Oversensing
oversensing of a 50 Hz signal
The usual characteristics observed in patients presenting with oversensing of a 50 Hz signal include 1) exposure to an emitting source at the time of the episode (for instance a poorly insulated household appliance), and 2) short, regular, uninterrupted signals throughout the cardiac cycle. By changing the sweep speed, the Boston Scientific defibrillators enable the demonstration of a characteristic sinusoidal waveform with a 20-ms separation between each signal; in double or triple chamber defibrillators, the signals are usually visible on the atrial, the RV and LV and the high-voltage channels. Their amplitude is greater on the high-voltage than on the sensing channel. They are managed by a) identifying their source, and b) avoiding the use of poorly grounded devices.
A mode of response to noise can be programmed, which is activated when the noise is prolonged and diagnosed by the device. This algorithm is triggered by signals sensed in the noise window of the ventricular blanking period. The tracing displays the AN, VN, RVN or LVN noise markers, while the markers AP-Ns, VP-Ns, RVP-Ns and LVP-Ns indicate asynchronous pacing in response to the sensing of continuous noise. An asynchronous (AOO, VOO or DOO) or an inhibitory (OOO) pacing mode can
also be programmed.
oversensing and use of an electrical scalpel
To prevent the delivery of inappropriate therapies due to the use of an electrical scalpel by the surgeon, 2 choices were available: 1) place a magnet over the pulse generator; the response to the magnet can be programmed in the a) OFF position (the magnet has no effect), in which case the electrograms (EGM) are recorded, though
no therapy is delivered, or b) Tachy mode (no therapy and no EGM recording), or 2) change the programming; the Tachy mode can be programmed on monitor only or on OFF. An Electrocautery Protection Mode is also available when an electrical scalpel is in use. It prevents the delivery of inappropriate therapies (Tachy mode in OFF position) though also enables asynchronous pacing (AOO, VOO or DOO) which might be an advantage in pacemaker-dependent patients. Oscilloscopic monitoring must continue throughout the procedure, as the patient is no longer protected by the defibrillator. The application of a magnet should not preclude a complete postoperative interrogation of the device.
Lead dysfunction
Abnormal impedance or a tracing showing intermittently short and disorganized cycles of variable amplitude, is suggestive of lead dysfunction. Remote monitoring enables an early diagnosis, which can prevent the delivery of inappropriate therapies. An early diagnosis may prevent a succession of inappropriate charges, which, even when borted, consume energy and can markedly shorten the lifespan of the device. A particular motion of the arm or shoulder can reproduce oversensing. A low impedance value suggests an insulation breakdown, while a higher value suggests a rupture of the lead conductor. It is important to detect sudden variations in the impedance curves, even when their values remain within normal limits.
Oversensing of P-waves
Oversensing of the atrial activity occurs preferentially when the defibrillating electrode of an integrated bipolar lead is straddling the tricuspid valve. Oversensing of the atrial arrhythmia by the ventricular channel may cause pauses, as the oversensed rapid rhythm inhibits ventricular pacing. Patients may present with recurrent light-headedness due to these episodes. A first remedial choice consists of lowering the ventricular sensitivity to prevent oversensing of the atrial activity. This programming might, however, be associated with a risk of a true VF undersensing. A new induction of VF would be required to confirm the reliable detection with the programming of this new sensitivity value. A second option is to place a new lead.
Oversensing of diaphragmatic myopotentials
Oversensing of diaphragmatic myopotentials is suggested by the presence of low-amplitude signals, best visible at the level of the sensing channel and often absent at the level of the high-voltage channel. Oversensing occurs most often after a long diastole or a ventricular paced event, when gain and sensitivity are maximum. It is more likely in recipients of integrated bipolar lead (the sensing anode is the defibrillation electrode of the lead) implanted at the RV apex. In pacemaker-dependent patients, ventricular pacing may be followed by prolonged oversensing that inhibits pacing and caused syncope. The programming choices commonly consist of lowering the ventricular sensitivity and/or increasing the back up rate. A second option is to place a new lead.
T-wave oversensing
T-wave oversensing is very infrequent in Boston Science devices. The incidence is probably much lower than with devices from other manufacturers. Many physicians following patients implanted with a Boston Science device have never observed a single case of T-wave oversensing. The band-pass filter of 20-85 Hz designed to filter out low frequency T-wave and high frequency muscle noise, the nominal sensitivity of 0.6 mV and the characteristics of the sensing circuit may be the main reasons
to explain this very low rate. The possibilities of reprogramming are limited in case of these infrequently described cases of T-wave oversensing (no possibility of reprogramming the upper threshold, the decay delay, the filters...). The only possibility is to alter the ventricular sensitivity and to verify the accurate detection of a true VF
episode.
