Hypothesis
Directly recorded tonic activity in the cervical vagus nerve would correlate with standard heart rate variability metrics, which are used clinically as a proxy for vagal tone.
What failed
Chronic recordings from the left cervical vagus of anaesthetised and awake rats, made with carbon nanotube yarn electrodes, showed no correlation between tonic vagal activity and common HRV measures. A breathing-linked rise in vagal firing during inspiration was also uncorrelated with HRV.
Broken assumption
Heart rate variability is a faithful indirect measure of vagus nerve activity.
Why
The paper does not identify what HRV does track; it concludes that the link between HRV and 'vagal tone' needs to be reinterpreted.
From the published paper: Marmerstein et al. Scientific Reports, 2021. ↗CC BY