Dual chamber discrimination, sinus tachycardia, V>A and atrial refractory periods
This 66-year-old man suffering from severe ischemic cardiomyopathy received a Teligen dual chamber defibrillator for the treatment of episodes of slow VT. He was seen for a routine follow-up visit.
Initial diagnosis of SVT based on the Rhythm ID discriminator and absence of therapy delivery. Diagnosis of VT in a second stage and delivery of 5 bursts of ATP.
- probable sinus tachycardia with 1:1 atrioventricular conduction; cycles classified VS alternating with cycles classified VT-1;
- 8 out of 10 criterion fulfilled;
- at the end of Duration, no therapy was delivered; the rhythm was stable and the vectors during tachycardia were correlated with the reference vector;
- continuation of the cycle-by-cycle analysis;
- probable change in the time when the R wave was sensed by the ventricular lead, explaining the variability of the ventricular cycles. Rhythm considered unstable; since the vectors remained correlated, no therapy was delivered;
- Stability criterion again fulfilled;
- probable atrial extrasystole with recurrent irregularity of the rhythm;
- another atrial extrasystole; because the PR was relatively long, the atrial extrasystole and sensing of the R wave by the ventricular channel occurred simultaneously. Since the extrasystole fell in the post ventricular atrial refractory periods it was not sensed;
- the compensatory pause following the non-sensed atrial extrasystole caused a pseudo-slowing of the atrial rhythm compared with the ventricular rhythm. Hence, when comparing the atrial rate with the ventricular rate of the 10 previous cycles, the V>A criterion was fulfilled. Since this criterion trumps all others, the decision was made to deliver therapy;
- delivery of first burst of ATP;
- persistence of sinus tachycardia;
- second burst of ATP;
- third burst of ATP;
- fourth burst of ATP;
- fifth burst of ATP;
- slowing of the sinus rate below the VT-1 zone and end of episode.
This was an episode of sinus tachycardia initially discriminated accurately. When the vectors are correlated, the device withholds the therapies. However, it was mislead secondarily by the failure to sense an atrial extrasystole during a period of refractoriness. The V>A criterion takes precedence over all others, including the vectors analysis. The post ventricular atrial refractory periods are short, though are not programmable. A likely solution is to avoid an overlap between the detection zones and the patient’s zone of physiologic rate acceleration, even if that seemed most challenging, since this patient developed repetitive episodes of slow VT. Another likely option is to favor the vectors analysis, which, in this patient, seemed quite feasible, by de-activating the V>A criterion, responsible in this case for the delivery of inappropriate therapies.