Abstract
We present noninvasive, quantitative in vivo measurements of methemoglobin formation and reduction in a rabbit model using broadband diffuse optical spectroscopy (DOS). Broadband DOS combines multifrequency frequency-domain photon migration (FDPM) with time-independent near infrared (NIR) spectroscopy to quantitatively measure bulk tissue absorption and scattering spectra between 600 nm and 1,000 nm. Tissue concentrations (denoted by brackets) of methemoglobin ([MetHb]), deoxyhemoglobin ([Hb-R]), and oxyhemoglobin ([HbO2]) were determined from absorption spectra acquired in "real time" during nitrite infusions in nine pathogen-free New Zealand White rabbits. As little as 30 nM [MetHb] changes were detected for levels of [MetHb] that ranged from 0.80 to 5.72 microM, representing 2.2 to 14.9% of the total hemoglobin content (%MetHb). These values agreed well with on-site ex vivo cooximetry data (r2= 0.902, P < 0.0001, n = 4). The reduction of MetHb to functional hemoglobins was also carried out with intravenous injections of methylene blue (MB). As little as 10 nM changes in [MB] were detectable at levels of up to 150 nM in tissue. Our results demonstrate, for the first time, the ability of broadband DOS to noninvasively quantify real-time changes in [MetHb] and four additional chromophore concentrations ([Hb-R], [HbO2], [H2O], and [MB]) despite significant overlapping spectral features. These techniques are expected to be useful in evaluating dynamics of drug delivery and therapeutic efficacy in blood chemistry, human, and preclinical animal models.
Highlights
METHEMOGLOBINEMIA IS amyl nitrite (AN) ALTERED hemoglobin condition resulting from the oxidation of the ferrous moiety within the hemoglobin molecule
Cooximetry is generally the preferred laboratory technique for diagnosis of methemoglobinemia, but, because it relies on the absorption spectra of a few wavelengths for the calculation of MetHb concentration, false positive readings can result from the presence of other pigments, such as methylene blue (MB) or sulfhemoglobin, which have high absorption at the methemoglobin absorption peak of 630 nm [11, 32]
It was determined that broadband diffuse optical spectroscopy (DOS) was able to detect as little as 30 nM [MetHb] changes in tissue
Summary
METHEMOGLOBINEMIA IS AN ALTERED hemoglobin condition resulting from the oxidation of the ferrous moiety within the hemoglobin molecule. This conversion leads to abnormal oxygen affinity, reduced oxygen-carrying capacity, and tissue hypoxia. Traditional diagnostic modalities for hypoxemic patients include pulse oximetry, arterial blood-gas analysis, and cooximetry. Cooximetry is generally the preferred laboratory technique for diagnosis of methemoglobinemia, but, because it relies on the absorption spectra of a few wavelengths for the calculation of MetHb concentration, false positive readings can result from the presence of other pigments, such as methylene blue (MB) or sulfhemoglobin, which have high absorption at the methemoglobin absorption peak of 630 nm [11, 32].
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