Sense-B-noise: an enigmatic cause for inappropriate shocks in subcutaneous implantable cardioverter defibrillators

Abstract

Aims: Subcutaneous implantable cardioverter defibrillators (S-ICDs) are well established. However, inappropriate shocks (IAS) remain a source of concern since S-ICDs offer very limited troubleshooting options. In our multicentre case series, we describe several patients who experienced IAS due to a previously unknown S-ICD system issue.

Methods and results: We observed six patients suffering from this novel IAS entity. The IAS occurred exclusively in primary or alternate S-ICD sensing vector configuration (therefore called 'Sense-B-noise'). IAS were caused by non-physiologic oversensing episodes characterized by intermittent signal saturation, diminished QRS amplitudes, and disappearance of the artefacts after the IAS. Noise/oversensing could not be provoked by manipulation, X-ray did not show evidence for lead/header issues and impedance measurements were within normal limits. The pooled experience of our centres implies that up to ∼5% of S-ICDs may be affected. The underlying root cause was discussed extensively with the manufacturer but remains unknown and is under further investigation.

Conclusion: Sense-B-noise is a novel cause for IAS due to non-physiologic signal oversensing, arising from a previously unknown S-ICD system issue. Sense-B-noise may be suspected if episodes of signal saturation in primary or alternate vector configuration are present, oversensing cannot be provoked, and X-ray and electrical measurements appear normal. The issue can be resolved by reprogramming the device to secondary sensing vector.

Keywords: Device failure; Inappropriate shock; Oversensing; S-ICD; Subcutaneous defibrillator.

Figure 1

Inappropriate shock due to oversensing (Case #1). The episode begins with a sudden artefact driving the signal into saturation (1). Subsequently, the cardiac signal becomes largely unrecognizable with low-amplitude signals (2), and the S-ICD starts to charge (3, charging artefact). The shock delivery is ensued by normal QRS signal amplitudes (4).

Figure 2

Patient Case #2. Panel (A) inappropriate shock due to artefact oversensing. Signal saturation episodes are visible (asterisks), the low-amplitude QRS becomes better visible after the IAS. Panel (B) similar to Figure 1, the episode is triggered by a sudden artefact (1). The ensuing artefact-rich signal shows episodes of signal saturation and oversensing.

Figure 3

Patient Case #3. Panel (A) patient with IAS inducing ventricular fibrillation. The shock is preceded by an artefact-rich signal with episodes of signal saturation (asterisks). Panel (B) shows the following second shock. Panel (C) remote monitoring of the patient in secondary vector. Episodes as shown in panel (A) were not registered anymore. However, intermittent T-wave oversensing (1) and QRS undersensing (2) occurred. Myopotential noise was appropriately classified by the device (3).

Figure 4

Patient Case #4. Panel (A) heavily diminished ECG signal amplitudes causing oversensing and shock delivery. Panel (B) similar episode 2 days later. Both episodes (panel B and C) showing QRS amplitude restoration after shock delivery.

Figure 5

Simplified flowchart for the diagnosis and management of inadequate S-ICD shocks of non-cardiac origin. Initial diagnostic steps (yellow boxes) mostly allow identifying the cause for the IAS (red boxes). Dashed arrows indicate diagnostic key hints. If the findings remain unclear or unsuspicious, the manufacturer’s technical service should be involved. Green boxes show general management recommendations. Abbreviations: AC, alternating current; IAS, inadequate shock.