Heart Rate Variability and Sensitivity to Experimentally Induced Pain: A Replication

To the Editor, The neural systems involved in autonomic control and perception of pain are closely coupled, and extensive interactions between these sets of neural structures can be observed. The functional interaction of these systems is an important component of the pain regulatory process. Blood pressure and heart rate are both products of autonomic reactivity, and have been studied to specify the relationship between pain stimuli and autonomic reactions. Heart rate variability (HRV), by contrast, attempts to tease out the relative contributions of sympathetic and parasympathetic activity underlying autonomic reactivity, and therefore bears great potential to study autonomic nervous system (re)activity in experimental pain research. However, while several experimental studies have explored the effect of experimentally induced pain on HRV, only 2 previous studies have treated resting HRV as an independent variable, to investigate interindividual differences in pain sensitivity. Appelhans and Luecken found that HRV was associated with ratings of pain unpleasantness but not pain intensity in subjects receiving painful cold stimulation. In a recent study published in Pain Practice, NahmanAverbuch et al. found that the relationships between parasympathetic function, pain perception, and pain modulation were prominent only in individuals with higher anxiety, where higher parasympathetic arousal was associated with higher ratings of tonic heat pain and a more efficient conditioned pain modulation capacity (ie, a decrease in sensitivity to pain). In this letter, we aimed to replicate previous findings by Appelhans and Luecken to further elucidate the association of pain sensitivity and resting autonomic function. Data for this analysis was taken from the first session of a previous study investigating the 2-week test-retest reliability of the cold pressor task (CPT). Healthy undergraduate students were recruited at the SRH University Heidelberg from September 2012 through January 2013. Participants provided written informed consent and completed several questionnaires. Self-rated health (S-RH) was measured using the question “How do you rate your current health status? ” on a 0 (“very bad”) to 6 (“excellent”) scale. Only subjects indicating an S-RH of ≥ 3 (“fair”) were included in the trial. The sample of participants included in this analysis consisted of 28 female and 6 male, healthy undergraduate students. The age range was 20 to 31 years (mean 22.85 years, SD = 2.46). Each participant was given a day of the week (Monday through Friday) and a time (between 9:00 am and 6:00 pm) when assessments would be made. Date and time of measurements were recorded by a protocol. Before the CPT, HRV was measured. Sociodemographic variables were assessed using a self-developed questionnaire. All participants received class credits or an allowance of 20€ for completion of the study. Heart rate variability was measured for 5 minutes in a sitting position immediately before hand immersion into cold water. A Polar RS800CX portable device (Polar Electro Inc., Bethpage, NY, U.S.A.) using a transmitter consisting of a stable polyamide case with electrodes attached to an elastic belt fixed to the chest of the subjects was used to record interbeat intervals (IBIs) at a sampling frequency of 1,000 Hz, providing a temporal resolution of 1 millisecond for each R–R interval. Device-specific software (Polar ProTrainer 5) was used to transfer recordings to a PC. IBI data (.txt files) were exported and analyzed using Kubios HRV (Biosignal Analysis and Medical Imaging Group, University Kuopio, Finland, Version 2.0). In line with the previous study, an autoregressive model was fitted to the RR-interval series, and the area under the power spectral density function within the low-frequency (LF; 0.04 to 0.15 Hz) and high-frequency bands (HF; 0.15 to 0.4 Hz) was derived, and underwent natural log transformation. The peak frequency of HRV power within the HF band was used as an estimate of respiratory rate. Cold pain sensitivity was assessed by immersing the nondominant hand up to the wrist in an acrylic glass tank with circulating water to prevent local warming. Water temperature was controlled constantly at 4°C (mean = 4.32°C; SD = 0.25°C) with a chilling device, and water pump, and measured with 3 digital thermometers at different spots. Subjects were told to keep their hands open rather than closed in a fist while they were immersed in the water. Before the immersion the subject was told to keep his or her hand in the water until the pain became intolerable, with a cutoff time of 4 minutes. The latencies to the first pain sensation (pain threshold) and to the intolerable pain (pain tolerance) were measured with a stopwatch in seconds. The