Evaluation of Electrical and Optical Plethysmography Sensors for Noninvasive Monitoring of Hemoglobin Concentration

Completely noninvasive monitoring of hemoglobin concentration has not yet been fully realized in the clinical setting. This study investigates the viability of measuring hemoglobin concentration noninvasively by evaluating the performance of two types of sensor using a tissue phantom perfused with a blood substitute. An electrical sensor designed to measure blood volume changes during the cardiac cycle was used together with an infrared optical sensor for detection of erythrocyte-bound hemoglobin. Both sensors demonstrated sensitivity to changes in pulse volume (plethysmography). The electrical sensor produced a signal referred to as capacitance plethysmograph (CPG) a quantity which was invariant to the concentration of an infrared absorbing dye present in the blood substitute. The optical sensor signal (photoplethysmograph) increased in amplitude with increasing absorber concentration. The ratio PPG:CPG is invariant to pulse pressure. This quantity is discussed as a possible index of in vivo hemoglobin concentration

Novel idea to monitor and measure blood hemoglobin noninvasively

Measuring blood hematocrit noninvasively is reviewed in this paper. Although there is an inclination to measure the hematocrit by determining the bioelectrical impedance of the blood, in vitro experimental methods still remain practically inapplicable. The blood sample size is determined when blood samples are examined. Determining the impedance and volume of blood is the biggest challenge in measuring the hematocrit noninvasively without drawing a blood sample. Calculating the blood impedance in vivorequires developing an impedance measurement using a multi-frequency method and also calculating the change in pressure simultaneously during the heart’s pulsatile cycle.Keywords: Blood, hematocrit, measuremen

Doctor of Philosophy

dissertationSubcellularly resolved, excitable changes (i.e., those induced by electrical or chemical stimuli) in membrane capacitance, influenced by factors including integralmembrane protein activity, lipid densities and membrane-bound water content, may be used to elucidate nonconductive ion-channel conformational state changes, lipid-raft locations and drug-membrane binding processes. However, membrane capacitance has proven difficult to measure, partially because of bandwidth limitations associated with glass/quartz pipettes used during conventional electrophysiology. To address these challenges, techniques introduced in this thesis integrate the principles of extracellular radio frequency (RF) recording with conventional two-electrode voltage clamp (TEVC) to 1) spatially resolve effective membrane capacitance and 2) monitor excitable changes in effective membrane capacitance. Furthermore, this thesis also introduces a new multielectrode method to approximate electrode-electrolyte interfacial impedance, which might prove useful in electric impedance spectroscopic or electric impedance tomographic applications. Specific contributions include the following: 1) A method that simultaneously estimates double-layer and interelectrode (chamber) impedances, in the linear regime of electrode voltage-current sensitivity, during extracellular electrode-based measurements. This method estimates impedance parameters by applying a nonlinear least-squares regression to measurements between various groups or pairs of a three-electrode system and, unlike previous double-layer approximation methods, can be done without the use of multiple calibration solutions or moveable electrode configurations. 2) A platform capable of visualizing the spatial distribution of membrane capacitance, using extracellular RF electrode recordings, around a single cell. The proof-of-concept for this technique is demonstrated with dielectric maps around polarized Xenopus oocyte membranes. 3) Development and characterization of a platform to enable RF impedancebased measurements around voltage-clamped ShakerB-IR-expressing Xenopus oocytes. Data indicated that the platform was most sensitive to effective changes in oocyte dielectric at 300 kHz and 500 kHz. 4) Temporal characterization of changes in voltage-sensitive RF membrane capacitance associated with ShakerB-IR activation (expressed in Xenopus oocytes) and ShakerB-IR-Cu2+ interactions. Results indicate that extracellular RF-impedance-based measurements can temporally and spatially elucidate changes in excitable cell-membrane capacitance and could supplement conventional electrophysiological techniques to provide a broader understanding of cellular biophysics