PMC9458922_01
(A) The onset of VT after a short-long-short (SLS) sequence. A single ventricular premature beat (coupling interval VS 388 ms) occurs during normal sinus rhythm (52 bpm). The post-extrasystolic pause of 1,713 ms ends with a single ventricular paced beat (VP) at the lower rate limit (35 beats/min) initiating a VT with slight cycle length irregularities at onset (VS 415-380 ms) until a stable cycle length of 350 ms (170 bpm) is present. (B) The interval plot of the VT episode with the programmed detection windows (VT threshold 375 ms/VF threshold 273 ms). A sudden onset may be declared "gradual" if the rate transition is five or more beats before the episode onset (third consecutive fast beat = VT 338). Slow "VS" beats (415-400-380 ms) delay the declaration of the episode onset. The pre-onset interval buffer comprises 11 intervals leading to episode onset. The baseline interval average was calculated to be 858 ms (four-interval average beginning with the sixth interval prior to the pivot interval: 918 + 388 + 1,713 + 415 ms = 3,434 ms/4 = 858 ms). The pivot interval is the VT interval at onset with the largest decrease (VS 380-VT 308 ms = 72 ms pivot delta). The second phase interval is determined as the second longest (338 ms) of the four intervals beginning with the pivot interval. The baseline interval average minus the second phase interval was 520 ms. The onset interval was determined as the percentage of the baseline interval average (72/858 = 8%) not exceeding the programmed threshold. Consequently, onset was classified as "gradual" (Gradl V-Epsd), whereas the stability criterion was met. Having the combination onset (9%) AND stability (30 ms) programmed together with SRD OFF, adequate ATP therapy was inhibited until the VT was terminated by external cardioversion after 48 min Strips display the right ventricular electrocardiogram (EGM) on top, the shock EGM below, and the marker channel on the bottom.
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F1
The VT was preceded by an SLS sequence when a premature ventricular complex (VS coupling interval 388 ms) was followed by a long pause (1,713 ms) ending with a paced ventricular beat (VP, Figure 1).