Atrial sensing failure

This tracing reveals an intermittent atrial undersensing. The quality of the sensing of a signal is contingent on a certain number of its physical characteristics:

1. its rate spectrum: the rate of a signal is expressed in hertz (Hz), and is the inverse of its period. A pacemaker amplifies entrant signals comprised in a rate range on average between 10 and 70 Hz corresponding to the cardiac depolarization signals. The signals located below and beyond this zone are filtered and are therefore rendered less or not detectable by the system. In the ventricular channel, the R waves have a rate spectrum between 10 and 30 Hz and are amplified. In contrast, the T waves whose spectrum is less than 5 Hz are filtered. Similarly, signals originating from the atrium and collected in the ventricle usually have a very low rate, and are most often filtered. The rate spectrum of signals of muscular origin, such as the pectoral muscle (myopotentials), is superimposed on that of the P waves and the R waves. In the unipolar configuration, the sensing field extends from the distal electrode of the pacing lead to the pulse generator which is placed on or under the pectoral muscle with an increased risk of interference by the sensing of muscular signals during certain physical efforts;
2. its slope: this parameter depicts the variation in amplitude of the cardiac signal as a function of time, expressed in mV/ms. The pacemaker senses the portion of the fastest signal which corresponds to the transit of the depolarization front facing the electrode. If the depolarization signal is fragmented, as is sometimes the case for an extrasystole, the slopes of its various components are often slower, with an increased risk of undersensing. At implantation, the pacing leads should ideally be positioned at a site where the depolarization slope is at least 1mV/ms in the ventricle and at least 0.5mV/ms in the atrium. The measurement of the slope of the signal is contingent on its processing and in particular on the filters used. These differ depending on the measuring system, the external device for determining the thresholds or the prosthesis that will be permanently connected. The observed differences can vary significantly. Direct recording of the signal during implantation can, however, be useful in searching for the site that allows observing the broadest intrinsic deflection. Intrinsic deflection of an endocardial signal almost never occurs at the beginning of the corresponding signal on the surface ECG. For example, the sensing of ventricular depolarization in a patient with full right bundle branch block is particularly late. Similarly in the atrium, atrial sensing can occur at the end of the P wave of the surface ECG;
3. its amplitude: the amplitude of the signal measured by the pacemaker corresponds to the amplitude of the signal which remains detectable by the pacing system after its rate analysis and the determination of the slope. It is expressed in mV. This is the parameter used at the end of the signal processing chain to determine the sensitivity level of the system. Programming a pacemaker with a sensitivity of 4 mV means that only signals with an amplitude greater than this 4 mV value are detectable once the signal has been processed (satisfactory rate and slope). All signals of lower amplitude will not be taken into account. Increasing the programmed sensitivity value (e.g. to 12 mV) means reducing the sensing capacity of the system, now requiring a signal amplitude of more than 12 mV for this signal to be detectable. Altogether, during the implantation of a pacing lead, attention must be given to obtaining a signal corresponding to the bandwidth of the pacemaker, the intrinsic deflection of which is the fastest possible and of high amplitude. Amplitude levels of at least 5 mV in the ventricle and at least 2 mV in the atrium are typically targeted.

The sensing of the P wave presents certain peculiarities. Indeed, the amplitude of the sensed atrial signal may vary according to the position of the patient and as a function of the respiratory cycle. It is therefore necessary to set a sufficient margin to avoid the problems of atrial undersensing on exertion which, in a patient with an atrioventricular block, may cause a sudden drop in ventricular pacing rate. In addition to the lack of proper monitoring of P waves upon exertion, atrial undersensing may have a pro-arrhythmogenic effect if atrial pacing occurs in an atrial vulnerable period with a risk of atrial arrhythmia induction.