PV[O]H: Noninvasive Enabling Technology, New Physiological Monitoring, and Big Data.

INTRODUCTION: Measures of normal and abnormal physiology are interrelated and vary continuously. Our ability to detect and predict changes in physiology in real time has been limited in part by the requirement for blood sampling and the lack of a continuous data stream of various "signals", i.e., measurements of vital signs. It is important to determine which signals are most revealing for detection and treatment of, e.g., internal bleeding, managing fluid balance for mission/combat readiness, and hydration. Although our current algorithm for PV[O]H reflects changes in hematocrit and blood and plasma volumes, additional algorithms utilizing the whole raw PV[O]H data stream, along with other variables, can be constructed. We present a working prototype demonstrating that acceptable size, power, and complexity footprints for military needs can be achieved. Results of previous studies involving humans have demonstrated that PV[O]H can provide simultaneous, noninvasive, in vivo continuous monitoring of hematocrit, vascular volume, hemoglobin oxygen saturation, pulse rate, and breathing rate using a single light source with a reporting frequency of every 3 seconds. MATERIALS AND METHODS: We have engineered an instrument implementing the PV[O]H algorithm in which (1) single channel photodetectors replace multichannel detection; (2) optical filters replace gratings; (3) battery power is used; and (4) sufficient computation with input/output capability moderated by application specific graphical user interfaces, and compatible with all cloud, wireless environment, and local protocols is implemented. RESULTS: We have engineered a complete version of a two-probe PV[O]H system meeting military needs and have fabricated a first version. Testing of subsystems, calibration, and optical characterization of the optical probes are underway. CONCLUSIONS: Simultaneous noninvasive continuous monitoring of peripheral vessels using a previous PV[O]H system demonstrates large, physiology revealing data sets. The technologies enable the methodical search for relevant physiological signals allowing the use of discriminant analysis, Bayesian approaches, and artificial intelligence to create predictive algorithms enabling timely interventions in medical care and troop training.

Noninvasive Blood and Tissue Analysis: Raman Spectroscopy, One Perspective for Monitoring of Glucose and Beyond.

Noninvasive in vivo blood and tissue analysis remains a challenge to medical technology epitomized by the ongoing quest to replace fingerstick self-monitoring of blood glucose. Recent developments warrant comment on near-term prospects for using Raman spectroscopy to meet that challenge. These developments combined with 20 years of experimentation with noninvasive blood and tissue analysis suggest that it may be possible and practical to perform noninvasive in vivo glucose analysis with improvements in (1) the enabling technologies for making Raman measurements and (2) an underlying anatomical-physiological model of how in vivo spectroscopic measurements are made and interpreted. We review the substantial progress made toward meeting the challenge and the personal, public health, and economic implications of these ongoing efforts.

Coupled Turbidity and Spectroscopy Problems: A Simple Algorithm for Volumetric Analysis of Optically Thin or Dilute, In Vitro Bacterial Cultures in Various Media.

An approach binary spectronephelometry (BSN) to perform real-time simultaneous noninvasive in situ physical and chemical analysis of bacterial cultures in fluid media is described. We choose to characterize cultures of Escherichia coli (NC), Pseudomonas aeruginosa (PA), and Shewanella oneidensis (SO) in the specific case of complex media whose Raman spectrum cannot be unambiguously assigned. Nevertheless, organism number density and a measure of the chemical makeup of the fluid medium can be monitored noninvasively, simultaneously, and continuously, despite changing turbidity and medium chemistry. The method involves irradiating a culture in fluid medium in an appropriate vessel (in this case a standard 1 cm cuvette) using a near infrared laser and collecting all the backscattered light from the cuvette, i.e., the Rayleigh-Mie line and the inelastically emitted light which includes unresolved Raman scattered light and fluorescence. Complex "legacy" media contain materials of biological origin whose chemical composition cannot be fully delineated. We independently calibrate this approach to a commonly used reference, optical density at 600 nm (OD600) for characterizing the number density of organisms. We suggest that the total inelastically emitted light could be a measure of the chemical state of a biologically based medium, e.g., lysogeny broth (LB). This approach may be useful in a broad range of basic and applied studies and enterprises that utilize bacterial cultures in any medium or container that permits optical probing in the single scattering limit.

In vivo, noncontact, real-time, PV[O]H imaging of the immediate local physiological response to spinal cord injury in a rat model.

We report a small exploratory study of a methodology for real-time imaging of chemical and physical changes in spinal cords in the immediate aftermath of a localized contusive injury. One hundred separate experiments involving scanning NIR images, one-dimensional, two-dimensional (2-D), and point measurements, obtained in vivo, within a 3 × 7 mm field, on spinal cords surgically exposed between T9 and T10 revealed differences between injured and healthy cords. The collected raw data, i.e., elastic and inelastic emission from the laser probed tissues, combined via the PV[O]H algorithm, allow construction of five images over the first 5 h post injury. Within the larger study, a total of 13 rats were studied using 2-D images, i.e., injured and control. A single 830-nm laser (100-µm diameter round spot) was spatially line-scanned across the cord to reveal photobleaching effects and surface profiles possibly locating a near surface longitudinal artery/vein. In separate experiments, the laser was scanned in two dimensions across the exposed cord surface relative to the injury in a specific pattern to avoid uneven photobleaching of the imaged tissue. The 2-D scanning produced elastic and inelastic emission that allowed construction of PV[O]H images that had good fidelity with the visually observed surfaces and separate line scans and suggested differences between the volume fractions of fluid and turbidity of injured and healthy cord tissue.

Optical interference probe of biofilm hydrology: label-free characterization of the dynamic hydration behavior of native biofilms.

Biofilm produced by Escherichia coli (E. coli) or Pseudomonas aeruginosa (P. aeruginosa) on quartz or polystyrene is removed from the culture medium and drained. Observed optical interference fringes indicate the presence of a layer of uniform thickness with refractive index different from air-dried biofilm. Fringe wavelengths indicate that layer optical thickness is < 20 ?? ? m or 1 to 2 orders of magnitude thinner than the biofilm as measured by confocal Raman microscopy or fluorescence imaging of the bacteria. Raman shows that films have an alginate-like carbohydrate composition. Fringe amplitudes indicate that the refractive index of the interfering layer is higher than dry alginate. Drying and rehydration nondestructively thins and restores the interfering layer. The strength of the 1451-nm near infrared water absorption varies in unison with thickness. Absorption and layer thickness are proportional for films with different bacteria, substrates, and growth conditions. Formation of the interfering layer is general, possibly depending more on the chemical nature of alginate-like materials than bacterial processes. Films grown during the exponential growth phase produce no observable interference fringes, indicating requirements for layer formation are not met, possibly reflecting bacterial activities at that stage. The interfering layer might provide a protective environment for bacteria when water is scarce.

Coupled turbidity and spectroscopy problems: a simple algorithm for the volumetric analysis of optically thin or dilute two-phase systems.

We report an algorithm for measuring the phase volume fraction and solute concentration of a two-phase system, applicable to either optically thin or optically dilute spatially homogeneous systems. Probing light is directed into the sample, and the elastically scattered light (EE) is collected as one signal and the inelastically scattered light (IE) collected as another signal. The IE can be pure fluorescence or Raman or an unresolved combination of the two. As the IE and the EE are produced by fundamentally different processes, they are independent. The algorithm, derived from radiation transfer theory, shows that phase volume and concentration are linear functions of the EE and IE. The parameters are derived from a training set. We present examples of how the algorithm performs when the assumption of spatial homogeneity is violated and when light-induced photochemistry causes changes in the IE. Although this is a generally valid algorithm with many potential applications, its use is discussed briefly in the context of blood and tissue analysis since the algorithm was originally designed for noninvasive in vivo probing of human skin.

Analyzing near-infrared scattering from human skin to monitor changes in hematocrit.

Probing tissue with near-infrared radiation (NIR) simultaneously produces remitted fluorescence and Raman scattering (IE) plus Rayleigh∕Mie light scattering (EE) that noninvasively give chemical and physical information about the materials and objects within. We model tissue as a three-phase system: plasma and red blood cell (RBC) phases that are mobile and a static tissue phase. In vivo, any volume of tissue naturally experiences spatial and temporal fluctuations of blood plasma and RBC content. Plasma and RBC fractions may be discriminated from each other on the basis of their physical, chemical, and optical properties. Thus, IE and EE from NIR probing yield information about these fractions. Assuming there is no void volume in viable tissue, or that void volume is constant, changes in plasma and RBC volume fractions may be calculated from simultaneous measurements of the two observables, EE and IE. In a previously published analysis we showed the underlying phenomenology but did not provide an algorithm for calculating volume fractions from experimental data. Now, we present a simple analysis that allows monitoring of fluid fraction and hematocrit (Hct) changes by measuring IE and EE, and apply it to some experimental in vivo measurements.

Raman spectroscopic investigation of spinal cord injury in a rat model.

Raman spectroscopy was used to study temporal molecular changes associated with spinal cord injury (SCI) in a rat model. Raman spectra of saline-perfused, injured, and healthy rat spinal cords were obtained and compared. Two injury models, a lateral hemisection and a moderate contusion were investigated. The net fluorescence and the Raman spectra showed clear differences between the injured and healthy spinal cords. Based on extensive histological and biochemical characterization of SCI available in the literature, these differences were hypothesized to be due to cell death, demyelination, and changes in the extracellular matrix composition, such as increased expression of proteoglycans and hyaluronic acid, at the site of injury where the glial scar forms. Further, analysis of difference spectra indicated the presence of carbonyl containing compounds, hypothesized to be products of lipid peroxidation and acid catalyzed hydrolysis of glycosaminoglycan moieties. These results compared well with in vitro experiments conducted on chondroitin sulfate sugars. Since the glial scar is thought to be a potent biochemical barrier to nerve regeneration, this observation suggests the possibility of using near infrared Raman spectroscopy to study injury progression and explore potential treatments ex vivo, and ultimately monitor potential remedial treatments within the spinal cord in vivo.

On probing human fingertips in vivo using near-infrared light: model calculations.

We probe volar-side fingertip capillary beds with near-infrared laser light and collect Raman, Rayleigh, and Mie scattered light and fluorescence. The results are interpreted using radiation transfer theory in the single-scattering approximation. The surface topography of the skin is modeled using the Fresnel equations. The skin is treated as a three-layer material, with a mean-field treatment of tissue composition and related optical properties. The model, with a reasonable choice of tissue parameters, gives a remarkably accurate account of the features of actual measurements. It predicts the optimal values for the incident angle of the laser beam and the distance between beam and detector. It explains the correlated temporal changes in the intensities of elastically and inelastically scattered light caused by heart-driven pulses and why they are out of phase. With appropriate boundary conditions, the model can be used to discuss the scattering from ridged skin extruded conformally into an aperture in a metal surface under constant light pressure. The probing results suggest an inherent regularity and similarity in the anatomy and composition of surface and subsurface tissues of a wide range of skin types.

Simultaneous, noninvasive observation of elastic scattering, fluorescence and inelastic scattering as a monitor of blood flow and hematocrit in human fingertip capillary beds.

We report simultaneous observation of elastic scattering, fluorescence, and inelastic scattering from in vivo near-infrared probing of human skin. Careful control of the mechanical force needed to obtain reliable registration of in vivo tissue with an appropriate optical system allows reproducible observation of blood flow in capillary beds of human volar side fingertips. The time dependence of the elastically scattered light is highly correlated with that of the combined fluorescence and Raman scattered light. We interpret this in terms of turbidity (the impeding effect of red blood cells on optical propagation to and from the scattering centers) and the changes in the volume percentages of the tissues in the irradiated volume with normal homeostatic processes. By fitting to a model, these measurements may be used to determine volume fractions of plasma and RBCs.

Effect of hemoglobin concentration variation on the accuracy and precision of glucose analysis using tissue modulated, noninvasive, in vivo Raman spectroscopy of human blood: a small clinical study.

Tissue modulated Raman spectroscopy was used noninvasively to measure blood glucose concentration in people with type I and type II diabetes with HemoCue fingerstick measurements being used as reference. Including all of the 49 measurements, a Clarke error grid analysis of the noninvasive measurements showed that 72% were A range, i.e., clinically accurate, 20% were B range, i.e., clinically benign, with the remaining 8% of measurements being essentially erroneous, i.e., C, D, or E range. Rejection of 11 outliers gave a correlation coefficient of 0.80, a standard deviation of 22 mg/dL with p<0.0001 for N=38 and places all but one of the measurements in the A and B ranges. The distribution of deviations of the noninvasive glucose measurements from the fingerstick glucose measurements is consistent with the suggestion that there are at least two systematic components in addition to the random noise associated with shot noise, charge coupled device spiking, and human factors. One component is consistent with the known variation of fingerstick glucose concentration measurements from laboratory reference measurements made using plasma or whole blood. A weak but significant correlation between the deviations of noninvasive measurements from fingerstick glucose measurements and the test subject's hemoglobin concentration was also observed.