Heart Rate Variability as an Indicator of Nociceptive Pain in Disorders of Consciousness?

Heart rate variability is thought to reflect the affective and physiological aspects of pain and is emerging as a possible descriptor of the functional brain organization contributing to homeostasis. To investigate whether the short-term Complexity Index (CIs), a measure of heart rate variability complexity is useful to discriminate responses to potentially noxious and nonnoxious stimulation in patients with different levels of consciousness. Twenty-two patients (11 minimally conscious state [MCS], 11 vegetative state/unresponsive wakefulness syndrome [VS/UWS]) and 14 healthy controls (HC) were enrolled. We recorded the electrocardiographic response and calculated the CIs before (baseline), during, and after nonnoxious and noxious stimulation. Mann-Whitney and Wilcoxon's tests were used to investigate differences in CIs according to the level of consciousness (i.e., HC vs. patients and VS/UWS vs. MCS) and the three conditions (i.e., baseline, nonnoxious, noxious). The correlation between the three conditions and the Coma Recovery Scale-Revised was investigated by Spearman's correlations. We observed higher CIs values in HC as compared with patients during the baseline (P < 0.034) and after the noxious stimulation (P < 0.0001). We also found higher values in MCS versus VS/UWS patients after the noxious condition (P < 0.001) and lower values in the noxious versus nonnoxious condition solely for the VS/UWS group (P < 0.007). A correlation was found between CIs in noxious condition and Coma Recovery Scale-Revised scores. Our results suggest a less complex autonomic response to noxious stimuli in VS/UWS patients. Such method may help to better understand sympathovagal response to potentially painful stimulation in brain-injured patients.

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