The sample size was estimated using the diagnostic performance obtained from the pivotal study. The spectral-based four-variable model using DF, HL-PkR , HL-PSDR, and the number of shocks delivered before ROSC, showed a diagnostic sensitivity of 0.94. Looking for a 95% confidence interval and 5% precision, and assuming that 61% patients will show FNP according to our previous research, a total 143 individuals (both prospective and retrospective cohorts) suitable for analysis must be included.