Heart Rate Variability as an indicator of nociceptive pain in disorders of consciousness?

Background: Heart rate variability (HRV) has been proposed as an indicator of nociceptive pain processing1 although its reliability as pain indicator remains under debate. The objective was to study the interest of an HRV complexity analysis2 method as an indicator of nociceptive pain processing in severely brain-injured patients with disorders of consciousness.
Methods: Twenty-two patients (11 in minimally conscious state [MCS], 11 in a vegetative state/unresponsive wakefulness syndrome [VS/UWS]) and 14 healthy subjects [HS] were included in this study. We administered a non-noxious and a noxious stimulation while recording the electrocardiographic response was recorded before, during, and following non-noxious and noxious stimulation. The short-term Complexity Index (CIs) was calculated. Mann-Whitney and Wilcoxon’s test were used to investigate differences in CIs according to the level of consciousness (i.e., HS vs patients and VS/UWS vs MCS) and the three conditions (i.e., baseline, non-noxious, noxious). The correlation between the three conditions and the Coma Recovery Scale-Revised3 were investigated by Spearman’s correlations.
Results :We observed higher CIs values in HS as compared with patients during baseline and following the noxious stimulation. We also found higher CIs values in MCS vs VS/UWS patients following the noxious condition and lower values in the noxious vs non-noxious condition solely for the VS/UWS group. A correlation was found between CIs in noxious condition and Coma Recovery Scale-Revised scores.

Conclusion: our data suggest a less complex autonomic response to noxious stimuli in VS/UWS patients4. Such analysis may help to better understand sympathovagal response to potentially painful environmental stimulation in brain-injured patients.