Heart rate variability (HRV) is in inverse association with inflammatory markers, which is, for this reason, an indicator of the anti-inflammatory reactivity of the immune system. (1) In the most severe COVID-19 cases with poor outcome an increased neutrophil and a decreased lymphocyte and monocyte number was measured. (2)(3) It is a striking peculiarity that HRV – modulated by stress levels – is positively correlated with monocytes and lymphocytes and negatively correlated with neutrophils. (4) This association may be used for improving the immunological profile by increasing the depressed HRV of severely affected patients. Central proinsulin C-peptide may be one of the most interesting candidates for increasing low HRV. (5) It may increase splenic lymphocyte proliferation that is suppressed by the sympathetic nerve (6)

1. https://pubmed.ncbi.nlm.nih.gov/30872091/
2. https://www.medrxiv.org/content/10.1101/2020.02.18.20024364v1
3. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7108125/
4. https://pubmed.ncbi.nlm.nih.gov/31408866/
5. https://pubmed.ncbi.nlm.nih.gov/8781764/
6. https://pubmed.ncbi.nlm.nih.gov/11151760/