Spectral analysis of the microcirculatory laser Doppler signal at the Hoku acupuncture point

We aimed to characterize the frequency spectra of skin blood flow signals recorded at Hoku, an important acupuncture point (acupoint) in oriental medicine. Electrocardiogram (ECG) and laser Doppler flowmetry signals were measured simultaneously in 31 trials on seven volunteers aged 21–27 years. A four-level Haar wavelet transform was applied to the measured 20 min laser Doppler flowmetry (LDF) signals, and periodic oscillations with five characteristic frequency peaks were obtained within the following frequency bands: 0.0095–0.021 Hz, 0.021–0.052 Hz, 0.052–0.145 Hz, 0.145–0.6 Hz, and 0.6–1.6 Hz (defined as FR1-FR5), respectively. The relative energy contribution in FR3 was significantly larger at Hoku than at the two non-acupoints. Linear regression analysis revealed that the relative energy contribution in FR3 at Hoku significantly increased with the pulse pressure (R² = 0.48; P < 0.01 by F-test). Spectral analysis of the flux signal revealed that one of the major microcirculatory differences between acupoints and non-acupoints was in the different myogenic responses of their vascular beds. This information may aid the development of a method for the non-invasive study of the microcirculatory characteristics of the acupoint.