Dual chamber discrimination, atrial fibrillation and inappropriate therapies

N° 8
Manufacturer Boston Scientific Device ICD

This 62-year-old man presenting with dilated cardiomyopathy underwent implantation of a Fortify triple chamber defibrillator after an episode of aborted sudden cardiac death. He was seen for evaluation of palpitation followed by an electrical shock.

Graph and trace
  1. AFib with rapid and variable atrioventricular conduction; alternation between paced (RVP-Tr) and spontaneous cycles in the sinus (RVS) or VT (VT) zones; 
  2. V-Epsd with diagnosis of gradual onset (8 out of 10 cycles in the VT zone), without abrupt change in the ventricular rate, hence the absence of sudden onset (Gradl);
  3. at the end of Duration, diagnosis of AFib with atrioventricular conduction, based on irregular du ventricular rhythm; no therapy was delivered;
  4. the 6 out of 10 criterion was no longer fulfilled on this cycle;
  5. the 8 out of 10 and Duration criteria were again fulfilled and AFib with atrioventricular conduction was diagnosed. The atrial rate surpassed the AFib Rate Threshold and the rhythm was unstable; hence, no therapy was delivered;
  6. continuation of the cycle-by-cycle analysis and inhibition of the therapies;
  7. stabilization of the ventricular rhythm, now classified as stable (stable); suspicion of dual (Afib + VT) tachycardia and decision to treat;
  8. first burst of ATP;
  9. unsuccessful burst; redetection in the VT zone;
  10. second burst of ATP;
  11. third burst of ATP;
  12. fourth burst of ATP;
  13. fifth burst of ATP;
  14. sixth burst of ATP;
  15. charge of the capacitors;
  16. end of charge;
  17. delivery of 21-J electrical shock;
  18. probable termination of the atrial arrhythmia; probable sinus tachycardia with atrioventricular conduction and PVCs.

This was very likely an episode of conducted AFib, accurately identified initially by the device. The atrial rhythm was rapid, the ventricular rhythm was considered unstable and the therapies were withheld. In a second stage, the ventricular rhythm stabilized, a typical occurrence when the ventricular rate of conducted AFib surpasses 170 bpm. When in doubt between conducted AFib that has become regular and dual tachycardia, the device favors sensitivity over specificity and delivers the therapies. Ventricular ATP does not reliably treat irregular supraventricular arrhythmias, whereas the termination of an atrial tachycardia with 1:1 atrioventricular conduction by a sequence of ventricular ATP is not uncommon. No additional discrimination is programmed for redetection. Since the ventricular rate remains rapid, the therapies were delivered in sequence until termination of the episode by an electrical shock.

In this patient, the stability criterion, programmed at a nominal value of 20 ms, was misleading. Its sensitivity could be lowered and specificity increased by increasing the value to 30 or 40 ms, for example, as the adjustment of discriminators consists invariably of finding a balance between sensitivity and specificity. 

Message to remember

Initial diagnosis of SVT based on the Onset/Stability discrimination and absence of treatment. Diagnosis of VT in a second stage with delivery of 3 bursts of ATP, followed by 3 more bursts and, finally a 21 J electrical shock.