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Probing volar side fingertip capillary beds with 830 nm light produces remission spectra containing Rayleigh and Raman scattered light, and fluorescence, allowing continuous monitoring of intravascular plasma volume and hematocrit using the FRD-PVOH algorithm. During dialysis, Raman emission by polyatomic electrolytes i.e., phosphate tracks with fluid removal i.e., the change in intravascular plasma volume and in agreement with simultaneous hematocrit measurement of extracorporeal blood in the dialysis unit using the FDA approved CritLine. The variation of Raman features associated with urea in interstitial fluid and plasma suggests urea is involved in chemistry in the skin compartment i.e., in the extravascular space causing its clearance to lag the removal of electrolytes and water. Consistent with known skin conditions induced by chronic kidney disease and dialysis, we speculate that routine excess urea in the interstitial fluid destabilizes hydrogen bonding networks associated with keratin bundles in both viable keratinocytes and stratum corneum, exposing disulfide linkages, making them vulnerable to reduction by other species in the interstitial fluid. Oral administration of furosemide removes more water than electrolytes relative to the proportions removed by dialysis leading to solubility stress and striking variations of the Raman spectra. These results reinforce the notion that the various compartments in the human body do not drain at equal rates during dialysis and that real time Raman and FRD-PVOH monitoring-based feedback during hemodialysis could reduce the frequency of adverse events and thus improve outcomes.
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