Tracing

Atrial pacing (Ap) and ventricular sensing (Vs); at the beginning of the tracing, Ap-Vs intervals oscillating around 400 ms and therefore lower than the programmed value of the long PR at rest (350 + 100 ms); then prolongation of the Ap-Vs interval exceeding the programmed value of the long PR at rest (>450 ms) over 6 consecutive cycles; switching to DDD mode (vertical line) and ventricular pacing (AV cycles) with programmed AV delay.

Comments

On a surface electrocardiogram, the PR interval corresponds to the delay between the depolarization of the first atrial cell and the depolarization of the first ventricular cell. The PR interval can be divided between the P wave, whose width denotes the depolarization time of the 2 atria, and the PR segment, measured between the end of the P wave and the beginning of the R wave, and representing the conduction delay in the atrioventricular node and the His-Purkinje network. The PR segment is most often isoelectric, the signals generated by the traversed structures not being of sufficient amplitude to be detected on the surface electrocardiogram.

The first-degree atrioventricular block (AV block) corresponds to a simple prolongation of the PR interval which exceeds the physiological values; there is an equal number of P waves and QRS complexes and a fixed, constant PR space, exceeding 200 ms in adults; if sinus function is normal, there is no bradycardia (ventricles at the same rate as the atria). The term AV block is therefore not appropriate since, technically-speaking, it is not a block (interruption of conduction) but rather a slowing of conduction.

The SafeR mode was designed to provide ADI pacing with continuous monitoring of atrioventricular conduction and switching to DDD mode when the pacemaker detects impaired conduction. There are 4 switching criteria corresponding to the different degrees of atrioventricular block (AVB I, AVB II, AVB III, pause). A certain number of parameters are programmable, 3 of which pertain to the AVB I criterion such as to allow a physiological response in patients exhibiting a paroxysmal prolongation of the PR interval. The maximum and minimum PR values ​​are programmable as well as the possibility of programming this parameter in response to exercise only (switching possible only on exertion) or at rest + exercise (switching possible at rest and on exertion). The pacemaker switches to the DDD mode when the PR interval exceeds the programmed limit value for 6 consecutive cycles (non-programmable value). The programmed value corresponds to the limit value on an intrinsic atrial activation (P). For a paced atrium (A), the limit value corresponds to the sensed value + 100 ms (non programmable).

What value should be programmed in terms of maximum PR duration? From what length of time should a PR interval be considered too long and should ventricular pacing be preferred? The answer to these questions is complex and must integrate various parameters: presence of symptoms, left ventricular ejection fraction, duration of the QRS complex. In an asymptomatic patient, with normal ejection fraction and narrow QRS, it may be desirable to favor the intrinsic condition so as to avoid asynchronous activation induced by right ventricular pacing as well as increased energy consumption despite suboptimal filling in relation to the long PR. The parameter can then be programmed for exercise alone or for rest + exercise with relatively long PR values (long max PR at 350-400 ms and long PR at 250 ms).

In SafeR mode, the pacemaker switches when the PR interval exceeds a programmable value over 6 consecutive cycles; the limit value on atrial pacing is 100 ms longer than on a sensed atrial event.