Thermoregulation is a mammalian physiological function fulfilled in large part by autonomic control of blood flow. We demonstrate the variation in hematocrit (Hct) and intravascular volume (VV) in the peripheral circulation when the external means of maintaining the initial thermal disequilibrium is removed using a PV[O]H device capable of noninvasively measuring both Hct and VV with unprecedented sensitivity, accuracy and precision on a 3 second timescale. Calibrated using an FDA approved device now in standard use for monitoring Hct during dialysis, the PV[O]H detection limit for measuring Hct variation is ±0.03 where 45% is normal. Observing the return to thermal equilibrium at 2 separate anatomic locations, we observe the return to normal homeostasis in a matter of a few minutes. Heat induced vasodilation results in an antecedent increase in plasma volume in greater proportion than for red blood cells into the dilated capillaries. At equilibrium homeostasis i.e. when there is no externally maintained thermal gradient we observe periodic fluctuations in the peripheral Hct and VV on a roughly 15 second to 1.5 minute timescale.
We previously reported a new algorithm “PV[O]H” for continuous, noninvasive, in vivo monitoring of hematocrit changes in blood and have since shown its utility for monitoring in humans during 1) hemodialysis, 2) orthostatic perturbations and 3) during blood loss and fluid replacement in a rat model. We now show that the algorithm is sensitive to changes in hemoglobin oxygen saturation. We document the phenomenology of the effect and explain the effect using new results obtained from humans and rat models. The oxygen sensitivity derives from the differential absorption of autofluorescence originating in the static tissues by oxy and deoxy hemoglobin. Using this approach we show how to perform simultaneous, noninvasive, in vivo, continuous monitoring of hematocrit, vascular volume, hemoglobin oxygen saturation, pulse rate and breathing rate in mammals using a single light source. We suspect that monitoring of changes in this suite of vital signs can be provided with improved time response, sensitivity and precision compared to existing methodologies. Initial results also offer a more detailed glimpse into the systemic oxygen transport in the circulatory system of humans.
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