Monitoring heart rate variability to assess experimentally induced pain using the analgesia nociception index: A randomised volunteer study.

BACKGROUND: Pain assessment using a numerical rating scale (NRS) is considered good clinical practice, but objective assessment in noncommunicating patients is still a challenge. A potential solution is to monitor changes in heart rate variability transformed into the analgesia nociception index (ANI), that offers a noninvasive means of pain quantification.

OBJECTIVES: The aim was to measure magnitudes, descending slopes and time courses of ANI following expected and unexpected painful, nonpainful and sham experimental stimuli and compare these with pain intensity as assessed by NRS in conscious human volunteers. We expected a negative correlation between ANI and NRS after painful stimuli.

DESIGN: Randomised stimuli and placebo-controlled, single-blinded study.

SETTING: Experimental pain simulation laboratory, Bochum, Germany.

PARTICIPANTS: Twenty healthy male students, (mean ± standard deviation; 24.2 ± 1.9 years) recruited via local advertising, were consecutively included.

INTERVENTION: ANI values were continuously recorded. After resting, four stimuli were applied in a random order on the right forearm (unexpected and expected electrical pain, expected nonpainful and sham stimuli). Blinded volunteers were asked to rate all four stimuli on NRS.

MAIN OUTCOME MEASURES: ANI means (0-100), amplitudes, maxima, minima and slopes with NRS pain intensity scores (0-10).

RESULTS: Resting alert volunteers showed ANI values of 82.05 ± 10.71. ANI decreased after a random stimulus (maximal decrease of 25.0 ± 7.3%), but different kinds of stimuli evoked similar results. NRS scores (median; interquartiles) were significantly (P = 0.008) higher after expected (5.25; 3.5-6.75) compared with unexpected (4.50; 3.0-5.0) pain stimuli. No correlation was found between ANI and NRS.

CONCLUSION: ANI did not allow a differentiation of painful, nonpainful or sham stimuli in alert volunteers. Therefore, ANI does not exclusively detect nociception, but may be modified by stress and emotion. Thus, we conclude that ANI is not a specific, robust measure for assessment of pain intensity.