IL6 and hydrops fetalis: A study on the prognosis of coronavirus disease 2019 (COVID-19) patients based on HRV [16] concluded that a decrease in the activity of the autonomic nervous system due to sympathetic nerve depletion (e.g., the low standard deviation of RR intervals) and dominance of the parasympathetic nervous system (e.g., low normalized HF component) were associated with poor prognosis, increased mortality, and elevated interleukin 6 levels [16].