Acute pain increases heart rate: differential mechanisms during rest and mental stress.

The main aim was to investigate if acutely stressed subjects have abnormal heart rate variability responses to acute pain. Efferent cardiac autonomic activity was assessed by analyzing RR interval variation in 26 male volunteers. Heart rate variability was measured as mean and standard deviation of normal RR intervals (mean RR, SDNN) and by power spectral analysis where high frequency (HF) and low frequency (LF) power were used as indexes of vagal function and of sympatho-vagal interaction, respectively. Coefficient of component variance in the LF and HF bands (CCV-LF, CCV-HF) was estimated to adjust for possible influences of different mean RR levels on power amplitude. Subjects received painful and non-painful sural nerve stimulations during rest, during attention to pain, and during mental stress. Our results show that pain significantly decreased mean RR and increased LF power and CCV-LF during rest and during attention to pain. SDNN, HF power, and total power were not affected by pain. During mental stress, pain significantly decreased mean RR but failed to affect other heart rate variability parameters. We conclude that acute pain induced efferent cardiac sympathetic activation during rest and during attention to pain as LF power and CCV-LF increased without alterations of pure vagal heart rate variability measures. During mental stress, pain inhibited mean RR without changing heart rate variability measures suggesting that pain does not increase efferent cardiac sympathetic activity during mental stress. Pain induced decrease of mean RR during mental stress may be caused by the release of catecholamines into the systemic circulation.