Elucidation of Speckleplethysmography as an Alternative to Photoplethysmography for Deriving Blood Pressure
Abstract
Speckleplethysmography (SPG) is a recently realized biosignal to measure heart rate variability using a technique called laser speckle contrast imaging (LSCI). Compared to electrocardiography (ECG) and photoplethysmography (PPG), which are the signals used for deriving continuous non-invasive blood pressure (BP), SPG is under explored and researched, especially concerning its relationship with BP. Studies into the relationship between SPG and PPG have revealed that SPG has a higher signal to noise ratio, less sensitivity to melanin content in skin for recording, and cleaner readings in colder environments than PPG. Additionally, a clear correlation is demonstrated between the two signals not only in their natureof blood flow measurement but in their signal peaks relative to ECG. SPG has also been proven to be low cost in components to record and is able to be adapted onto mobile or wearable platforms, making it ideal for future remote and continuous systems. To demonstrate the potential for SPG to be a viable substitute for PPG in BP measurement, several experiments were conducted using a custom phone system and compared to simultaneously recorded PPG and ECG signals. From these signals, the pulse transit time (PTT) was calculated from the peak to peak time delay between the ECG waveform and the PPG and SPG signals. The PTT values from the ECG-SPG and ECG-PPG pairings were used to estimate BP and compared to a commercial gold standard device: the Omron inflatable cuff. From the proposed experiment, SPG was able to be used as a comparable substitute for PPG in PTT based BP estimation as its results produced a similar or smaller error than the BP predicted from PPG. This experiment shows the potential to create an alternative BP system that is simple, accessible, and cost effective for users and researchers alike. To build upon the findings in this thesis, more experiments with larger datasets and different mathematical models capturing the relationship between BP and PTT will be conducted to further explore the connection between SPG and BP.