Europace Advance Access originally published online on April 19, 2008
Europace 2008 10(8):1009-1011; doi:10.1093/europace/eun103
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CASE REPORTS
Far-field oversensing of atrial signals: an unusual cause for very short V–V intervals and inappropriate implantable cardioverter defibrillator therapy
Abteilung für Kardiologie und Pneumologie, Herzzentrum, Universitätsmedizin Göttingen, Robert-Koch-Strasse 40, 37075 Göttingen, Germany
Manuscript submitted 16 March 2008. Accepted after revision 28 March 2008.
* Corresponding author. Tel: +49 551 39 6046; fax: +49 551 39 10267. E-mail address: dirkvollmann2000{at}aol.com
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A 36-year-old man presented for an unscheduled follow-up after he had experienced multiple shock discharges of his implantable cardioverter defibrillator (ICD) one day earlier while playing soccer. The initial ICD system had been implanted in 1997 for a dilated cardiomyopathy with a left ventricular ejection fraction of 30% and non-sustained ventricular tachycardias (VTs). The current dual-chamber ICD (EnTrustTM, Medtronic Inc., MN, USA) had been implanted in 2004 together with a Medtronic model 5076 atrial lead, whereas the ICD lead (Medtronic Sprint model 6943) had not been replaced since the initial implant in 1997.
Several device alert observations were noticed upon device interrogation. First, the sensing integrity counter (SIC), which cumulatively counts very short (120–130 ms) ventricular (V–V) intervals and thereby typically indicates transient oversensing of electrical noise, had increased to 42 552 within the last 3 days. Second, all therapies including shocks had been delivered without success for a VT/ventricular fibrillation (VF) episode. The episode counter showed that eight shocks had been delivered for one VF episode at the preceding afternoon. Analysis of the stored episode plot and electrogram revealed inappropriate detection of very short V–V intervals and thus VT/VF during sinus tachycardia because of intermittent far-field oversensing of atrial signals (Figure 1). The measurement of sensing and pacing performance revealed a significant decrease in R-wave amplitude sensing from 
5 mV at last follow-up to 2.0–2.5 mV (Figure 2) and no ventricular capture at 8 V/1.5 ms. Furthermore, pacing impedance had jumped from 1200 
at the last measurement to >3000 , whereas impedance within the high voltage circuit had decreased from 80 to 53 . Far-field oversensing of atrial signals was also observed during normal sinus rhythm at the time of follow-up (Figure 2) when ventricular sensitivity was unchanged at 0.3 mV. No evidence for oversensing of electrical noise was found during different provocation manoeuvres and upon analysis of all stored episode data. On chest X-ray, the tip of the ICD lead was still near the right ventricular apex with no evidence for lead dislocation or for a structural lead defect. On the basis of the sum of all findings (incomplete) ICD lead failure was suspected and the lead was replaced. Unfortunately, the old lead could not be explanted. Three months later, no evidence for system malfunction was found during the routine follow-up.
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Implantable cardioverter defibrillator lead failure is a significant problem that may result in potentially fatal inappropriate device therapy. If the structural lead defect is incomplete at first, the electrical integrity of the system may be lost only for brief moments. If this is the case, lead failure will not necessarily manifest with inappropriate ICD therapy because of oversensing of electrical noise and is unlikely to be detected by single measurements of lead impedance and pace–sense performance. The SIC enhances the early detection of ICD lead failure in this situation because it also detects very brief, sporadic episodes of noise oversensing.1
,2 The algorithm continuously counts the number of short V–V intervals (120 and 130 ms) that are typically seen during oversensing of electrical noise. An increment in the total SIC to more than 300 events since last follow-up is considered as a specific indicator of system dysfunction.1,2
We previously reported that ICD leads with integrated bipolar sensing appear to be more susceptible for false positive increases of the SIC (in the absence of lead failure).2 Recently, we also found that in a normally functioning true bipolar ICD lead, an increase of the SIC may occur if intermittent T-wave oversensing and premature ventricular complexes co-exist.3 The present case illustrates another so far unidentified cause for an increase of the SIC in a true bipolar ICD lead, namely far-field oversensing of atrial signals during sinus tachycardia. In general, oversensing of atrial signals in the ventricular channel is a rare event that has been linked to integrated bipolar sensing and to the automatic adjusting gain feature that is present in most ICD devices.4,5 Interestingly, oversensing of atrial signals in the present case occurred with an ICD lead for true bipolar sensing, at a time point when the lead sensing and pacing function acutely worsened, probably due to incomplete conductor fracture and/or an insulation defect. Under these circumstances, automatic gain adjustment ensured the detection of low amplitude ventricular signals, but also contributed to the oversensing of atrial far-field signals. Unfortunately, since the old ICD lead was not explanted, the exact structural defect that caused the present form of inappropriate sensing could not be clarified.
In summary, this case illustrates that far-field oversensing of atrial signals may indicate impending ICD lead failure and can cause very short ventricular sensed intervals as quantified by the SIC.
Conflict of interest: none declared.
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[1] Gunderson BD, Patel AS, Bounds CA, Shepard RK, Wood MA, Ellenbogen KA. An algorithm to predict implantable cardioverter-defibrillator lead failure. J Am Coll Cardiol (2004) 44:1898–902.
[2] Vollmann D, Erdogan A, Himmrich E, Neuzner J, Becker D, Unterberg-Buchwald C, et al. Patient alert to detect ICD lead failure: efficacy, limitations, and implications for future algorithms. Europace (2006) 8:371–6.
[3] Vollmann D, Luthje L, Zabel M. Unusual cause for an increase of the sensing integrity counter in a patient with inappropriate implantable cardioverter-defibrillator therapy. Europace (2007) 9:275–7.
[4] Curwin JH, Roelke M, Ruskin JN. Inhibition of bradycardia pacing caused by far-field atrial sensing in a third-generation cardioverter defibrillator with an automatic gain feature. Pacing Clin Electrophysiol (1996) 19:124–6.[CrossRef][Medline]
[5] Vollmann D, Luthje L, Gortler G, Unterberg C. Inhibition of bradycardia pacing and detection of ventricular fibrillation due to far-field atrial sensing in a triple chamber implantable cardioverter defibrillator. Pacing Clin Electrophysiol (2002) 25:1513–6.[CrossRef][Medline]
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