Preliminary Study on Establishing a Heart Rate Variability-based Method for Objectively Evaluating Bone Metastasis Pain

The aim of this study was to establish an objective evaluation method for pain due to bone metastasis, based on heart rate variability (HRV). In this prospective study, patients who underwent radiotherapy for painful bone metastases were enrolled. Pain was assessed using a numerical rating scale (NRS), and anxiety and depression were evaluated using the Hospital Anxiety and Depression Scale (HADS). Autonomic and physical activities were evaluated by measuring HRV using a wearable device. NRS, HADS, and R-R interval (RRI) values were obtained upon starting, completing, and 3-5 weeks after radiotherapy. Between July 2020 and July 2021, 11 patients were enrolled. The median average NRS score was 5 (range=2-10). HADS-assessed median anxiety and depression scores were 8 (range=1-13 and 2-21). For patients with an NRS score ≥4, NRS score was significantly associated with low-frequency/high-frequency (LF/HF) component ratio (p=0.03). Heart rate during physical activity was significantly higher than resting heart rate; however, mean resting LF/HF was significantly higher than LF/HF during physical activity. During rest, excluding patients with a HADS depression score ≥7 in an NRS score 1-3, there was a trend for a positive correlation between the NRS score and the mean LF/HF (p=0.07). HRV measurements can objectively evaluate pain due to bone metastasis. However, we must consider that the effects of mental status, such as depression, on LF/HF also affect HRV in patients with cancer with mild pain.