A special kind of plot - Cobalt VR

Patient

Male implanted with a primary prevention single-chamber defibrillator (Cobalt VR) for ischaemic cardiomyopathy; multiple episodes of NSVT stored in memory.

Trace

1- What diagnosis is suggested by the interval plot?
The plot shows a characteristic ‘railroad track’ appearance; initially the rhythm is regular with intervals around 600 ms, which is compatible with a sinus tachycardia; then the plot shows a ’railroad track’ appearance alternating between 2 intervals, the first at around 250 ms, the second at around 350 ms; it is important to note that the sum of these 2 intervals (250 +350 ms) is equal to the initial value of 600 ms.
These 2 elements (railroad track appearance + sum of intervals equal to the initial value) are very suggestive of oversensing of a physiological cardiac signal (T wave, P wave, double counting of the R wave); the duration of the intervals (250 and 350 ms) makes the hypothesis of double counting of the R wave very unlikely (one of the two intervals would be much shorter, bordering on the ventricular blanking period); an analysis of the EGM helps clarify the diagnosis.

2- What is your final diagnosis?
The EGM shows 2 alternating signals of different morphology, a high-frequency “sharp” signal corresponding to the QRS complex and a lower-frequency, “softer” signal corresponding to the T wave; there is also an alternating pattern to the duration of the intervals: a short cycle (260-270 ms) and a longer cycle (300-320 ms).
The 2 intervals fall within the VT zone programmed between 330 and 250 ms (TS markers); however, the oversensing is intermittent and the counters are not filled, which explains the NSVT classification.
Therefore, the final diagnosis is T wave oversensing during sinus rhythm

Take home message

Analysis of the interval plot can sometimes suggest certain diagnoses; analysis of the EGM can confirm them.
A systematic analysis of episodes classified as NSVT by the device sometimes leads to unexpected diagnoses, such as T wave oversensing.
T wave oversensing is associated with a typical appearance of alternation between 2 morphologically different signals that include a high-frequency signal (R wave) and a low-frequency signal (T wave); for each cardiac cycle, the device not only counts the R wave, but also the T wave as a second additional signal, resulting in a doubling of the heart rate; the magnitude of the variation in cycle length (RT intervals and TR intervals) is generally more pronounced for slow heart rates (short RT intervals and long TR intervals) but is less pronounced during exercise (similar RT and TR intervals) and in patients with a long QT interval.
Although unrelated to the problem described in this trace, the programming in this patient is surprising, the use of the VT counter (which implements consecutive cycles and resets following a single undersensed event ) for such short cycle lengths (250 ms intervals) being very questionable …

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