Because of the high amplitude of atrial
Because of the high amplitude of atrial signals, atrial sensitivity was decreased from 0.5mV to 0.7mV to 1.0mV. As the sensitivity decreased, atrial sensing was restored gradually. With the sensitivity of 1.0mV, normal atrial sensing with mode switching was restored completely. The sensitivity was programmed to 1.4mV. The resultant surface EGM and AEGM at this setting are shown in Fig. 1B. The Sensing Assurance function was turned off because atrial sensitivity was limited to 0.5mV when the feature was enabled. This reprogramming led to the accurate detection of AF with mode switching to DDIR. Subsequent ambulant follow-up at 6, 12, and 24 months demonstrated continued normal sensing during AF.
Discussion The mechanism of PAUS during AF seen in this case report was most probably caused by repetitive activation of the quiet timer blanking period . Current pacemakers are equipped with a mechanism called “quiet timer blanking” that is intended to allow the sense amplifier circuit response, known as “ringing,” to die down before bringing the sense amplifier back online. The normal quiet timer interval in Kappa pacemaker ranges from 50 to 100ms (nonprogrammable). Thus, if large and/or wide signals and/or high levels of postpace polarization are sensed, these quiet timer blanking periods may cover the entire sensing window. Furthermore, with a short sensed atrial dna-pkcs length (CL), the blanking timer may be set repetitively, resulting in disabling of atrial sensing . In the present case, the combination of high sensitivity (0.5mV; lower programmed value indicates being more sensitive) and the characteristics of the patient′s atrial signal (amplitude of 5.6–8.0mV and CL of 160–170ms) repetitively reset the quiet timer blanking interval. Accurate sensing and mode switching behavior were restored when the sensitivity was decreased to 1.0mV. The Kappa series are also equipped with an analog noise filter (ANF) in order to sense cardiac signals when there is low-level background noise of continuous frequency interference, such as 50/60Hz. The behavior of ANF depends on the amplitude of the signal compared with the amplitude of the background noise. Changing the sensitivity setting does not change the behavior of ANF. Thus, it is unlikely that ANF was involved in the atrial undersensing observed in the present case. PAUS was first described using a sheep model by Willems et al. . The study indicated that undersensing was present if the pacemaker was programmed to be 4 or more times more sensitive than the sensing threshold . To date, a total of 6 cases, including the present case, have been reported [2–5] (Table 2). The atrial rhythm was AF in 5 patients and atrial flutter in 1 patient. The atrial CL was <180ms in all patients except in patient 2, who was in atrial flutter with a CL of 220ms and relatively high sensed amplitude. In all patients, the minimum sensed atrial amplitude was >4mV during the tachycardia. Normal atrial sensing was successfully restored in 5 of 6 patients by decreasing the atrial sensitivity. The ratio of the sensed atrial amplitude to the sensing threshold to avoid PAUS ranged from 4 to 13, which roughly agrees with the results of the animal study . In addition, it should be noted that PAUS was not completely avoided even at 2mV sensitivity in a patient who had the highest sensed atrial amplitude of 10mV during atrial fibrillation with a short CL of 160ms . All 6 patients with PAUS had Medtronic pacemakers (Table 2). The sensitivity setting of the pacemaker is implemented by 2 means: variable-gain amplifiers or adjustable-threshold voltage comparators . The variable gain amplifier is the first stage of the sensing circuitry, and it will change the gain of the sense amplifier according to the set sensitivity. The design of Medtronic pacemakers may differ from that of pacemakers from other manufacturers. However, discussing the technical aspect of the sensing circuitry is not the purpose of this paper. In 1 study, patients with a dual-chamber pacemaker and AF were investigated for the incidence of PAUS by increasing the atrial sensitivity in a stepwise fashion . PAUS was seen in 9 of 71 (13%) patients with 6 different pacemakers from 5 different manufacturers. The study indicated that, technically, PAUS may universally occur; however, further accumulation of data is needed to clarify if the Medtronic pacemakers have a propensity for the behavior in real-world clinical practice.