Recent studies regarding adolescents found a negative correlation between HF-HRV and IL-6, and a positive correlation between LF-HRV and IL-6 [53]; negative associations between rMSSD, HF-HRV, cardiac vagal index, and soluble IL-6 receptor (sIL6R), while positive associations between LF/HF, cardiac sympathetic index, and sIL6R [54]. Here, IL6 is linked to hydrops fetalis.