The study population was 494 patients, 130 (26.3%) of whom were enrolled in the ICD-EGMs study and 364 (73.7%) of whom were enrolled in the PROSE-ICD study. Clinical characteristics of the patients are presented in . During the mean follow-up of 3.4±1.9 years, 58 patients (3.7% per person-year of follow-up) experienced VT/VF with appropriate ICD therapies. MMVT events were detected in 46 patients (9.3%), and PVT/VF events were detected in 12 patients (2.43%). Two patients had both MMVT and PVT/VF events. SVT events were detected in 25 patients (5.1%) in isolation, and an additional 16 patients had both MMVT and SVT events. There was no overlap between the PVT/VF and SVT patient sets (). Patients with PVT/VF had lower left ventricular ejection fraction, and ischemic cardiomyopathy patients with PVT/VF events were less likely to be revascularized than other subgroups (). We observed no other differences in clinical variables.
Clinical characteristics of patients with and without arrhythmia events
Flow diagram showing the distribution of spontaneous events in study participants. NSVT=non-sustained ventricular tachycardia
is a study flow chart. Of 1386 events extracted from the memories of 83 patients’ ICDs, 1022 were excluded due to ventricular pacing or frequent ventricular ectopy before the onset of a VT/VF event, or aborted without ICD therapy VT/VF events, and 364 events were analyzed. There were 213 episodes of MMVT with a cycle length of 326±60 ms, 70 episodes of PVT/VF with a cycle length of 258±73 ms, and 81 episodes of SVT with a cycle length of 418±130 ms. On average 148±75 beats were analyzed at baseline, 6±3 beats before VT/VF event onset, and 6±4 beats before SVT event onset.
Baseline relative intracardiac J-point amplitude, and J-point amplitude on surface ECG
The baseline relative intracardiac FF EGM J-point amplitude did not differ significantly between patients who sustained VT/VF during follow-up and those who did not (−0.095±0.237 vs. −0.145±0.889; p=0.468). A positive relative intracardiac J-point amplitude ≥0.005 was found in 52 (10.5%) patients at baseline, who had a significantly higher rate of VT/VF events during follow-up than did patients with an isoelectric or negative J-point (intracardiac J-point amplitude < 0.005) [23.1% vs. 10.0%, P=0.047].
We found significant J-point elevation on inferior ECG leads in 16 (3.24%) patients, on lateral leads in 22 (4.45%) patients, and on both inferior and lateral leads in 5 (1.0%) patients. Although there was a trend toward more frequent VT/VF among patients with baseline J-point elevation on surface ECG, the difference did not reach statistical significance.
To investigate correlations between J-point amplitude on surface ECG and on FF EGM, we performed linear regression analysis only on PROSE-ICD study data. We took this step to diminish bias in measurements due to the dynamic nature of the J-point amplitude. Only one patient had significant J-point elevation on both the surface ECG and the FF EGM, and there was no correlation between the baseline relative intracardiac J-point amplitude and the relative J-point amplitude on surface ECG ().
Scatterplots of baseline relative intracardiac FF EGM J-point amplitude by relative J-point amplitudes on 12 ECG leads.
Surface ECG and intracardiac J-point amplitude did not differ significantly among clinical subgroups (). At baseline in patients with subsequent PVT/VF there was weak negative correlation between heart rate and relative J-point amplitude (r= − 0.134; P=0.03). Interestingly, in patients with subsequent SVT there was positive correlation (r=0.327; P=0.02). There were no significant correlations between relative J-point amplitude and heart rate immediately before arrhythmia onset.
Baseline intracardiac J-point amplitude, by demographics and clinical characteristics
Surface ECG filtering experiment results
Bandpass filtering (3–60 Hz) of the ECG signal did not change the absolute J-point amplitude (), but significantly attenuated relative J-point amplitude (). Bland-Altman analysis23
() showed a modest agreement between the regular signal (high-pass filter 0.05 Hz) and the filtered (bandpass 3–60 Hz) ECG relative J-point amplitude (Bias −0.057 [−0.160; 0.055]). Importantly, non-filtered and filtered relative J-point amplitude significantly correlated (Pearson’s r
= 0.60; P<0.0001; ).
Pre-onset relative J-wave amplitude
illustrates data recorded in a patient who experienced PVT during follow-up. There was no J-point elevation on the baseline surface ECG (), but there was a small J-wave on baseline FF EGM () and a dramatically enlarged J-wave immediately before the onset of PVT (). We observed characteristic changes in T-wave morphology before PVT/VF events. Intracardiac FF EGM T-wave is biphasic as it represents a filtered signal. Although the typical baseline FF EGM T-wave morphology is characterized by first negative and second positive phases (, ), the T-wave immediately before the onset of PVT was frequently (in 65.7%) characterized by the first positive phase, but second negative phase (). As compared with baseline, changes in T-wave morphology and augmented intracardiac J-wave were observed before the onset of PVT events. Relative intracardiac J-point elevation > 0.005 was seen before 58 out of 70 (82.9%) PVT/VF events. In contrast, a predominantly negative or isoelectric J-point was observed before the onset of MMVT and SVT events. Relative intracardiac J-point elevation > 0.005 was seen before 108 out of 213 MMVT events (50.7%), before 57 out of 81 SVT events, and before 67 out of 308 baseline recordings (21.8%).
Figure 5 Representative example of an FF EGM and a surface ECG recorded in the study patient with a spontaneous sustained polymorphic ventricular tachycardia (PVT) event during follow-up: A. Baseline FF EGM in sinus rhythm. Small J-wave is visible. B. FF EGM immediately (more ...)
In paired analysis () relative intracardiac J-point amplitude was significantly greater before MMVT and PVT/VF than before the baseline and SVT events, but no significant differences in these parameters were observed when comparing baseline to pre-SVT events. There was a trend toward more frequently observed elevated intracardiac J-point amplitude before PVT/VF in post-MI patients with ischemic cardiomyopathy, as compared to patients with non-ischemic cardiomyopathy [5/7 Post-MI patients (71%) vs. 1/5 NICM patients (20%), P=0.08].
Paired comparison of relative J-point amplitude
Mixed-effects logistic regression
In a mixed-effects logistic regression model, adjusted for multiple episodes per patient, each 10% increase in relative J-point amplitude increases the odds of having VT/VF by 13% [OR 1.13 (95% CI 1.07–1.19); P<0.0001] and increases the odds of having PVT/VF by 27% [OR 1.27 (95% CI 1.11–1.46); P=0.001].