Effects of cold stimulation on the harmonic structure of the blood pressure and photoplethysmography waveforms

Cold stimulation (CS) accompanied by vasoconstriction can increase the stiffness of the arterial system. The vascular responses during CS have been studied using photoplethysmography (PPG), but most have focused on time-domain waveform indexes. Focusing on the radial arterial blood pressure (BPW) and the finger PPG waveforms, we aimed to determine if harmonic index could help to noninvasively characterize the changes in arterial pulse transmission following mild CS.

Trials were measured on male healthy volunteers (n=29); mild CS was applied by placing a bag filled with 2000 cc of water at a temperature of 19-21°C around the right lower arm. For each experiment, we recorded a 3-min baseline-data sequence, applied local mild CS and recorded a 3-min effect sequence, and then recorded another 3-min effect sequence immediately after stopping CS. BPW and PPG spectra were used to calculate the amplitude proportion (C(n)) and phase angle (P(n)) for each harmonic (for n=1-10) from averages of all the pulses during the measurement period.

Several harmonic indexes were prominently increased following CS, including C(4)-C(10) and P(3)-P(10) for the BPW and C(5)-C(10) and P(3) and P(4) for PPG waveforms.

It was demonstrated that the application of mild CS significantly changes several harmonic-analysis indexes of the BPW and the PPG waveforms. By providing detailed information about the pulse transmission of each frequency component, harmonic analysis may improve the ability to detect arterial elastic properties induced by CS, other forms of external stimulation, or pathological factors. It may be pertinent to the development of medical devices for application in telemedicine.