The use of automatic gain control and auto-adjusting sensitivity algorithms throughout the cardiac cycle, optimizes the capacities of the device to appropriately sense VF signals but also increases the risk of myopotential oversensing at the end of diastole, when sensitivity reaches its maximum.
Myopotential oversensing is rare but was more commonly observed in pacemaker-dependent patients implanted with older Boston Scientific ICDs connected to an integrated bipolar lead positioned at the RV apex. Being paced continuously is a variable associated with an increased risk of diaphragmatic oversensing since after pacing, the time at maximal sensitivity is prolonged in devices that utilize Automatic Gain Control especially at slow heart rates. Integrated bipolar leads allow for a wider sensing antenna as the large area of the RV shocking coil is shared as the anode for sensing; this could explain a higher incidence of inappropriate detection. In pacing-dependent patients, the risk of PM inhibition during oversensing episodes is also increased. Clinically, it may result in syncope from inhibition of pacing followed by the delivery of an inappropriate shock.
Oversensing of diaphragmatic myopotentials presents as low-amplitude, high frequency signals most of the time detected exclusively in the near-field channel with no alteration in the shock electrogram. The 2 main particularities of these signals are that the changes in amplitude sometimes follow a respiratory cycle and that they can be reproduced with specific provocation maneuvers as deep inspiration/expiration, Vasalva and coughing.
When a defibrillator senses these myopotentials, inhibition of pacing and/or inappropriate detection of VF may ensue. Oversensing does not occur until automatic gain control increases the gain sufficiently and is more likely after long diastolic intervals or after ventricular paced events. The occurrence of a large R-wave reduces the level of sensibility and interrupts at least temporarily the oversensing explaining why prolonged oversensing is more common in pacemaker-dependent patients.
Oversensing of diaphragmatic myopotentials may be corrected by reducing ventricular sensitivity. Verification of reliable VF sensing at the new level of sensitivity must be confirmed. In pacemaker-dependent patients, pacing at a faster rate may reduce oversensing. In rare cases, a surgical revision is required with implantation of a new high-voltage true bipolar lead (DF4 systems) or of a pace/sense lead (DF1 systems) away from the diaphragm (usually positioned at the RV septum).
The incidence of this problem has decreased in Boston Scientific devices since the introduction of a noise-rejection algorithm. However, to prevent the occurrence of diaphragmatic oversensing, it seems logical, during implantation, to select a septal site instead of the apex when choosing an integrated bipolar lead. Provocative maneuvers can be performed during implantation (forced inspiration if performed during local anesthesia or increased volumes if patient is ventilated) but they may not accurately predict the occurrence of oversensing when the patient will exercise in a supine position.