INTRODUCTION

Ventricular fibrillation (VF) is normally a sustained condition, and electrical shock is required to end it. However, we do sometimes see rhythms that appear to be VF on the electrocardiograph, but which stop spontaneously. These rhythms have been described as polymorphic ventricular tachycardia or non-sustained VF,1 and their definition is a question of semantics, since from a clinical point of view and according to the electrocardiograph, they are nearly indistinguishable. The reasons that determine the prolongation or spontaneous termination of these arrhythmias are unknown but profoundly interesting, since they could provide information regarding VF mechanisms and aid in the development of new algorithms in implantable cardioverter defibrillators (ICDs) to avoid treating self-resolving fibrillations. The purpose of this study is to evaluate the spectral characteristics of sustained and non-sustained VF episodes in patients fitted with an ICD.

Patients and Episodes

We reviewed the information from all of the patients whose defibrillators were being monitored in 2 tertiary hospitals. The study includes those patients who had logged at least one sustained fibrillation episode and another non-sustained fibrillation episode, whether it was spontaneous or induced. VF2 is defined as ventricular tachyarrhythmia with changeable and irregular morphology and a mean cycle length <300 ms. VF is considered to be sustained when it is present during at least 5 s and when a shock is required to end it; it is non-sustained when it lasts >3 s and terminates spontaneously (the duration >3 s is required to permit a reliable mathematical analysis of the signal oscillations). The electrical signal analysed during VF is obtained from the digital electrogram stored in the device.

Signal Processing

The analysis period spanned the 3 initial seconds of the VF and its onset was established as the moment when the recorded signal became free from interferences produced by the shock or the 50 Hz current employed in induction. Monopolar configuration was used to register the potential difference between the active ICD case and the electrode tip, as this is an electrical source that is not as limited as the distal bipole. Each episode that the device classified as VF was visually analysed to confirm that it was indeed a fibrillation rhythm. P(f) spectral representation was obtained using Welch's periodogram cancelling the baseline, Hamming window (256 samples), a 50% overlap and 1024 resolution samples. The spectrum thus obtained was normalised in the module in order to have a unitary area. As frequency parameters, we estimated the dominant frequency(fd), corresponding to the absolute maximum of the spectrum module, and the peak potentials for the fd and the second and third harmonics: Pn(fd), Pn(f2), and Pn(f3), respectively. As a means of organisation we used the organisational index, defined as the quotient of the potential contained in the band corresponding to 75% of the dominant frequency and its harmonics and the total potential for the band in question (Figure).

Figure. Schema of the spectrum of the ventricular fibrillation signal and the calculated spectral parameters.

Statistical Analysis

Variables are described as the mean (standard deviation). The Student t test was used to compare means for paired data (P<.05 being significant).

RESULTS

Of the 243 patients fitted with ICDs who were evaluated, 56 presented at least one episode of VF with spontaneous termination and one ended using shock; 5 received the shock before 5 s, for which reason their significance is doubtful and they were excluded from the study. The 51 remaining patients (44 males; aged, 61.7 [12.8]) constituted the analysed population. Two sustained VF episodes and 3 non-sustained VF episodes were spontaneous, and the rest were induced during implantation (by applying 50 Hz to 44 patients and shock on T to 2 others). Given that the number of patients whose VF was induced by shock on T is quite small, this variable was not analysed in this study. Ninety-four point one percent of the patients had ischaemic cardiopathy, and in 24 the LVEF was <30%. The total duration of the episodes that were analysed was 8.7 (1.7) s for sustained episodes and 6.3 (2.5) s for those that ended spontaneously. The overlap in the duration of sustained and non-sustained episodes is due to the variations in condenser charging time according to the programmed level of energy, the state of the battery and the device make and model. Spectral parameters of both groups are shown in Table. The sustained episodes present a significantly larger fd than the non-sustained ones (4.6 [0.7] compared with 4.3 [0.6] Hz respectively). Powers for fd, the harmonics, and the OI were similar in both groups.

DISCUSSION

Our results indicate that sustained VF episodes in patients fitted with an ICD have a higher fd than non-sustained episodes in the first 3 s of arrhythmia. However, we did not find significant differences in either the potentials or the organisational spectral parameters.

Although polymorphic ventricular tachycardias can prolong and become VF, the episodes often end spontaneously. Classifying these episodes as polymorphic tachycardia, torsades de pointes or non-sustained VF is a problem of terminology, since there are no clear morphological or frequency criteria that would allow us to distinguish between these conditions using electrocardiography.1 Spectral analysis of these rhythms indicates that there is a certain degree of underlying organisation that becomes translated as the appearance of defined spectral peaks.3-5 Mäkikallio et al2 analysed 7 patients fitted with similar ICDs to those used by our patients. The analysis employed non-linear dynamic methods and the team found better organisation in the first beats of self-resolving episodes compared with in sustained ones, although the mean cycle of the intervals, which corresponds to fd, does not show significant differences. The low number of patients analysed, the use of bipolar electrograms and the different algorithms that were used could explain this discrepancy. In a preliminary analysis of 18 patients with ICD, using manual cycle measurement, it has been described that the non-sustained VF cycle was larger than that of sustained episodes, reaching statistical significance at the start of the sixth interval.6 The present study confirms these findings in a higher number of patients by using spectral analysis.

Limitations of the Study

Although our study includes a larger number of patients than do similar studies described in the literature, it is still relatively small. It is possible that a larger sample would have shown significant differences in the organisation of the arrhythmia. On the other hand, most of the episodes were induced using a 50 Hz current that affects the EGM, for which reason it is not possible to determine the exact moment of VF induction, and its relation with the analysed period is only approximate. Thirdly, spectral analysis was limited to a single derivation, obtained from electrodes situated in the right ventricle, and so we did not have information about the characteristics of the VF signal in different zones. Obtaining simultaneous records from various spots on both ventricles could significantly improve the sensitivity of the analysis. Finally, some episodes that were included in the sustained VF group because treatment was received might have resolved spontaneously without applying shock. This limitation is inherent to the design of the study and can only be remedied by mapping fibrillation during extracorporeal circulation.

To conclude, during the first 3 s, although spectral characteristics of sustained and non-sustained VF episodes are similar, they do have differences that could be used in designing algorithms for the early detection of self-resolving VF in patients fitted with an ICD.

This study was funded in part by Boston Scientific research projects and the Spanish National R&D&i plan (TEC2007-68096C02-TCM).

Correspondence: Dr. J.J. Sánchez Muñoz.
Periodista Antonio Herrero, 25 B, esc. 6, 3.o M. 30007 Murcia. España.
E-mail: jjsanchezmunoz@gmail.com

Received February 22, 2008.
Accepted for publication July 21, 2008.