RESUMO
OBJECTIVE: When patients cannot indicate pain, physiological parameters may be useful. We tested whether heart rate variability (HRV) parameters, as reflection of sympathetic and vagal tone, can be used to quantify pain intensity. DESIGN: Prospective study. SUBJECTS AND SETTING: A standardized heat stimulus was applied to the forearm in 75 healthy volunteers during three study periods of 2 minutes. METHODS: Before and after each application, pain intensity was measured by a visual analog scale (VAS) and inter beat interval (IBI) was recorded. Standard deviation of normal to normal beat intervals (SDNN) of the IBI, the power of the low (LF, 0.07-0.14 Hz) and high frequency (HF, 0.15-0.50 Hz) band, and LF/HF ratio were calculated. Log transformation resulted in normal distribution. Correlation between HRV parameters and pain intensity was assessed by Pearson's correlation coefficient. RESULTS: Data from 73 volunteers (44 women) could be analyzed. The mean age was 30 ± 11 years. Compared with baseline, during all heat periods, pain intensity measured by VAS increased from 2 ± 3 mm, 3 ± 5 mm, and 2 ± 4 mm, to 40 ± 20 mm, 42 ± 21 mm, and 44 ± 22 mm, respectively. Log transformed SDNN (lnSDNN) and LF (lnLF) decreased; lnSDNN from 4.0 ± 0.4 to 3.9 ± 0.5, P = 0.002; 4.0 ± 0.4 to 3.9 ± 0.5, P = 0.016; and 4.1 ± 0.4 to 3.9 ± 0.4, P = 0.004, respectively; lnLF from 6.3 ± 1.0 to 6.1 ± 1.2, P = 0.001; 6.4 ± 1.0 to 6.2 ± 1.1, P = 0.019; and 6.5 ± 1.0 to 6.2 ± 1.1, P = 0.020, respectively. No correlation of any HRV parameter with VAS score was found. CONCLUSION: HRV parameters may detect responses to heat pain, but are not suitable to assess pain intensity.
