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Relationships between pain intensity and heart rate variability in patients after abdominal surgery: a pilot study.

BACKGROUND
A link between postoperative pain intensity and heart rate variability (HRV) had not been well established. This study aimed to investigate the correlation between post-operative pain intensity and HRV.


METHODS
The subjects in this cross-sectional correlation study comprised of patients who had undergone abdominal surgery in a regional teaching hospital in central Taiwan during the period July 2009 - November 2009. The visual analogue scale (VAS) and the short-form McGill pain questionnaire (SF-MPQ) were used to measure post-operative pain. HRV was measured as the standard deviation of normal RR interval, and by power spectral analysis that included high frequency (HF), low frequency (LF), very low frequency power, and LF/HF ratio.


RESULTS
A total of 34 subjects were included in this study. We found that the day after the surgery, the mean VAS score was 47.50 ± 20.98 and the mean SF-MPQ score was 18.06 ± 8.90, indicating a moderate degree of pain. Moderate to severe degrees of tenderness were reported by 70.6% of the patients, moderate to severe degrees of gnawing pain were experienced by 67.7% of the patients, moderate to severe degrees of tiring-exhaustion pain were reported by 64.7% of the patients, and 41.2% of the patients who experienced moderate to severe pain believed that the pain was punishing-cruel. The standard deviation of normal RR interval and high frequency values obtained from male patients or married patients were higher than female patients or unmarried (P < 0.05). The correlation of the standard deviation of normal RR interval, high frequency, very low frequency value and patient's age were negative (P < 0.05). The total SF-MPQ pain scores positively correlated with the LF/HF ratio (P < 0.05).


CONCLUSIONS
The multidimensional pain assessment tool (SF-MPQ) reflects better the patients' post-operative pain than the single-dimensional assessment tool (VAS). HRV positively correlated with SF-MPQ scores in patients after abdominal surgery.