Extracorporeal life support (ECLS) is used in intensive care units (ICUs) as heart-lung bypass for critically ill patients to support either inadequate heart or lung function. Decisions to discontinue ECLS are typically based on clinical judgment and patient trajectory during trial-off support. We investigated an optical measurement of muscle oxygenation (MOx) as an indicator for adequacy of circulatory function during trials-off ECLS. Clinicians were queried prior to trial-off as to whether the patient was deemed: ready, might be ready, or not likely ready to discontinue ECLS. MOx was determined using an optical analysis developed in our laboratory. Optical spectra were acquired from infants using a fiber-optic probe affixed to the arm or leg. Five infants were studied during 6 trials-off ECLS. Mean initial MOx was 96.6+/-8.5% (n=6). In trials resulting in discontinuation of ECLS, MOx was > 94%. In those remaining on ECLS, MOx was lower during the trial off at all time points. Mean MOx trended lower (75.1+/-23.5%), in the first 6 minutes for those not removed from ECLS, compared with those for whom ECLS was subsequently discontinued (97.2+/-3.8%). Lactate trended higher in subjects remaining on ECLS (3.4+/-0.8) compared with those removed from support (1.7+/-0.7). Clinician predictions prior to trial-off did not correlate with ultimate decision for discontinuing ECLS. Although preliminary, we believe that MOx may be useful to assist in objectively assessing adequacy of circulatory function and may be helpful in the early determination of readiness for discontinuation of ECLS.
Optical spectroscopy is being used increasingly in medical applications to noninvasively investigate tissues below the skin. In order to assure adequate sampling of tissues underlying the skin, photon penetration depth must be known. Photon penetration in tissues has been studied with near-infrared (NIR) light, but experimental study of visible light propagation in tissue has been limited. In this study, a micro-motion system coupled with a reflectance spectroscopy system was used to determine the penetration depth of visible-range and NIR photons (535-800 nm) in phantoms composed of Intralipid and hemoglobin. An absorbing target was placed at intervals of 0.1mm along a 15mm line perpendicular to and bisecting the line between the ends of the source and detector optical fiber bundles. Comparisons between detected light intensities at different target positions were used to determine the most probable photon path depths at 576 nm and at 760 nm. Scattering coefficients, hemoglobin concentrations, and source-detector separations were varied to evaluate their effects on the penetration depth of photons. Results from phantoms containing Intralipid only showed that the most-probable penetration depth at 576 nm was comparable to that at 760 nm. Larger sourcedetector separations resulted in deeper photon penetration depths for both spectral regions. Changes in scattering over a 4-fold range did not affect the photon path depth appreciably. In the presence of hemoglobin with a source-detector separation of 13 mm, the most probable depth of photon penetration in the visible range was greater than 2.5 mm, and was within 1 mm of the most probable depth of photon penetration in the NIR. This study demonstrates the feasibility of using the visible and NIR regions in transcutaneous reflectance spectroscopy.
Myoglobin is an important intracellular oxygen transport molecule in muscle. Oxygen binding to myoglobin can be determined spectroscopically due to differences in absorption of oxymyoglobin and deoxymyoglobin. Myoglobin oxygenation can be used as a measure of intracellular oxygen tension in muscle. We sought to determine the effects of differences in temperature and pH on myoglobin absorption spectra in the near-infrared spectral region. Transmission spectra were taken of pure solutions of oxymyoglobin and deoxymyoglobin at 10°, 20°, 30°, and 40°C at pH values of 6.0, 7.0, and 8.0 (n=4). In second derivative spectra at 40°C, the deoxymyoglobin peak near 760 nm was shifted by 0.9-1.2 nm toward longer wavelengths relative to 10°C at constant pH. Differences in pH did not result in statistically significant shifts in this peak at constant temperature. Estimations of myoglobin saturation from myoglobin spectra with intermediate saturations were obtained by least squares (LS) and partial least squares (PLS) analyses. Both algorithms estimate myoglobin saturation with small root mean square errors (<1e-6) when component spectra and calibration set spectra are at the same temperature as test spectra (n=100). However, when spectra at 20°C or 40°C were used as component spectra in LS with test spectra at 30°C (all at pH 7.0), errors were 0.8% and 1.4%, respectively. PLS analysis of 30°C test spectra using 20°C or 40°C calibration set spectra yielded errors of 1.6% and 1.5%, respectively. When the PLS analysis is endpoint corrected, these errors become vanishingly small. These results demonstrate that peak shifts due to temperature are potential sources of error if calibration and test spectra differ by 10°C. These errors can be minimized by appropriate spectral analytic methods.
Optical spectroscopy has previously ben used to investigate mitochondrial function in muscle, but spectral overlap of the absorbing species in muscle has limited the accuracy of these determinations. This report demonstrates a multispectral approach to determine cytochrome c redox state in the isolated perfused heart using partial least squares analysis. Optical spectra were acquired in the visible region from the surface of the isolated perfused guinea pig heart and interpreted using reference spectra obtained from in vitro solutions. Calibration spectra were acquired form separate light scattering solutions that contained cytochrome c or myoglobin in both oxidized and reduced states. Cytochrome c redox state and myoglobin oxygen saturation were determined separately form each spectrum obtained form 16 guinea pig hearts during baseline perfusion with oxygenated buffer, during 90 seconds of ischemia and during recovery following the ischemic period. A brief delay between myoglobin deoxygenation and cytochrome c reduction was determined, demonstrating that both processes could be determined simultaneously form the same spectra. This approach may lead to improved determinations of mitochondrial function in the isolated perfused heart.
Spectroscopic measurement of myoglobin oxygenation has been made in the presence of hemoglobin using visible light. Although the spectral changes with oxygen binding are smaller, near infrared wavelengths have deeper tissue penetration. This offers an attractive means for making noninvasive muscle oxygenation measurements. Visible and near-infrared spectra of myoglobin with decreasing oxygen saturation in the presence of both oxyhemoglobin and deoxyhemoglobin were obtained. Fractional oxygen saturations were determined from the visible region. The near-infrared spectra were divided into calibration and test sets and analyzed by the method of multiple linear regression. Baseline offsets were reduced by preprocessing the data using spectral second derivatives to enhance the significant features of the spectra. This method of analysis demonstrated good correlation of the near- infrared spectra with the calibration saturation data. These results show that myoglobin oxygen saturation measurements in vitro can be made in the presence of hemoglobin as an interferant, using near-infrared spectroscopy.
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