Photoplethysmographic Signals to Predict the Success of Lumbar Sympathetic Blockade for Lower Extremity Pain and Cold Hyperalgesia

Abstract

Background: Although lumbar sympathetic blockade (LSB) is a valuable modality to manage pain in the lower extremities, it is difficult if LSB will result in pain alleviation before the procedure. In this study, we investigated how signals of photoplethysmography (PPG) change during LSB. Furthermore, we established which patients would respond to LSB through a power spectrum analysis (PSA). Methods: A prospective, observational study was performed with 38 patients who had single leg pain with cold hyperalgesia. Bilateral temperature and PPG signals were recorded during LSB. PPG data including low- (LF) and high-frequency (HF) DC signals were analyzed blindly by a technician. The PSA was attempted using a fast Fourier transform periodogram blindly by a technician. Results: Eight (22.1%) patients had excellent pain-relieving response to LSB, and they were considered as having sympathetically mediated pain (SMP). AC signal changes of PPG occurred immediately after drug administration even before temperature changes were noted. DC signals were decreased slowly with a linear fashion. The PSA of DC signals showed that significantly lower LF/HF ratios in SMP than in sympathetically independent pain (SIP), both before LSB (2.81 ± 1.60 vs. 4.99 ± 3.12, P = 0.038) and after LSB (1.77 ± 0.82 vs. 3.21 ± 2.03, P = 0.023). Conclusions: PPG can be used as an early indicator of a successful LSB in the clinical setting. Indeed, PPG signals can be used to distinguish SMP from SIP before performing LSB.