Noninvasive Cellular Oxygenation Measurement During Graded Hypoxia Using Visible-Near-Infrared Spectroscopy.

In critically ill patients, direct knowledge of intracellular pO2 would allow for identification of cellular hypoxia, which when prolonged leads to organ failure. We have developed a visible-near-infrared optical system that noninvasively measures myoglobin saturation, which is directly related to intracellular pO2, from the surface of the skin. We used an animal model of graded hypoxia from low levels of inspired oxygen (n = 5) and verified that low intracellular pO2 is correlated with high steady-state serum lactate values. In addition, the pO2 gradient between arterial blood and inside muscle cells was 83 mm Hg at 21% O2, but fell to 24 mm Hg at 8% O2. Continuous myoglobin saturation measurement in skeletal muscle displayed the same trends as cerebral oxygenation with no lag in changes over time, demonstrating its relevance as a measure of systemic oxygenation.

Assessment of muscle oxygenation in children with congenital heart disease.

BACKGROUND: Adaptive responses to congenital heart disease result in altered muscle perfusion and muscle metabolism. Such changes may be detectable using noninvasive spectroscopic monitors. AIMS: In this study we aimed to determine if resting muscle oxygen saturation (MOx) is lower in children with acyanotic or cyanotic congenital heart disease than in healthy children and to identify differences in muscle oxygen consumption in children with cyanotic and acyanotic congenital heart disease. METHODS: Using a custom fiber optic spectrometer system, optical measurements were obtained from the calf or forearm of 49 patients (17 with acyanotic congenital heart disease, 18 with cyanotic congenital heart disease, and 14 control). Twenty additional control patients were used to develop the analytic model. Spectra were used to determine MOx at baseline, during arterial occlusion, and during reperfusion. The rate of muscle desaturation during arterial occlusion was also evaluated. Two-sample t-tests were used to compare each heart disease group with the controls. RESULTS: Patients with acyanotic and cyanotic congenital heart disease had lower baseline MOx than controls. Baseline MOx was 91.3% (CI 85.9%, 96.7%) for acyanotic patients, 91.1% (CI 86.3%, 95.9%) for cyanotic patients, and 98.9% (CI 96.7%, 101.1%) for controls. Similarly, MOx was lower in the acyanotic and cyanotic groups than the controls after reperfusion (84.6% [CI 74.1%, 95.1%] and 82.1% [CI 74.5%, 89.7%] vs 98.9% [96.5%, 101.3%]). The rate of decline in oxygenation was significantly greater in cyanotic patients versus controls (0.46%/s (CI 0.30%, 0.62%/s) vs 0.17%/s (0.13%, 0.21%/s)). CONCLUSION: This study demonstrates that muscle oxygenation is abnormal in children with both cyanotic and acyanotic congenital heart disease. This suggests that noninvasive monitoring of muscle oxygenation may provide valuable information in situations where children with congenital heart disease may be at risk of hemodynamic compromise.

Muscle oxygenation as an indicator of shock severity in patients with suspected severe sepsis or septic shock.

PURPOSE: The aim of this pilot study was to evaluate the potential of a new noninvasive optical measurement of muscle oxygenation (MOx) to identify shock severity in patients with suspected sepsis. METHODS: We enrolled 51 adult patients in the emergency department (ED) who presented with possible sepsis using traditional Systematic Inflammatory Response Syndrome criteria or who triggered a "Code Sepsis." Noninvasive MOx measurements were made from the first dorsal interosseous muscles of the hand once potential sepsis/septic shock was identified, as soon as possible after admission to the ED. Shock severity was defined by concurrent systolic blood pressure, heart rate, and serum lactate levels. MOx was also measured in a control group of 17 healthy adults. RESULTS: Mean (± SD) MOx in the healthy control group was 91.0 ± 5.5% (n = 17). Patients with mild, moderate, and severe shock had mean MOx values of 79.4 ± 21.2%, 48.6 ± 28.6%, and 42.2 ± 4.7%, respectively. Mean MOx for the mild and moderate shock severity categories were statistically different from healthy controls and from each other based on two-sample t-tests (p < 0.05). CONCLUSIONS: We demonstrate that noninvasive measurement of MOx was associated with clinical assessment of shock severity in suspected severe sepsis or septic shock. The ability of MOx to detect even mild septic shock has meaningful implications for emergency care, where decisions about triage and therapy must be made quickly and accurately. Future longitudinal studies may validate these findings and the value of MOx in monitoring patient status as treatment is administered.

Muscle Oxygenation as an Early Predictor of Shock Severity in Trauma Patients.

INTRODUCTION: We evaluated the potential utility of a new prototype noninvasive muscle oxygenation (MOx) measurement for the identification of shock severity in a population of patients admitted to the trauma resuscitation rooms of a Level I regional trauma center. The goal of this project was to correlate MOx with shock severity as defined by standard measures of shock: systolic blood pressure, heart rate, and lactate. METHODS: Optical spectra were collected from subjects by placement of a custom-designed optical probe over the first dorsal interosseous muscles on the back of the hand. Spectra were acquired from trauma patients as soon as possible upon admission to the trauma resuscitation room. Patients with any injury were eligible for study. MOx was determined from the collected optical spectra with a multiwavelength analysis that used both visible and near-infrared regions of light. Shock severity was determined in each patient by a scoring system based on combined degrees of hypotension, tachycardia, and lactate. MOx values of patients in each shock severity group (mild, moderate, and severe) were compared using two-sample t tests. RESULTS: In 17 healthy control patients, the mean MOx value was 91.0 ±â€Š5.5%. A total of 69 trauma patients were studied. Patients classified as having mild shock had a mean MOx of 62.5 ±â€Š26.2% (n = 33), those classified as in moderate shock had a mean MOx of 56.9 ±â€Š26.9% (n = 25) and those classified as in severe shock had a MOx of 31.0 ±â€Š17.1% (n = 11). Mean MOx for each of these groups was statistically different from the healthy control group (P < 0.05).Receiver operating characteristic analyses show that MOx and shock index (heart rate/systolic blood pressure) identified shock similarly well (area under the curves [AUC] = 0.857 and 0.828, respectively). However, MOx identified mild shock better than shock index in the same group of patients (AUC = 0.782 and 0.671, respectively). CONCLUSIONS: The results obtained from this pilot study indicate that MOx correlates with shock severity in a population of trauma patients. Noninvasive and continuous MOx holds promise to aid in patient triage and to evaluate patient condition throughout the course of resuscitation.

Muscle oxygenation measurement in humans by noninvasive optical spectroscopy and Locally Weighted Regression.

We have developed a method to make real-time, continuous, noninvasive measurements of muscle oxygenation (Mox) from the surface of the skin. A key development was measurement in both the visible and near infrared (NIR) regions. Measurement of both oxygenated and deoxygenated myoglobin and hemoglobin resulted in a more accurate measurement of Mox than could be achieved with measurement of only the deoxygenated components, as in traditional near-infrared spectroscopy (NIRS). Using the second derivative with respect to wavelength reduced the effects of scattering on the spectra and also made oxygenated and deoxygenated forms more distinguishable from each other. Selecting spectral bands where oxygenated and deoxygenated forms absorb filtered out noise and spectral features unrelated to Mox. NIR and visible bands were scaled relative to each other in order to correct for errors introduced by normalization. Multivariate Curve Resolution (MCR) was used to estimate Mox from spectra within each data set collected from healthy subjects. A Locally Weighted Regression (LWR) model was built from calibration set spectra and associated Mox values from 20 subjects using 2562 spectra. LWR and Partial Least Squares (PLS) allow accurate measurement of Mox despite variations in skin pigment or fat layer thickness in different subjects. The method estimated Mox in five healthy subjects with an RMSE of 5.4%.

Direct optical measurement of intraoperative myocardial oxygenation during congenital heart surgery.

This study demonstrates use of novel technology to measure cellular oxygenation during corrective congenital heart surgery. Cellular oxygenation was measured using a custom-designed optical probe placed on the free wall of the right ventricle. Cellular oxygenation, determined from myoglobin saturation, was calculated using multiwavelength analysis. Timing of bypass, aortic cross-clamp, infusion of cardioplegic solution, and length of intensive care unit (ICU) stay were recorded. Baseline cellular oxygenation was approximately 50% just before aortic cross-clamp and decreased to approximately 20% during cardioplegia. Cellular oxygenation remained low throughout cardioplegia and returned toward baseline after bypass. In four cases, cellular oxygenation did not return as quickly to baseline as in the other three cases. Among the four patients demonstrating slow recovery, the average ICU length of stay was 2.25 days compared with an average stay of 1.33 days for those patients exhibiting rapid cellular oxygenation recovery (p = 0.06). The slow recovery group had an average cross-clamp time of 40.1 ± 28.4 minutes, compared with 26.0 ± 8.5 minutes for the fast recovery group (p = 0.34). This study demonstrates for the first time that myocyte cellular oxygenation can be measured intraoperatively during cardiac surgery. Measurement of cellular oxygenation may be useful for improving myocardial preservation techniques.

Simultaneous optical spectroscopic measurement of hemoglobin and myoglobin saturations and cytochrome aa3 oxidation in vivo.

A method to simultaneously measure oxygenation in vascular, intracellular, and mitochondrial spaces from optical spectra acquired from muscle has been developed. In order to validate the method, optical spectra in the visible and near-infrared regions (600-850 nm) were acquired from solutions of myoglobin, hemoglobin, and cytochrome oxidase that included Intralipid as a light scatterer. Spectra were also acquired from the rabbit forelimb. Three partial least squares (PLS) analyses were performed on second-derivative spectra, each separately calibrated to myoglobin oxygen saturation, hemoglobin oxygen saturation, or cytochrome aa3 oxidation. The three variables were measured from in vitro and in vivo spectra that contained all three chromophores. In the in vitro studies, measured values of myoglobin saturation, hemoglobin saturation, and cytochrome aa3 oxidation had standard errors of 5.9%, 7.4%, and 12.2%, respectively, with little cross-talk between the in vitro measurements. In the progression from normal oxygenation to ischemia in the rabbit forelimb, hemoglobin desaturated first, followed by myoglobin, while cytochrome aa3 reduction occurred last. The ability to simultaneously measure oxygenations in the vascular, intracellular, and mitochondrial compartments will be valuable in physiological studies of muscle metabolism and in clinical studies when oxygen supply or utilization are compromised.

Mitochondrial function in vivo: spectroscopy provides window on cellular energetics.

Mitochondria integrate the key metabolic fluxes in the cell. This role places this organelle at the center of cellular energetics and, hence, mitochondrial dysfunction underlies a growing number of human disorders and age-related degenerative diseases. Here we present novel analytical and technical methods for evaluating mitochondrial metabolism and (dys)function in human muscle in vivo. Three innovations involving advances in optical spectroscopy (OS) and magnetic resonance spectroscopy (MRS) permit quantifying key compounds in energy metabolism to yield mitochondrial oxidation and phosphorylation fluxes. The first of these uses analytical methods applied to optical spectra to measure hemoglobin (Hb) and myoglobin (Mb) oxygenation states and relative contents ([Hb]/[Mb]) to determine mitochondrial respiration (O2 uptake) in vivo. The second uses MRS methods to quantify key high-energy compounds (creatine phosphate, PCr, and adenosine triphosphate, ATP) to determine mitochondrial phosphorylation (ATP flux) in vivo. The third involves a functional test that combines these spectroscopic approaches to determine mitochondrial energy coupling (ATP/O2), phosphorylation capacity (ATP(max)) and oxidative capacity (O2max) of muscle. These new developments in optical and MR tools allow us to determine the function and capacity of mitochondria noninvasively in order to identify specific defects in vivo that are associated with disease in human and animal muscle. The clinical implication of this unique diagnostic probe is the insight into the nature and extent of dysfunction in metabolic and degenerative disorders, as well as the ability to follow the impact of interventions designed to reverse these disorders.

Accurate myoglobin oxygen saturation by optical spectroscopy measured in blood-perfused rat muscle.

Optical spectra were acquired from myoglobin and hemoglobin solutions and from the tibialis anterior muscle of Sprague-Dawley rats in the visible region (515 to 660 nm). Validation studies were performed on the in vitro spectra to demonstrate that partial least squares analysis of second-derivative spectra yields accurate measurements of myoglobin saturation in the presence of varying hemoglobin concentrations and saturations. When hemoglobin concentrations were varied between 0.25 and 4 times that of myoglobin, myoglobin saturations were measured with a root mean squared error (RMSE) of 4.9% (n = 56) over the full range from 0 to 1. Myoglobin saturations were also shown to be largely unaffected by hemoglobin saturation. RMSE values of only 1.7% (n = 77) were found when hemoglobin saturations were varied independently from myoglobin saturations. These in vitro validation studies represent the most complete and rigorous done to date using partial least squares analysis on myoglobin and hemoglobin spectra. Analysis of reflectance spectra from the rat hind limb yielded accurate measures of volume-averaged myoglobin fractional saturation in the presence of hemoglobin in vivo. Hemodilution showed that myoglobin fractional saturation measurements in the rat leg are not sensitive to changes in hematocrit, thereby confirming the results from solutions in vitro. Decreases in optical density of 11.3 +/- 3.0% (n = 3) were achieved while myoglobin saturation decreased by only 3.1 +/- 3.8%. Myoglobin saturation was significantly increased when the fraction of inspired O(2) was increased, showing that manipulations of myoglobin saturation are detectable and that myoglobin is not fully saturated in resting muscle. Together, these in vitro and in vivo studies show that cellular oxygenation derived from myoglobin fractional saturation can be measured accurately with little cross-talk from hemoglobin in the visible wavelength region, thereby extending optical spectroscopic studies of cellular and vascular oxygenation beyond the near-infrared regions previously studied.

Optical spectroscopy demonstrates elevated intracellular oxygenation in an endotoxic model of sepsis in the perfused heart.

Recent clinical studies of patients with sepsis have shown that the delivery of adequate oxygen alone does not necessarily result in improved organ function or survival. This study was undertaken to determine if optical spectroscopy could detect higher intracellular oxygenations in isolated, perfused guinea pig hearts that have been treated with endotoxin (lipopolysaccharide [LPS]) than in controls. Four hours after intraperitoneal injection with LPS, adult guinea pigs were anesthetized, and hearts were excised and perfused in the Langendorff manner. Six control and eight LPS-exposed guinea pigs were studied. Myoglobin oxygen saturation was determined from analysis of optical reflectance spectra acquired from the left ventricular free wall. Myoglobin saturation was significantly higher at baseline with LPS than in controls (96.0% +/- 0.8% vs. 89.4% +/- 1.7%, P < 0.001). At the end of 30 s of ischemia, myoglobin saturation decreased to 15% +/- 1% in controls, but to only 60% +/- 7% in the LPS group. Myocardial performance was determined by measured left ventricular developed pressure, which was significantly depressed in the LPS-exposed hearts relative to controls (30 +/- 4 mmHg vs. 67 +/- 9 mmHg, P < 0.001). Myocardial oxygen consumption, calculated from measurements of arterial and venous PO2 and coronary flow, was lower in LPS hearts relative to controls (0.199 +/- 0.021 mL oxygen x min(-1) x g(-1) vs. 0.157 +/- 0.006 mL oxygen x min(-1) x g(-1)). In this model of sepsis in the perfused guinea pig heart, intracellular oxygenation was higher and oxygen consumption was lower than in controls. Cellular dysfunction seen in sepsis may be caused by compromised oxygen use rather than insufficient oxygen delivery. Optical spectroscopy has the potential to noninvasively monitor patients and their responses to therapy.

Myocardial oxygenation and adenosine release in isolated guinea pig hearts during changes in contractility.

Previous work from this laboratory using near-infrared optical spectroscopy of myoglobin has shown that approximately 20% of the myocardium is hypoxic in buffer-perfused hearts that are perfused with fully oxygenated buffer at 37 degrees C. The present study was undertaken to determine cardiac myoglobin saturation in buffer-perfused hearts when cardiac contractility was increased with epinephrine and decreased during cardiac arrest with KCl. Infusion of epinephrine to achieve a doubling of contractility, as measured by left ventricular maximum pressure change over time (dP/dt), resulted in a decrease in mean myoglobin saturation from 79% at baseline to 65% and a decrease in coronary venous oxygen tension from 155 mmHg at baseline to 85 mmHg. Cardiac arrest with KCl increased mean myoglobin saturation to 100% and coronary venous oxygen tension to 390 mmHg. A previously developed computer model of oxygen transport in the myocardium was used to calculate the probability distribution of intracellular oxygen tension and the hypoxic fraction of the myocardium with an oxygen tension below 0.5 mmHg. The hypoxic fraction of the myocardium was approximately 15% at baseline, increased to approximately 30% during epinephrine infusion, and fell to approximately 0% during cardiac arrest. The coronary venous adenosine concentration changed in parallel with the hypoxic fraction of the myocardium during epinephrine and KCl. It is concluded that catecholamine stimulation of buffer-perfused hearts increases hypoxia in the myocardium and that the increase in venous adenosine concentration is a reflection of the larger hypoxic fraction of myocardium that is releasing adenosine.