Comparison of feature-based indices derived from photoplethysmogram recorded from different body locations during lower body negative pressure
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Objective
Various time domain features, including dicrotic notch ( dic ), in photoplethysmogram (PPG), and the pulse transit time (PTT) determined using the simultaneously recorded electrocardiogram (ECG), are believed to have a critical role with many potential clinical applications. However, the dependence of these parameters on PPG sensor location is less well known.
Approach
Three pulse oximetry probes (Xhale) were put simultaneously on the ear, nose, and finger in 36 healthy volunteers recruited for the lower body negative pressure (LBNP) experiment. The end point of the experiment is either the end of the -75 mmHg phase or development of hypoperfusion symptoms, whichever came first. Various features in the time domain of the recorded PPG signals were analyzed from the ear, nasal, and finger PPG at different LBNP phases. The simultaneously recorded finger PPG and ECG (Nellcor) is utilized to evaluate the dependence of PTT on the PPG sensor location.
Results
The signal quality is different among PPG signals recorded from different sites with the nasal and ear PPGs having the highest and lowest qualities respectively. Most feature-related indices evaluated from different sites are different, except pulse rate (PR). Specifically, the ratios of detectable dic are different, with finger and nasal PPGs being the highest and lowest respectively, and when dic is detectable, the e point and dic are also significantly different. Pulse rate variability-related indices and PTT exhibit differences when evaluated from different body locations, while we cannot conclude the behavior of PTT during different LBNP experimental phases.
Conclusion
Indices derived from PPG signals depend on where the PPG sensor is placed on the body. Researchers should exercise caution when extrapolating conclusions from PPG recordings taken at one body location to those obtained from a different location. This is particularly crucial in the digital health era, characterized by the proliferation of mobile devices equipped with PPG sensors positioned across various body sites.
