Dean E. Myers

Noninvasive method for measuring local hemoglobin oxygen saturation in tissue using wide gap second derivative near-infrared spectroscopy.

A simple continuous wave near-infrared algorithm for estimating local hemoglobin oxygen saturation in tissue (%StO2) is described using single depth attenuation measurements at 680, 720, 760, and 800 nm. Second derivative spectroscopy was used to reduce light scattering effects, chromophores with constant absorption, baseline/instrumentation drift, and movement artifacts. Unlike previous second derivative methods which focused primarily on measuring deoxyhemoglobin concentration; a wide 40 nm wavelength gap used for calculating second derivative attenuation significantly improved sensitivity to oxyhemoglobin absorption. Scaled second derivative attenuation at 720 nm was correlated to in vitro hemoglobin oxygen saturation to generate a %StO2 calibration curve. The calibration curve was insensitive to total hemoglobin, optical path length, and optical scattering. Measurement error due to normal levels of carboxyhemoglobin, methemoglobin, and water absorption were less than 10 %StO2 units. Severe methemoglobinemia or edema combined with low blood volume could cause StO2 errors to exceed 10 StO2 units. Both a broadband and commercial four-wavelength spectrometer (InSpectra) measured %StO2. The InSpectra tissue spectrometer readily detected limb ischemia on 26 human volunteers for hand, forearm, and leg muscles. A strong linear correlation, r2>0.93, between StO2 and microvascular %SO2 was observed for isolated animal hind limb, kidney, and heart.

Dynamic near-infrared spectroscopy measurements in patients with severe sepsis.

This study evaluated near-infrared spectroscopy (NIRS)-derived measurements in hemodynamically stable patients with severe sepsis, as compared with similar measurements in healthy age-matched volunteers. Prospective, preliminary, observational study in a surgical intensive care unit and clinical research center at a university health center. We enrolled 10 patients with severe sepsis and 9 healthy age-matched volunteers. For patients with severe sepsis, we obtained pulmonary artery catheter and laboratory values three times daily for 3 days and oxygen consumption values via metabolic cart once daily for 3 days. For healthy volunteers, we obtained all noninvasive measurements during a single session. We found lower values in patients with severe sepsis (versus healthy volunteers), in tissue oxygen saturation (StO2), in the StO2 recovery slope, in the tissue hemoglobin index, and in the total tissue hemoglobin increase on venous occlusion. Patients with severe sepsis had longer StO2 recovery times and lower NIRS-derived local oxygen consumption values versus healthy volunteers. In our preliminary study, NIRS provides a noninvasive continuous method to evaluate peripheral tissue oxygen metabolism in hemodynamically stable patients with severe sepsis. Further research is needed to demonstrate whether these values apply to broader populations of patients with systemic inflammatory response syndrome, sepsis, severe sepsis, and septic shock.

Assessment of tissue oxygen saturation during a vascular occlusion test using near-infrared spectroscopy: the role of probe spacing and measurement site studied in healthy volunteers

IntroductionTo assess potential metabolic and microcirculatory alterations in critically ill patients, near-infrared spectroscopy (NIRS) has been used, in combination with a vascular occlusion test (VOT), for the non-invasive measurement of tissue oxygen saturation (StO2), oxygen consumption, and microvascular reperfusion and reactivity. The methodologies for assessing StO2 during a VOT, however, are very inconsistent in the literature and, consequently, results vary from study to study, making data comparison difficult and potentially inadequate. Two major aspects concerning the inconsistent methodology are measurement site and probe spacing. To address these issues, we investigated the effects of probe spacing and measurement site using 15 mm and 25 mm probe spacings on the thenar and the forearm in healthy volunteers and quantified baseline, ischemic, reperfusion, and hyperemic VOT-derived StO2 variables.MethodsStO2 was non-invasively measured in the forearm and thenar in eight healthy volunteers during 3-minute VOTs using two InSpectra tissue spectrometers equipped with a 15 mm probe or a 25 mm probe. VOT-derived StO2 traces were analyzed for base-line, ischemic, reperfusion, and hyperemic parameters. Data were categorized into four groups: 15 mm probe on the forearm (F15 mm), 25 mm probe on the forearm (F25 mm), 15 mm probe on the thenar (T15 mm), and 25 mm probe on the thenar (T25 mm).ResultsAlthough not apparent at baseline, probe spacing and measurement site significantly influenced VOT-derived StO2 variables. For F15 mm, F25 mm, T15 mm, and T25 mm, StO2 ownslope was -6.4 ± 1.7%/minute, -10.0 ± 3.2%/minute, -12.5 ± 3.0%/minute, and -36.7 ± 4.6%/minute, respectively. StO2 upslope was 105 ± 34%/minute, 158 ± 55%/minute, 226 ± 41%/minute, and 713 ± 101%/minute, and the area under the hyperemic curve was 7.4 ± 3.8%·minute, 10.1 ± 4.9%·minute, 12.6 ± 4.4%·minute, and 21.2 ± 2.7%·minute in these groups, respectively. Furthermore, the StO2 parameters of the hyperemic phase of the VOT, such as the area under the curve, significantly correlated to the minimum StO2 during ischemia.ConclusionsNIRS measurements in combination with a VOT are measurement site-dependent and probe-dependent. Whether this dependence is anatomy-, physiology-, or perhaps technology-related remains to be elucidated. Our study also indicated that reactive hyperemia depends on the extent of ischemic insult.

Tissue hemoglobin index: a non-invasive optical measure of total tissue hemoglobin.

IntroductionThe tissue hemoglobin index (THI) is a hemoglobin signal strength metric provided on the InSpectra™ StO2 Tissue Oxygenation Monitor, Model 650. There is growing interest regarding the physiologic meaning of THI and whether a clinically useful correlation between THI and blood hemoglobin concentration exists. A series of in vitro and in vivo experiments was performed to evaluate whether THI has potential utility beyond its primary purpose of helping InSpectra™ device users optimally position a StO2 sensor over muscle tissue.MethodsThe THI and tissue hemoglobin oxygen saturation (StO2) were measured using the InSpectra™ StO2 Tissue Oxygenation Monitor, Model 650, with a 15 mm optical sensor. A THI normal reference range was established in the thenar eminence (hand) for 434 nonhospitalized human volunteers. In 30 subjects, the thenar THI was also evaluated during 5-minute arterial and venous blood flow occlusions, and with blood volume exsanguination in the hand induced with an Esmarch bandage. In addition, correlation of the THI to blood total hemoglobin concentration (Hbt) was studied in five pigs whose Hbt was isovolumetrically diluted from 13 to 4 g/dl systemically and 0.5 g/dl locally in the hind limb. The sensitivity and specificity of the THI to measure tissue hemoglobin concentration (THC) were characterized in vitro using isolated blood tissue phantoms.ResultsIn human thenar tissue, the average THI was 14.1 ± 1.6 (mean ± standard deviation). The THI extrapolated to 100% blood volume exsanguination was 3.7 ± 2.0 units presumably from myoglobin. On average, the THI increased 1.5 ± 1.0 units with venous occlusion and decreased 4.0 ± 2.0 units with arterial occlusion. In porcine hind limbs, the THI weakly correlated with Hbt (r2 = 0.26) while ΔTHI during venous occlusion had a stronger correlation (r2 = 0.62). In vitro tests indicated that THI strongly correlated (r2 > 0.99) to phantom THC and was insensitive to StO2 changes.ConclusionsSteady-state THI values do not reliably indicate Hbt. The THI is a reproducible quantitative index for THC, and THI trends can discriminate between arterial or venous blood flow occlusions. The THI magnitude permits the estimation of myoglobins contribution to StO2.

A wide gap second derivative NIR spectroscopic method for measuring tissue hemoglobin oxygen saturation.

We describe a four wavelength continuous wave near-infrared algorithm for estimating hemoglobin oxygen saturation in tissue using single depth attenuation measurements. The 40 nm gap second derivative algorithm and in vitro calibration method provided StO2 measurements of reasonable accuracy that were applied across a variety of tissues and probe spacings with no measured or assumed values for optical pathlength or optical scattering.

Noninvasive Tissue Oxygen Saturation Measurements Identify Supply Dependency

BACKGROUND Hemorrhagic shock can lead to multiple organ failure and death. We have previously shown that noninvasive measurement of tissue oxygen saturation (StO(2)) has predictive value for outcomes in patients suffering hemorrhagic shock. Our study objectives were twofold: (1) to compare invasive and noninvasive measurements of local and systemic tissue hemoglobin oxygenation and (2) to compare the effects of various physiologic conditions seen in patients in hemorrhagic shock on tissue hemoglobin oxygenation. MATERIALS AND METHODS We studied pigs in controlled conditions mimicking shock induced by one of the following: hypothermia, isovolemic hemodilution, or manipulations of vascular tone. We obtained both invasive and noninvasive measurements in a hind limb of StO(2), tissue hemoglobin index, femoral artery and venous flows, blood pressures, temperature, pH, pO(2), pCO(2), oxygen saturation, lactate, hemoglobin, and base excess. In all cases, we measured baseline values in both experimental and control hind limbs. RESULTS We found that tissue hemoglobin oxygenation did not vary significantly over relevant physiologic temperatures. Under all physiologic conditions tested, we found supply-dependent oxygen consumption at oxygen levels less than 7 mL O(2)/min/kg. Similarly, we found that local oxygen delivery in animals subjected to varying degrees of isovolemic hemodilution or altered vascular tone was correlated with supply-dependent oxygen consumption, as measured by local noninvasive StO(2). CONCLUSIONS Noninvasive StO(2) measurements are valid and durable over a wide range of physiologic conditions and correlate with invasively-measured oxygen delivery.