-triggered recording, proper visualization of P waves, R waves and T waves facilitates the differentiation between ventricular tachycardia and supraventricular tachycardia but also between sinus pause and atrioventricular block episode. In contrast, automatic recording is based solely on the analysis of ventricular rhythm analysis and on R wave counting, the specificity of which can be altered by oversensing of P waves or T waves by the device. It is therefore essential to obtain a proper balance between sensing of all R waves without oversensing of P or T waves in order to avoid memory saturation (episodes diagnosed as bradycardia if undersensing, episodes diagnosed as tachycardia if oversensing) while effectively visualizing the P waves to allow differentiation between sinus dysfunction and atrioventricular block during an episode of bradycardia or asystole.  
2. reducing the risk of myopotential oversensing; to ensure the reliability of the automatic detection of episodes, it is important to minimize device movement within the subcutaneous pocket. Accordingly, the pocket should not be too large to avoid movement or too small to avoid the externalization of the device.

It should be noted that there are 2 different filter levels. For the sensing of QRS complexes and classification of intervals, the frequency band used by the device is between 10 and 40 Hz to avoid oversensing of low-frequency (T waves, P waves) and high-frequency (myopotentials and electromagnetic interference) signals. To facilitate the interpretation of the tracings by the physician from the programmer, different filters are used (0.5 to 40 Hz) in order to allow the visualization of low frequency signals while excluding high frequency signals (artifacts, etc.).

As can be seen on the second tracing, the P waves can be visualized on only a few complexes while the majority are filtered. If a ventricular pause occurs, it will be difficult or even impossible to differentiate between sinus pause and atrioventricular block.