Relationship Between Finger Photoplethysmographic Pulses and Skin Blood Perfusion

Introduction Photoplethysmography (PPG) measures are important in monitoring peripheral oxygen saturation (SpO₂). Another parameter is a derived quantity referred to as the peripheral perfusion index (PPI). It is calculated as the ratio of the peak-to-peak pulse amplitude of a PPG signal (PA_PPG) to the non-pulsating part of the total PPG signal. The PPI has been used as a marker of blood perfusion states in a variety of clinical settings but has not been systematically and directly compared to measures of local blood perfusion. This study's purpose was to investigate this issue to provide initial data on the relationship between finger skin blood perfusion, measured by laser Doppler blood perfusion flux (LDF) and PA_PPG. Methods Ten subjects (five male), recruited from medical students with an average age of 26 years, participated. While supine for 30 minutes, skin blood perfusion was recorded using laser Doppler flux (LDF) on the ring finger pulp of the non-dominant hand, and the photoplethysmography pulse (PPG) was recorded from the index finger of the same hand. The recorded data was searched sequentially manually to locate the first 30-pulse sequence in which the PPG amplitude of at least six PPG pulses was less than or equal to 60% of the maximum pulse amplitude in the sequence. The primary PPG parameter of interest was PA_PPG. For the LDF signal, the pulse amplitude is designated as PA_LDF, the total LDF for each pulse is designated as LDF_TOT, and the LDF pulsatile component is designated as PF. To investigate the relationship between LDF parameters and PA_PPG a linear regression analysis of each 30-pulse sequence was done with PA_PPG as the independent variable and each of the three LDF parameters individually (PA_LDF, LDF_TOT, and PF) as dependent variables.  Results There was a statistically significant direct relationship between PA_PPG and all three measures of blood perfusion (p<0.05). Correlation coefficients (R) varied among subjects but within-subject variations versus PA_PPG were similar, having mean values that ranged from 0.665 to 0.694. The results also provided evidence in support of a direct relationship between the LDF pulsatility index, defined as the ratio of PF to its mean value., and PA_PPG (R=0.779). Conclusions When finger PPG pulse amplitudes are measured in individual subjects there is a moderate-to-strong correlation between the PPG pulse amplitude changes and skin blood perfusion changes. This fact impacts the confidence in using the widely available PPG parameter, peripheral perfusion index, as an indicator of changes in tissue perfusion. However, differences in the PPG pulse amplitude among subjects were less reliable indicators of differences in blood perfusion among subjects. The findings also indicate that a related parameter, the LDF pulsatility index, is also highly correlated with the PPG pulse amplitude and may serve as a useful parameter for future clinical investigations.