Sensitivity, expressed in millivolts (mV), qualifies the pacemaker’s ability to correctly detect spontaneous cardiac events.

A pacemaker is equipped with input filters that specifically detect P waves in the atrium and R waves in the ventricle, based on analysis of three characteristics of these electrical signals: frequency spectrum, slope and amplitude.

Appropriate programming of the sensitivity level should enable detection of all spontaneous cardiac events occurring in the implanted chamber, while not detecting events of any other nature (cross-listening with detection of cardiac signals from the other chamber, myopotentials, interference, etc.).

Programming bipolar detection increases the specificity of detection compared with unipolar detection, by limiting the risk of listening to extracardiac signals or cross-listening, and enables high sensitivity values to be programmed (0.3 to 0.5 mV in the atrium, 2 to 3 mV in the ventricle). On the other hand, in unipolar configuration, the risk of cross-listening or listening to extracardiac signals means that lower sensitivity levels (1 to 1.5 mV in the atrium and 4 to 5 mV in the ventricle) have to be programmed, with a greater risk of under-detection.

The frequency spectrum

The frequency of a signal is expressed in hertz (Hz), and is the inverse of its period. A pacemaker amplifies incoming signals averaging between 10 and 70 Hz, corresponding to cardiac depolarization signals. Signals below and above this range are filtered out, making them less or non-detectable by the system.

Example: in the ventricular canal, R waves have a frequency spectrum between 10 and 30 Hz and are amplified. On the other hand, T waves with a spectrum of less than 5 Hz are filtered out. Similarly, signals of atrial origin collected in the ventricle usually have a very low frequency, and are usually filtered.

The frequency spectrum of signals originating from muscles such as the pectoral muscle (myopotentials), is superimposed on that of P waves and R waves. In unipolar configuration, the detection field extends from the distal electrode of the stimulation probe to the box, which is placed on or under the pectoral muscle, with an increased risk of interference due to the detection of signals originating from muscles during certain efforts.

Slope

This parameter describes the variation in amplitude of the cardiac signal as a function of time, expressed in mV/ms. The pacemaker detects the fastest part of the signal, corresponding to the depolarization front in front of the electrode.

If the depolarization signal is fragmented, as is sometimes the case in extrasystole, the slopes of its various components are often slower, with an increased risk of under-detection.

At implantation, pacing leads should ideally be positioned at a site where the depolarization slope is at least 1mV/ms in the ventricle and at least 0.5 mV/ms in the atrium. Measuring the slope of the signal depends on its processing, and in particular on the filters used. These differ according to the measurement system, the external threshold determination device or the prosthesis that will be permanently connected. The differences observed can be very significant. Direct recording of the signal at implantation can be useful, however, to find the site with the greatest intrinsic deflection.

The intrinsic deflection of an endocardial signal almost never occurs at the start of the corresponding signal on the surface ECG. For example, the detection of ventricular depolarization in a patient with complete right bundle-branch block is very late. Similarly, in the atrium, auriculogram detection may occur at the end of the P wave on the surface ECG.

Signal amplitude

The signal amplitude measured by the stimulator corresponds to the signal amplitude that remains detectable by the stimulation system after frequency analysis and slope determination. It is expressed in mV. This is the parameter used at the end of the signal processing chain to determine the system’s sensitivity level.

Programming a pacemaker with a sensitivity of 4 mV means that only signals whose amplitude is greater than this 4 mV value are detectable, once this signal has been processed (with a satisfactory frequency and slope). All signals of lower amplitude will be ignored. Increasing the programmed sensitivity value (to 12 mV, for example) means reducing the system’s detection capability, as this time a signal amplitude of more than 12 mV is required for the signal to be detectable.

All in all, when implanting a stimulation probe, the aim should be to obtain a signal corresponding to the stimulator’s bandwidth, with the fastest possible intrinsic deflection and high amplitude. Amplitude levels of at least 5 mV in the ventricle and at least 2 mV in the atrium are targeted.

Automatic detection

Traditionally, unlike defibrillators, pacemakers have operated with a stable, fixed sensitivity over the entire cardiac cycle. Increasingly, even if the constraints in terms of detection are not the same (the crucial need for a defibrillator to detect and treat very rapid, polymorphic and micro-volatile ventricular rhythm disorders), modern pacemakers enable adaptive sensitivity (variable sensitivity level depending on the amplitude of the R or P wave detected) with progressive increase in sensitivity over the cardiac cycle (possibility of detecting small amplitude signals without over-detecting the T wave).