Laser Speckle for Optical Monitoring of Pulsatile Blood Flow

Photoplethysmography (PPG) is the primary optical technique used for pulse oximetry and heart rate variability (HRV) measurements. The accuracy of PPG is limited by vasoconstriction, motion artifact, and increased melanin concentration. Recent work demonstrates the promise of PPG for the early detection of injured individuals at risk for hemorrhage. However, the inability to distinguish blood loss from other forms of blood volume loss (e.g., dehydration) limits its performance. A new pulsatile blood flow measurement based on laser speckle imaging, speckleplethysmography (SPG), can overcome multiple limitations associated with PPG. We developed an eight-layer tissue model to simulate both PPG and SPG signals via Monte Carlo methods. The simulations match our in vivo results measured with a two-camera set up for simultaneous imaging of PPG and SPG. Both in silico and in vivo data suggest SPG has a much larger signal-to-noise ratio than PPG, which may prove beneficial for noncontact, wide-field optical monitoring of cardiovascular health. We also compared SPG and PPG to ECG for estimation of HRV during an orthostatic challenge performed by 17 subjects. We found that SPG estimations of HRV are highly correlated to ECG HRV for both time and frequency domain parameters and provide increased accuracy over PPG estimations of HRV. The results suggest SPG measurements are a viable alternative for HRV estimation when ECG measurements are impractical.Lastly, we developed a novel, multi-wavelength coherent spatial imaging (CSI) prototype for comparisons to diffuse optical spectroscopy and diffuse correlation spectroscopy (DOS/DCS). CSI corrects relative measurements of blood flow and oxygenation with optical properties to create absolute measurements. CSI is significantly less expensive (~$200 off-the-shelf) and more portable than DOS/DCS (~$60,000 off-the-shelf). The small probe form factor enables convenient placement of the probe onto the human finger for continuous monitoring similar to current pulse oximeters. The preliminary in vitro phantom results from CSI match gold standard scattering and absorption results (R> 0.95 for all three wavelengths). The absolute measurements of SPG, oxygenation, and hemoglobin concentration are similar to the DOS/DCS results during an in vivo arterial occlusion. These preliminary results suggest CSI may be useful for continuous critical care monitoring